UBC Theses and Dissertations

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UBC Theses and Dissertations

Paramagentic impurity centres in alkali halides and strontium compounds Ng, Hok Nam 1971

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PARAMAGNETIC IMPURITY CENTRES I N A L K A L I HALIDES AND STRONTIUM COMPOUNDS b y NG HOK NAM B.Sc., U n i v e r s i t y o f A d e l a i d e ( A u s t r a l i a ) , I967 A T H E S I S SUBMITTED I N P A R T I A L FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e D e p a r t m e n t o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE U N I V E R S I T Y OF B R I T I S H COLUMBIA JUNE 1971 In present ing th i s thes i s in pa r t i a l f u l f i lmen t of the requirements fo r an advanced degree at the Un ivers i ty of B r i t i s h Columbia, | agree that the L ibrary sha l l make i t f r ee l y ava i l ab le for reference and study. I fu r ther agree that perm 1ssion for extens ive copying of th i s thes i s f o r s cho la r l y purposes may be granted by the Head of my Department or by h i s representat ives . It is understood that copying or pub l i ca t i on o f th i s thes i s f o r f i nanc i a l gain sha l l not be allowed without my wr i t ten permiss ion. Department of The Un ivers i ty o f B r i t i s h Columbia Vancouver 8, Canada Date ^yOTUL /'£~. / ? 7 J i i ABSTRACT Paramagnetic impurities containing oxygen were produced i n the reactions of KBr, KC1, NaCl and S r C l ^ with f l u o r i n e . Molecular oxygen, FOO r a d i c a l and C10 2 were I d e n t i f i e d by ESR as reaction by-products. The spectra assigned to CIC^ have predominant powder character even i n single c r y s t a l s of halides. The results are correlated with previous work i n t h i s laboratory. The o r i g i n of oxygen impurity i s suggested to be surface hydroxide ions i n KBr and KC1, and entrapped water i n NaCl and SrClg. Nucleation processes and other anomalous features observed i n these reactions by previous workers are explained by the presence of impurities. Oxygen was found to be incorporated into the S r C l ^ c r y s t a l s by r e c r y s t a l l i s a t i o n from the melt i n the presence of oxygen. It exists i n the form of superoxide ion Cv>" which occupies an i n t e r s t i t i a l p o s i t i o n between two l a t t i c e anions and i s associated with two anion vacancies. The molecular axis l i e s i n a [00l] d i r e c t i o n of the c r y s t a l and the degeneracy of the 2pTJ -molecular o r b i t a l i s l i f t e d by the g c r y s t a l f i e l d . The bonding between the Cv>~ ion and i t s neighbouring cations and anions i s discussed i n terms of a Cl^Sr-Og"*SrClg "complex". The o r b i t a l angular momentum reduction f a c t o r f o r i n SrClg has been calculated from the experimental g-factors and found to be anomalously large. i i i A survey was made on impurities incorporated into strontium compounds through processes of r e c r y s t a l l i s a t i o n from melts or from aqueous solutions. Strontium carbonate was i d e n t i f i e d i n m e l t - r e c r y s t a l l i s e d S r ( N 0 ^ ) 2 by Infrared Spectroscopy and X-ray powder method. It was produced by p a r t i a l decomposition of the n i t r a t e i n the presence of o _ atmospheric COg* i ° n w a s i d e n t i f i e d i n the r e c r y s t a l l i s e d material by ESR. ESR and i n f r a r e d studies p_ suggest that the NO^ ion substitutes a carbonate ion i n the SrCO^ l a t t i c e . Results from a s i m i l a r study on Ba^O^) also support t h i s conclusion. i v TABLE OF CONTENTS PAGE TITLE PAGE ABSTRACT TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES ACKNOWLEDGMENTS CHAPTER I INTRODUCTION 1 I. (A) DEFECTS AND IMPURITIES IN SOLID STATE 1 REACTION MECHANISMS a) I n t r o d u c t o r y Remarks 1 b) Previous Work on O x i d a t i o n of Met a l Halides 3 w i t h Halogens c) Scope of Present Work 9 I . (B) REVIEW OF ESR THEORY 12 a) Theory of the g - f a c t t i r s 12 b) Nuclear Hyperfine S t r u c t u r e 17 c) Powder Sp e c t r a 24 CHAPTER I I PARAMAGNETIC BY-PRODUCTS OF REACTIONS OF 28 •> ALKALI HALIDES AND STRONTIUM "CHLORIDE  WITH FLUORINE I I . (A) APPARATUS AND PROCEDURE 28 a) M a t e r i a l s 28 b) Handling of F l u o r i n e 31 c) Reaction Vessels and Procedure 33 i i i i v v i i i x i x i i V PAGE d) Spectroscopic Techniques 35 e) Preparation of Chlorine Dioxide 38 f) Heating of S r C l ^ i n Vacuo or Dry Nitrogen 40 II. (B) REACTIONS OF KBr WITH FLUORINE: RESULTS 43 AND DISCUSSION a) ESR Spectra hj b) Formation of F00« 46 I I . (C) REACTIONS OF NaCl, KC1 AND S r C l p WITH 50 FLUORINE: RESULTS a) ESR Spectra 50 b) Topotactic Relationship i n Reaction of 6l NaCl with F 2 c) S r C l 2 Reaction: I d e n t i f i c a t i o n of C10 2 63 i n Gas Phase d) S r C l 2 Reaction: C o r r e l a t i o n with Water 6? Retention I I . (D) REACTIONS OF NaCl, KC1 AND S r C l 2 WITH ?0 FLUORINE: DISCUSSION a) Orig i n of Impurity 70 b) Re c o n c i l i a t i o n of Anomalies 72 c) Orientation of the Radical 75 CHAPTER III QQ- SUPEROXIDE ION IN MELT-RECRYSTALLISED 82 STRONTIUM CHLORIDE II I . (A) APPARATUS AND PROCEDURE 82 a) Sample Preparation 82 b) Mounting of Cr y s t a l 83 v i PAGE I I I . ( B ) EXPERIMENTAL RESULTS 8 4 a) A n a l y s i s o f P o l y c r y s t a l l i n e S p e c t r u m 8 4 b) A n a l y s i s o f S i n g l e C r y s t a l S p e c t r a 86 I I I . ( C ) DISCUSSION OP RESULTS 92 a) I d e n t i f i c a t i o n o f S p e c t r a 92 b) S i t e a n d O r i e n t a t i o n o f t h e D e f e c t 101 c) B o n d i n g Scheme o f t h e D e f e c t 103 d) The g - f a c t o r a n d O r b i t a l A n g u l a r Momentum 1 0 8 R e d u c t i o n CHAPTER I V DEFECTS AND I M P U R I T I E S I N RECRYSTALLISED 115 AND X-IRRADIATED STRONTIUM COMPOUNDS I V . ( A ) APPARATUS AND PROCEDURE 115 a) S a m p l e P r e p a r a t i o n 115 b) I n f r a r e d S t u d i e s 116 c) X - r a y P o w d e r T e c h n i q u e s 117 d) X - i r r a d i a t i o n 117 I V . ( B ) EXPERIMENTAL RESULTS 119 a) Summary o f ESR I d e n t i f i c a t i o n o f R a d i c a l s 119 i n X - i r r a d i a t e d S t r o n t i u m Compounds b) The NO32" R a d i c a l I o n 127 I V . ( C ) DISCUSSION 1 4 8 a) The H o s t L a t t i c e o f NOg 2~ 1 4 8 2-b) F o r m a t i o n a n d S t a b i l i t y o f NO^ I n C a r b o n a t e 150 CHAPTER V CONCLUSION AND SUGGESTIONS FOR FUTURE WORK 157 V. (A) CONCLUDING REMARKS 157 v i i PAGE V. (B) SUGGESTIONS FOR FUTURE WORK 158 a) R e a c t i o n of S r C l 2 w i t h F 2 158 b) The 0 2 " I o n i n S r C l 2 159 c) The N 0 ^ ~ I o n i n a Carbonate L a t t i c e 160 BIBLIOGRAPHY 161 v i i i L I S T OF FIGURES FIGURE PAGE I . 1 M o d e l s o f t h e D e f e c t i n S r C l 2 ( a f t e r C a t t o n ) 7 I . 2 V a c a n c y Component o f " V e r n i e r - d e l o c a l i s e d " 8 D e f e c t M o d e l i n S r C l 2 , shown i n a ( 1 1 0 ) P l a n e I . 3 M.O. Scheme o f C l g " M o l e c u l e - I o n ( a f t e r K a n z i g ) 23 I . 4 T y p i c a l ESR P o w d e r S p e c t r a 27 I . 5 T y p i c a l F i r s t D e r i v a t i v e P o w d e r S p e c t r u m f o r 27 a n A x i a l l y S y m m e t r i c R a d i c a l w i t h One N u c l e u s o f S p i n 1 = 1 I I . 1 E v a p o r a t e d F i l m A p p a r a t u s 30 I I . 2 O u t l i n e o f F l u o r i n e H a n d l i n g S y s t e m 32 I I . 3 R e a c t i o n V e s s e l s 3^  I I . 4 M o u n t i n g o f S i n g l e C r y s t a l f o r ESR S t u d i e s 37 I I . 5 A p p a r a t u s f o r P r e p a r a t i o n o f C 1 0 2 39 I I . 6 S r C l 2 D r y i n g S y s t e m 42 I I . 7 ESR S p e c t r u m ( 77°K) o f K B r R e a c t e d w i t h F g 44 I I . 8 ESR S p e c t r u m a t 9 .43 GHz o f G a s e o u s P r o d u c t 4? o f R e a c t i o n K B r / F g I I . 9 ESR S p e c t r a ( 77°K) o f V a c u u m - e v a p o r a t e d KC1 51 R e a c t e d w i t h F 2 5 2 11.10 ESR S p e c t r u m ( 77°K) o f N a C l R e a c t e d w i t h F 2 56 11.11 ESR S p e c t r u m ( 77°K) o f S r C l 2 R e a c t e d w i t h F 2 57 11.12 Z e r o - l a y e r W e i s s e n b e r g P h o t o g r a p h o f N a C l 62 P a r t i a l l y R e a c t e d w i t h F 0 i x FIGURE PAGE 11.13 O p t i c a l A b s o r p t i o n S p e c t r a o f Gaseous 65 P r o d u c t s o f S r C l 2 / F 2 R e a c t i o n 66 11.14 I n t e n s i t y o f C 10 2 ESR S i g n a l v s . Temperature 69 o f Treatment o f S r C l 2 11.15 M o l e c u l e - f i x e d Axes Used f o r C 1 0 2 76 11.16 T r a p p i n g Model o f C10 p i n S r F L a t t i c e Shown 79 i n (110) P l a n e d I I I . 1 Powder Spectrum o f M e l t - r e c r y s t a l l i s e d S r C l g 85 I I I . 2 "Road-map" o f Resonance F i e l d s f o r R o t a t i o n 88 Around and C3 Axes 89 I I I . 3 M a g n e t i c I n e q u i v a l e n c e o f R a d i c a l S p e c i e s A 90 and B i n S r C l 2 P r o j e c t e d on a (110) P l a n e I I I . 4 S i n g l e C r y s t a l ESR S p e c t r a o f M e l t - 91 r e c r y s t a l l i s e d S r C l 2 I I I . 5 Energy L e v e l Scheme o f 0~ and 0 + w i t h 9k p - o r b i t a l s Degeneracy L i f t e d I I I . 6 E n e r g y L e v e l Scheme f o r Oxygen M o l e c u l a r Ions 97 w i t h U - l e v e l Degeneracy L i f t e d I I I . 7 Model o f 0 2 " i n S r C l 2 P r o j e c t e d on a (110) 104 P l a n e I I I . 8 L o c a l Symmetry o f t h e 0 2 " Ion i n S r C l 2 105 I I I . 9 TT- and 6-bonding o f t h e Sr-O-Sr Group 107 I I I . 1 0 M o l e c u l a r O r b i t a l Scheme o f 0 2 " Ion i n a 108 C r y s t a l F i e l d ( D 2 h ) IV. 1 Ap p a r a t u s f o r X - i r r a d i a t i o n a t 77°K 118 IV. 2 ESR Spectrum o f H Atom i n X - i r r a d i a t e d SrSO^ 122 IV. 3 Powder ESR S p e c t r a o f X - i r r a d i a t e d S r ( N 0 3 ) 2 123 IV. 4 ESR S p e c t r a o f M e l t - r e c r y s t a l l i s e d S r ( N 0 ) 130 X FIGURE PAGE I V . 5 ESR S p e c t r u m o f M e l t - r e c r y s t a l l i s e d B a ( N 0 3 ) 2 136 I V . 6 T h e o r e t i c a l H y p e r f i n e S p e c t r u m o f B a 137 I V . 7 I R S p e c t r a o f M e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) ? 139 I V . 8 P o w d e r X - r a y P h o t o g r a p h s o f M e l t - 147 r e c r y s t a l l i s e d S r ( N 0 3 ) 2 I V . 9 P r o j e c t i o n o f t h e A r a g o n i t e S t r u c t u r e A l o n g 156 b - a x i s S h o w i n g P s e u d o h e x a g o n a l A r r a n g e m e n t o f I o n s x i LIST OF TABLES TABLE PAGE 1 1 . 1 ESR Parameters o f the F 0 0 * R a d i c a l 45 1 1 . 2 Summary of ESR R e s u l t s f o r R e a c t i o n s o f 55 M e t a l H a l i d e s w i t h F 2 1 1 . 3 A n i s o t r o p i c ESR Parameters o f C 1 0 2 60 1 1 1 . 1 P r i n c i p a l g - f a c t o r s o f 0 ~ i n I o n i c C r y s t a l s 95 and G l a s s e s 1 1 1 . 2 T y p i c a l g - v a l u e s o f 0 ^ " i n V a r i o u s Host C r y s t a l s 98 1 1 1 . 3 g - f a c t o r s o f 0 2 + and N 2 ~ 99 1 1 1 . 4 ESR Parameters o f OH and H 0 2 R a d i c a l s 100 I I I . 5 S p e c t r o s c o p i c Parameters ~& , ^ and jl o f 0 2 109 C a l c u l a t e d f r o m E x p e r i m e n t a l g - v a l u e s I V . 1 . Summary o f S r Compounds I n v e s t i g a t e d f o r 121 ESR S i g n a l s IV.2 ESR R e s u l t s o f S r ( N 0 3 ) 2 X - i r r a d i a t e d a t Ambient 124 Temperature IV.3 ESR Parameters o f R a d i c a l NO^ 126 IV.4 ESR Parameters o f N 0 2 and NO^ 2" 131 IV.5 E l e c t r o n i c Parameters and Z 0 N 0 o f NO^ 2" and N 0 2 133 IV. 6 Fundamental V i b r a t i o n s o f the D-^ h N i t r a t e I o n 143 IV.7 A d d i t i o n a l I n f r a r e d A b s o r p t i o n Bands o f 145 M e l t - R e c r y s t a l l i s e d S r ( N 0 o ) P x i i ACKNOWLEDGMENTS I w o u l d l i k e t o e x p r e s s my s i n c e r e g r a t i t u d e t o my r e s e a r c h d i r e c t o r . D r . L.G. H a r r i s o n , f o r h i s c o n t i n u a l g u i d a n c e , i n t e r e s t a n d i n v a l u a b l e a d v i c e t h r o u g h o u t my w o r k . T h a n k s a r e a l s o due t o Dr . J . T r o t t e r f o r m a k i n g a v a i l a b l e h i s X - r a y d i f f r a c t i o n e q u i p m e n t , D r . R.C. Thompson f o r t h e u s e o f h i s r e f l e c t a n c e v i s i b l e s p e c t r o p h o t o m e t e r , Dr. F. Aubke f o r h i s v a l u a b l e a d v i c e o n h a n d l i n g o f c o r r o s i v e h a l o g e n compounds, D r s . P.C. C h i e h a n d D. Hughes f o r t h e i r h e l p f u l d i s c u s s i o n s on X - r a y d i f f r a c t i o n r e s u l t s , a n d t h e many i n d i v i d u a l s who, t h r o u g h o u t t h e c o u r s e o f t h i s r e s e a r c h , h a v e h e l p e d i n one way o r t h e o t h e r . The G r a d u a t e F e l l o w s h i p a w a r d e d b y t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a f o r t h e y e a r s 1969-1971 i s a l s o g r a t e f u l l y a c k n o w l e d g e d . CHAPTER I INTRODUCTION 1 I . ( A ) DEFECTS AND I M P U R I T I E S I N S O L I D STATE REACTION MECHANISMS a) I n t r o d u c t o r y Remarks I n t h e s t u d y o f k i n e t i c s a n d m e c h a n i s m s o f s o l i d s t a t e r e a c t i o n s a number o f c h e m i c a l a n d p h y s i c a l p r o c e s s e s h a v e t o be c o n s i d e r e d . C o n s i d e r a r e a c t i o n o f t h e t y p e : 2 A B ( s ) + C 2(g) —> 2 A C ( s ) + B 2 ( g ) o r A B ( s ) + C 2 ( g ) — > A C ( s ) + B C ( g ) . The f o l l o w i n g p r o c e s s e s may be i n v o l v e d : 1. D i f f u s i o n o f t h e r e a c t i n g g a s t o t h e s u r f a c e o f t h e s o l i d 2. A d s o r p t i o n o f t h e g a s on t h e s u r f a c e 3. N u c l e a t i o n a nd g r o w t h o f t h e n u c l e i 4. T r a n s p o r t o f t h e r e a c t i n g g a s t o t h e r e a c t i o n i n t e r f a c e o r i n t o t h e b u l k o f t h e s o l i d 5. T r a n s p o r t o f t h e p r o d u c t g a s t o t h e s u r f a c e 6. D e s o r p t i o n o f t h e p r o d u c t g a s a n d i t s d i f f u s i o n i n t o t h e m a i n b o d y o f t h e g a s . The k i n e t i c s o f d i f f u s i o n , n u c l e a t i o n a n d t r a n s p o r t o f m a t t e r i n t h e s o l i d h a s r e c e n t l y b e e n r e v i e w e d b y H a r r i s o n (*) . The r e l a t i v e i m p o r t a n c e o f t h e s e p r o c e s s e s d i f f e r s f r o m one r e a c t i o n t o a n o t h e r , a n d d e p e n d s o n t h e s t r u c t u r e , s u r f a c e a n d l a t t i c e d e f e c t s , e l e c t r o n s a n d h o l e s , d i s l o c a t i o n s a n d i m p u r i t i e s o f t h e s o l i d u n d e r s t u d y . D e f e c t s g e n e r a t e d d u r i n g t h e r e a c t i o n as 2 i n t e r m e d i a t e s may a l s o p l a y a d e c i s i v e r o l e . The i m p o r t a n c e (o) . o f t h e s e f e a t u r e s h a s b e e n p o i n t e d o u t b y J a c o b s v ' i n r e l a t i o n t o d e c o m p o s i t i o n r e a c t i o n s o f i n o r g a n i c a n d o r g a n i c s o l i d s , a n d b y H a r r i s o n i n a more g e n e r a l way ^ . The c h e m i c a l p u r i t y o f t h e s o l i d has t o o o f t e n b e e n a s s u m e d ; y e t t h e r o l e o f i m p u r i t i e s may be most i m p o r t a n t i n t h e n u c l e a t i o n p r o c e s s . N u c l e a t i o n h a s b e e n r e c o g n i s e d t o t a k e p l a c e a t d e f i n i t e s i t e s w h e r e t h e a c t i v a t i o n e n e r g y i s l e a s t . S u r f a c e i m p e r f e c t i o n s , p o i n t d e f e c t s , d i s l o c a t i o n s a n d c r a c k s a r e p o t e n t i a l s i t e s f o r n u c l e u s f o r m a t i o n . T h e r e i s a l s o a c c u m u l a t i n g e v i d e n c e t h a t i m p u r i t i e s o c c u r i n h i g h e r c o n c e n t r a t i o n s a t t h e s e s p o t s , a t l e a s t i n m e t a l s . T hus t h e i n d u c t i o n p e r i o d ( i n w h i c h n u c l e i a r e f o r m e d a n d g r o w ) r e q u i r e d f o r t h e d e c o m p o s i t i o n o f s i l v e r o x i d e i s r e d u c e d t o a n e g l i g i b l e p e r i o d when t h e o x i d e i s c o n t a m i n a t e d w i t h s i l v e r c a r b o n a t e ( 2 ) . The i n d u c t i o n p e r i o d o f ammonium p e r c h l o r a t e i s s h o r t e n e d b y e x p o s u r e t o r a d i a t i o n p r i o r t o t h e r m a l d e c o m p o s i t i o n . I m p u r i t i e s may be i n c o r p o r a t e d i n t o a s o l i d d u r i n g p r o c e s s e s o f p r e p a r a t i o n a n d r e c r y s t a l l i s a t i o n . I m p u r i t i e s n o t p u r p o s e l y a d d e d t o a s o l i d may be p r e s e n t i n s u c h m i n u t e amount t h a t t h e y a r e n o t d e t e c t a b l e b y o r d i n a r y ^ c h e m i c a l a n a l y s i s . D e f e c t c e n t r e s p r o d u c e d as r e a c t i o n i n t e r m e d i a t e s , i f p r e s e n t , a r e e x p e c t e d t o be l o w i n c o n c e n t r a t i o n as w e l l . The t e c h n i q u e s o f o p t i c a l a b s o r p t i o n a n d e l e c t r o n s p i n r e s o n a n c e a r e most s u i t a b l e f o r d e t e c t i o n a n d i d e n t i f i c a t i o n o f t h e s e i m p u r i t i e s a n d i n t e r m e d i a t e s 3 b e c a u s e o f t h e i r h i g h s e n s i t i v i t y . I n f a v o u r a b l e c a s e s t h e ESR t e c h n i q u e a l s o a l l o w s e l u c i d a t i o n o f t h e s t r u c t u r e s o f t h e s e i n t e r m e d i a t e s a n d t h e t r a p p i n g s i t e s o f i m p u r i t i e s , p r o v i d e d t h e y a r e p a r a m a g n e t i c . N o r m a l l y d i a m a g n e t i c s p e c i e s c a n s o m e t i m e s be made p a r a m a g n e t i c b y h i g h - e n e r g y i r r a d i a t i o n s o r c h e m i c a l r e a c t i o n s a n d t h e i d e n t i t y o f t h e o r i g i n a l i m p u r i t y be d e d u c e d f r o m t h e ESR r e s u l t s . b ) P r e v i o u s Work on O x i d a t i o n o f M e t a l H a l i d e s w i t h H a l o g e n s O x i d a t i o n r e a c t i o n s o f a l k a l i h a l i d e s b y a more e l e c t r o n e g a t i v e h a l o g e n g a s a t room t e m p e r a t u r e s h a v e b e e n e x t e n s i v e l y s t u d i e d I n t h i s l a b o r a t o r y , a n d i n d e p e n d e n t l y b y M o r r i s o n ' s g r o u p i n t h e N a t i o n a l R e s e a r c h C o u n c i l i n O t t a w a . M o r r i s o n a n d Nakayama s t u d i e d t h e r e a c t i o n o f s i n g l e c r y s t a l s o f K B r w i t h C l 2 , a n d f o u n d f r o m m i c r o s c o p i c a n d o t h e r m e a s u r e m e n t s t h a t r e a c t i o n was i n i t i a t e d a t p o i n t s o n t h e c r y s t a l s u r f a c e w h e r e l o c a l s t r a i n h a d b e e n c r e a t e d , a n d t h a t n u c l e a t i o n s i t e s i n c l u d e d c l e a v a g e s t e p s , s c r a t c h e s , p r e c i p i t a t e s o f i m p u r i t i e s a n d p e r h a p s o t h e r I r r e g u l a r i t i e s . P r e v i o u s w o r k i n t h i s l a b o r a t o r y h a s b e e n c e n t r e d on t h e r o l e s o f l a t t i c e a n d e l e c t r o n i c d e f e c t s ( v a c a n c i e s , t r a p p e d h o l e c e n t r e s , e t c . ) . B a i j a l i n h i s s t u d i e s o f o x i d a t i o n o f p r e s s e d p e l l e t s o f K I b y C l 2 f o u n d a n i n v e r s e r e l a t i o n s h i p b e t w e e n i n i t i a l c o n d u c t a n c e a n d r e a c t i v i t y o f t h e p e l l e t s . Two t y p e s o f r e a c t i o n s w e r e d i s t i n g u i s h e d d e p e n d i n g on t h e i n i t i a l c o n d u c t i v i t y o f t h e 4 p e l l e t s , w h i c h was t a k e n as a m e a s u r e o f t h e i n i t i a l c o n c e n t r a t i o n o f c a t i o n v a c a n c i e s . F o r p e l l e t s w i t h h i g h i n i t i a l c o n d u c t i v i t y no n u c l e a t i o n p r o c e s s was f o u n d , a n d t h e c o n d u c t i v i t y r e s u l t s w e r e i n t e r p r e t e d i n t e r m s o f t r a p p i n g o f p o s i t i v e h o l e s a t i s o l a t e d c a t i o n v a c a n c i e s d u r i n g o x i d a t i o n as t h e d o m i n a n t p r o c e s s . P o s i t i v e h o l e s w e r e e x p e c t e d t o f o r m f r o m r e m o v a l o f e l e c t r o n s f r o m t h e a n i o n b a n d i n t h i s t y p e o f o x i d a t i o n r e a c t i o n . F o r p e l l e t s w i t h l o w i n i t i a l c o n d u c t i v i t y , t r a p p i n g o f p o s i t i v e h o l e s was c o n s i d e r e d t o t a k e p l a c e c l o s e t o t h e g r a i n b o u n d a r i e s , l e a d i n g t o n u c l e a t i o n o f s o l i d i o d i n e . The n a t u r e o f t h e s e t r a p p e d h o l e s was n o t e s t a b l i s h e d , a l t h o u g h i n o x i d a t i o n o f s i n g l e c r y s t a l s o f K I o p t i c a l a b s o r p t i o n b a n d s w e r e f o u n d w h i c h a p p e a r e d t o c o r r e s p o n d t o known V - b a n d s p r o d u c e d i n K I b y d o p i n g w i t h I 2 . S i m i l a r V - b a n d s w e r e o b s e r v e d i n s i n g l e c r y s t a l s o f N a C l r e a c t e d w i t h F 2 . F o r p o l i s h e d c r y s t a l s o f N a C l t h e o v e r a l l e x t e n t o f t h e r e a c t i o n was r a t h e r s m a l l , a n d a b a n d a t 2150 A was o b s e r v e d . The e x t e n t o f r e a c t i o n was much g r e a t e r i f a s c r a t c h was made a c r o s s o t h e c r y s t a l , a n d t h e V - b a n d t h e n p e a k e d a t 2270 A. T h e s e two a b s o r p t i o n b a n d s were a t t r i b u t e d t o t h o s e o f V^ and V 2 ~ c e n t r e s , s u p p o s e d b y S e i t z t o c o n t a i n one a n d t w o t r a p p e d h o l e s r e s p e c t i v e l y . The V - b a n d o b s e r v e d b y C a t t o n i n s i n g l e c r y s t a l s o f K B r o x i d i s e d by C l p was s u g g e s t e d t o c o r r e s p o n d t o a V 2 - c e n t r e p r o d u c e d b y d o p i n g K B r w i t h B r 2 . 5 More c l o s e l y r e l a t e d t o t h e p r e s e n t w o r k a r e t h e ESR a n d o t h e r s t u d i e s b y C a t t o n a n d Rantamaa on t h e r e a c t i o n s o f N a C l , KC1 a n d S r C l 2 w i t h F 2 . An ESR s i g n a l was o b s e r v e d b y Adams a n d l a t e r s t u d i e d b y C a t t o n (9) i n t h e r e a c t i o n s o f v a c u u m - s u b l i m e d N a C l a n d KC1 w i t h F g , b u t n o t w i t h s i n g l e c r y s t a l s o r o r d i n a r y c o a r s e p o w d e r s . B o t h s p e c t r a i n N a C l a n d KC1 were v e r y s i m i l a r , a n d t h e h y p e r f i n e s t r u c t u r e was u n r e s o l v e d i n t h e c a s e o f N a C l b u t was p a r t l y r e s o l v a b l e i n KC1. T h e y w e r e I n t e r p r e t e d b y a m o d e l o f t h e d e f e c t as 3-l i n e a r C l ^ , i . e . , a n H - c e n t r e o r s o m e t h i n g v e r y s i m i l a r . The h i g h t e m p e r a t u r e s t a b i l i t y o f t h i s H - c e n t r e was a t t r i b u t e d t o a s p e c i a l p r o p e r t y o f t h e v a c u u m - s u b l i m e d m a t e r i a l w h i c h c o n s i s t s o f e s s e n t i a l l y p e r f e c t c r y s t a l s w i t h no s i t e s a v a i l a b l e t o a c c e p t e l e c t r o n s t r a n s f e r r e d f r o m t h e H - c e n t r e , a p r o c e s s t h o u g h t t o be r e s p o n s i b l e f o r t h e d e s t r u c t i o n o f H - c e n t r e s a b o v e 22°K ( 1 0 ) . The ESR s i g n a l a p p e a r e d o n l y a f t e r F^ was pumped o u t , a n d t h e r a t e o f g r o w t h was e x p o n e n t i a l i n t i m e i n d i c a t i n g a u t o - c a t a l y t i c f o r m a t i o n o f t h e d e f e c t . The d e c a y o f t h e s i g n a l was r e t a r d e d b y t h e p r e s e n c e o f C l 2 g a s , a n d t h i s was i n t e r p r e t e d i n t e r m s o f b l o c k i n g b y C l 2 o f s u r f a c e s i t e s a t w h i c h two d e f e c t s c o u l d c o m b i n e t o d e s o r b a C l ^ m o l e c u l e . ( 1 1 ) C a t t o n e x t e n d e d t h e s t u d i e s t o t h e g r o u p I I h a l i d e s A p a r a m a g n e t i c s p e c i e s was p r o d u c e d i n t h e r e a c t i o n o f S r C l 2 w i t h F 2 , b o t h i n p o w d e r s a m p l e s a n d s i n g l e c r y s t a l s . The g r o w t h o f t h e ESR s i g n a l s t o p p e d w i t h e v a c u a t i o n o f F The 6 spectra obtained i n single c r y s t a l specimens, although showing orien t a t i o n dependence, have powder character i n some of the lineshapes. The hyperfine structure could be associated with a single CI nucleus. The model of the defect responsible for t h i s s i g n a l was proposed to be a chlorine atom displaced from a normal anion s i t e along a [lOO] d i r e c t i o n towards a neighbouring anion vacancy (F i g . 1.1). The anisotropy of the spectra was considered to be consistent with the model i n which the degeneracy of the 3p-state of the CI atom was completely l i f t e d by the c r y s t a l f i e l d having orthorhombic symmetry. The unpaired electron could then occupy mainly the 3 P x - o r b i t a l along which d i r e c t i o n a maximum hyperfine s p l i t t i n g and a minimum g - s h i f t were observed. A neighbouring anion vacancy was postulated to explain why the CI atom did not i n t e r a c t with one or more neighbouring anions to form the C l ^ or V^-type molecular ion, which i s so well-known i n a l k a l i halides vand is the only hole centre i n a l k a l i n e earth fluor i d e s known from ESR ( 1 2 ) . The only known hole centre in S r C l 2 i s the V^-centre associated with four CI n u c l e i produced by X - i r r a d i a t i o n at 77°K ( 1 3 ) . The powder character of Catton's single c r y s t a l spectra was suggested to be a result of the CI atom occupying a continuous range of positions between two adjacent anion s i t e s (marked X i n F i g . I . l a ) . The CI atom was considered to approach the reaction interface along the z-axis, which may F i g u r e 1.1 M o d e l s o f t h e D e f e c t i n S r C l 0 ( a f t e r C a t t o n ) 8 b e t h o u g h t o f as t h e " r e a c t i o n c o o r d i n a t e " , a n d r e a c h t h e t o p o f a n e n e r g y b a r r i e r f o r m i g r a t i o n a t t h e s p e c i a l p o s i t i o n o f F i g . I . l b . The m o d e l was l a t e r m o d i f i e d ( ^ ) t o a v o i d p l a c i n g t w o e f f e c t i v e p o s i t i v e c h a r g e s a d j a c e n t t o e a c h o t h e r --a p o s i t i v e h o l e a n d an a n i o n v a c a n c y . The l o c a l i s e d a n i o n v a c a n c i e s w e r e r e p l a c e d w i t h a " v e r n i e r - d e l o c a l i s e d " m o d e l as shown i n F i g . 1.2. The d o m i n a n t t h e r m a l d i s o r d e r i n S r C L p i s o f t h e A n t i - F r e n k e l t y p e , i . e . , a n i o n v a c a n c i e s p l u s i n t e r s t i t i a l a n i o n s . B o t h t h e v a c a n c y a n d t h e i n t e r s t i t i a l w e r e c o n s i d e r e d t o be d e l o c a l i s e d b y " v e r n i e r -l i k e " d i s p l a c e m e n t a l o n g a [ 1 0 0 ] d i r e c t i o n , p r o v i d i n g a r a n g e o f d i s p l a c e d a n i o n s i t e s . The p o s i t i v e h o l e c o u l d t h e n o c c u p y an y o f t h e s e s i t e s , g i v i n g r i s e t o t h e p o w d e r l i n e s h a p e s . o o o o o _ o .. o O"" O "' O " O '"' o " o " o F i g u r e 1.2. V a c a n c y component o f " V e r n i e r - d e l o c a l i s e d " d e f e c t m o d e l i n S r C ^ , shown i n a ( 1 1 0 ) p l a n e . Open c i r c l e s , Sr^" 1" i n p l a n e ; d o t t e d c i r c l e s , S r 2 + a b o v e a n d b e l o w p l a n e ; X a n i o n s i t e s ; • a n i o n s ( a f t e r R e f . 1 4 a ) . 9 The g r o w t h o f t h e ESR s i g n a l was f o u n d b y Rantamaa v t o o b e y a p a r a b o l i c l a w ( t h e p l o t o f p e a k h e i g h t v s . s q u a r e r o o t o f t i m e i s l i n e a r ) u n d e r e x p e r i m e n t a l c o n d i t i o n s i n w h i c h t h e r a t e o f r e a c t i o n was c o n t r o l l e d b y g a s e o u s d i f f u s i o n down a n a r r o w t u b e t o t h e s o l i d s a m p l e . T h e r e was e v i d e n c e t h a t t h e s i g n a l was p r e s e n t o n l y i n t h e p r o d u c t l a y e r . The r e a c t i o n was f u r t h e r s t u d i e d g r a v i -m e t r i c a l l y i n a 2 - l i t r e s p h e r i c a l f l a s k , a n d t h e r a t e l a w was f o u n d t o o b e y t h e G i n s t l i n g - B r o u n s h t e i n e q u a t i o n f o r d i f f u s i o n t h r o u g h a s p h e r i c a l s h e l l o f s o l i d r e a c t i o n p r o d u c t h a v i n g a s h a r p i n t e r f a c e w i t h t h e r e a c t a n t . The p r o c e s s was d e d u c e d t o be g a s e o u s d i f f u s i o n i n c r a c k s i n t h e p r o d u c t l a y e r . The n u c l e a t i o n p r o c e s s was s t u d i e d a l s o b y R a n t a m a a b y m i c r o - p h o t o g r a p h i c t e c h n i q u e ( 1^" i) . The m a i n p a r t o f t h e i n d u c t i o n p e r i o d c o n s i s t e d o f g r o w t h o f n u c l e i , f o r m e d on e d g e s o f t h e c r y s t a l o r a t s c r a t c h e s made on i t s f a c e . Two t y p e s o f b e h a v i o u r w e r e o b s e r v e d , one i n w h i c h t h e n u c l e i g r e w w i t h a s t e a d y r a t e a n d t h e o t h e r i n w h i c h t h e r a t e c h a n g e d s h a r p l y a n d t w o l i n e a r r a t e s w e r e a p p a r e n t . The g r o w t h r a t e was s e e n t o a c c e l e r a t e a l o n g a c r a c k . c ) S c o p e o f P r e s e n t Work The r e s o l u t i o n o f t h e s p e c t r a o b t a i n e d i n N a C l , KC1 a n d S r C l g r e a c t e d w i t h f l u o r i n e was r a t h e r p o o r . Some e x p e r i m e n t a l f e a t u r e s o f t h e S r C l g r e a c t i o n a p p e a r e d t o be 10 a n o m a l o u s and d i f f i c u l t t o e x p l a i n b y t h e m o d e l f o r t h e d e f e c t p r o p o s e d i n p r e v i o u s w o r k , a n d t h e r e was g r o u n d f o r s u s p e c t i n g t h e p r e s e n c e o f i m p u r i t i e s ( s e e C h a p t e r I I , S e c t i o n s I I . ( G ) a) & H . ( D ) a) ) . To c l a r i f y t h e m a t t e r a n d i n p u r s u a n c e o f t h e r o l e s o f d e f e c t s and i m p u r i t i e s i n t h e s e r e a c t i o n s , i t was c o n s i d e r e d n e c e s s a r y t o r e - e x a m i n e t h e s e s p e c t r a a t l o w t e m p e r a t u r e s . The f i r s t p a r t o f t h i s w o r k d e a l s w i t h t h e r e s u l t s o f t h i s i n v e s t i g a t i o n . The i n v e s t i g a t i o n r e v e a l e d t h e p r e s e n c e a n d i m p o r t a n c e o f o x y g e n - c o n t a i n i n g i m p u r i t i e s . A number o f r e f e r e n c e s c a n be f o u n d i n t h e l i t e r a t u r e on i n c o r p o r a t i o n o f OH" a n d 02~ i o n s i n t h e l a t t i c e s o f a l k a l i h a l i d e s b y h e a t i n g t h e c r y s t a l s i n o x y g e n o r w a t e r v a p o r (*5-l8)^ R e l a t i v e l y l i t t l e i s known a b o u t t h e a l k a l i n e e a r t h compounds a l o n g t h i s l i n e . B o n t i n c k (*9) d e t e c t e d t h e p r e s e n c e o f h y d r o x y l i o n s a n d CaO a f t e r h e a t i n g C a P 2 i n t h e p r e s e n c e o f w a t e r v a p o r . The s e c o n d p a r t o f t h i s w o r k d e a l s w i t h t h e s t u d y o f a p a r a m a g n e t i c s p e c i e s f o u n d i n S r C l ^ r e c r y s t a l l i s e d f r o m t h e m e l t i n t h e p r e s e n c e o f o x y g e n . S r C l g was l a t e r d o p e d i n t h e m e l t w i t h h y d r o x i d e a n d o t h e r o x y a n i o n s , X - i r r a d i a t e d a n d s t u d i e d b y ESR i n an a t t e m p t t o e s t a b l i s h p o s s i b l e s o u r c e s o f o x y g e n i m p u r i t i e s o t h e r t h a n a t m o s p h e r i c o x y g e n . • The s t u d y was e x t e n d e d t o o t h e r a l l e g e d l y " p u r e " s t r o n t i u m compounds t o f i n d o u t w h a t i m p u r i t i e s may be i n t r o d u c e d i n t o t h e s e s y s t e m s t h r o u g h p r o c e s s e s o f r e c r y s t a l l i s a t i o n , e i t h e r i n t h e m e l t o r f r o m 11 aqueous solutions. One can perhaps anticipate that these Impurities may manifest themselves i n the form of substituent ions (e.g., 0 2 i n KC1), or i n creating s t r u c t u r a l defects (e.g., F or V-centres i n many a l k a l i h a l i d e s ) . These species may be stable radicals formed during processes of r e c r y s t a l l i s a t i o n , or may be made paramagnetic by high-energy i r r a d i a t i o n s , and thus become capable of detection by ESR. This constitutes the f i n a l part of the work of t h i s t h e s i s . 12 I . (B) REVIEW OF ESR THEORY The p r i n c i p a l t e c h n i q u e used i n t h i s work has been E l e c t r o n S p i n Resonance. I d e n t i f i c a t i o n o f r a d i c a l s p e c i e s and t h e i r r e l a t i o n s h i p w i t h the h o s t l a t t i c e w i l l be f r e q u e n t l y d i s c u s s e d , based on g - f a c t o r s and h y p e r f i n e s t r u c t u r e s . A b r i e f r e v i e w on t h e p e r t i n e n t t h e o r i e s i s thus deemed n e c e s s a r y . These t h e o r i e s have been w e l l - f o r m u l a t e d and comprehensive t r e a t m e n t s are p r o v i d e d i n many books and r e v i e w s , among w h i c h a re t h o s e by Pake S l i c h t e r ( 2 1 ) , C a r r i n g t o n (22) (23) (24} and McLachlan v , A t k i n s and Symons v ,Ay.scough v ' and Abragam and B l e a n e y a) Theory o f t h e g - F a c t o r s (25) An u n p a i r e d e l e c t r o n w i t h s p i n a n g u l a r momentum S has a magnetic moment JA$ a s s o c i a t e d w i t h i t . They ar e r e l a t e d by the e x p r e s s i o n where g i s t h e e l e c t r o n i c g - f a c t o r and /3 i s t h e Bohr magneton. F o r a f r e e e l e c t r o n g t a k e s t h e v a l u e o f g e = 2.0023 and i s i s o t r o p i c . F o r an e l e c t r o n i n an atomic o r m o l e c u l a r o r b i t a l however, i t may p o s s e s s o r b i t a l a n g u l a r momentum as w e l l . The " e f f e c t i v e " g - f a c t o r c a l c u l a t e d f r o m t h e measured magnetic moment and e q u a t i o n (1.1) w i l l now d e v i a t e f r o m t h e f r e e s p i n 13 value g g and becomes anisotropic. The i n t e r a c t i o n between the spin and o r b i t a l angular momenta and an external magnetic f i e l d H may be represented by the Hamiltonians f 4 1 + # 2 = gjK'S + /H-L . . . . . . . . (1.2) If the electron o r b i t a l ground state i s non-degenerate, a l l matrix elements involving L vanish; the o r b i t a l angular momentum i s s a i d to be quenched and the g-factor remains i s o t r o p i c and equal to g g . However, some o r b i t a l angular meomentum may be acquired by the electron through s p i n - o r b i t coupling, r e s u l t i n g i n a g-factor s h i f t . This e f f e c t of s p i n - o r b i t coupling may be represented by the Hamiltonian # 3 = Ml* S (1-3) where X i s c a l l e d the spi n - o r b i t coupling constant. In general the energies represented by the Hamiltonians 'f'f^s "f£> and "ff^ are a few orders of magnitude less than the o r b i t a l e l e c t r o n i c energy so that the problem may be treated by perturbation theory. The s p i n - o r b i t a l wave functions of the unpaired e l e c t r o n may be represented by , mg y . The e f f e c t of sp i n - o r b i t coupling represented b y ^ i s to admix the ground state wave function with excited states. By f i r s t order perturbation theory the modified wave functions w i l l be 14 where the summation q i s over the x,y and z di r e c t i o n s , the summation n i s over a l l the excited state wave functions, the summation m". Is over the spin states ( (L and Ji f o r S = ^ ) , E Q i s the o r b i t a l energy of and E n the o r b i t a l energy of Vj,. On applying the second perturbing Hamiltonian ^ to these modified wave functions, the matrix elements become = fi^.^i < t , ^ i \ L % ' \ f 0 , ms > • .i v v r H » '<^^ w - I^US»|-An, » u > ^ i ' , , , , , / + E . - E „ ;• < ^ ( « s I L i ' l ^ l m i > + 2nd order terms. The f i r s t term involves matrix elements <^)^ / L ^ ' / w h i c h are a l l zero f o r non-degenerate o r b i t a l ground states. Retaining only f i r s t order terms and noting that the perturbation i s diagonal i n m because the operator H • L does not include S ~ spin, one finds that the matrix elements within the unperturbed ground state become < ^ , m5 I A L-S + Ji H-L I ms> 14x + U. = X L-S + y3 H-L = P H-^J'S ( 1 . 5 ) 15 where A g n ' - ^ E. - En t 1 - 6 ' So t h e Zeeman i n t e r a c t i o n b e t w e e n t h e s p i n and o r b i t a l a n g u l a r momenta o f t h e e l e c t r o n and a n e x t e r n a l m a g n e t i c f i e l d i n t h e p r e s e n c e o f s p i n - o r b i t c o u p l i n g i s r e p r e s e n t e d b y # 2 = S-i'S-S + /3H-L + A L . S = /3 g H ' 1 - S + y 3 H - ^ g ' S = /^H-g-S ( i . 7 ) w h e r e 1 i s a u n i t m a t r i x o f d i m e n s i o n 3 x 3 a n d g = 2.0023 1 + i s t h e e f f e c t i v e g - t e n s o r . A few i m p o r t a n t q u a l i t a t i v e c o n c l u s i o n s may be d r a w n f r o m e q u a t i o n (1.6) on g - f a c t o r s h i f t s . - . i . The c o n t r i b u t i o n o f a c e r t a i n e x c i t e d s t a t e t o t h e g - s h i f t may be p o s i t i v e o r n e g a t i v e . I f t h e u n p a i r e d e l e c t r o n i s e x c i t e d t o a n e m p t y o r b i t a l o f h i g h e r e n e r g y , t h e n E n y EQ and t h e g - s h i f t w i l l be n e g a t i v e . I f t h e e x c i t e d s t a t e i s f o r m e d b y p r o m o t i n g a n e l e c t r o n f r o m a n i n n e r f i l l e d o r b i t a l t o t h e o r b i t a l o c c u p i e d b y t h e u n p a i r e d e l e c t r o n , t h e n E n ^ E o a n d t n e S " s n i f t w i l l be p o s i t i v e . 16 i i . The d i r e c t i o n a l c h a r a c t e r i s t i c of the matrix elements <t^ I L ^ / Y ^ ><f0 ) L^'llL. > renders the g - s h i f t s a n i s o t r o p i c and determines which excited states may be coupled to the ground state by spin-or b i t coupling. I f the o r b i t a l s represented by and the o r b i t a l angular momentum operator transform according to the Irreducible representations r 0(G), r n(G) and Tq(G) respectively under the symmetry group G, then according to Group theory the matrix elements ( V ^ . l L ^ l V ' o ) w i l l be zero unless the direct product of any two representations, say Tn(G) x r Q(G), is or contains the t h i r d , rq(G). i i i . The lowest-lying excited states make the largest contribution to the g - s h i f t due to t h e i r small energy separations from the ground state. Large g - s h i f t s are expected f o r paramagnetic molecules with degenerate e l e c t r o n i c ground states f o r the i s o l a t e d molecule, where the degeneracy i s l i f t e d by i n t e r a c t i o n of the molecule with i t s environment. Usually, there i s only one low-lying excited state which may be coupled to the ground state i n a s p e c i f i c d i r e c t i o n q and the summation over n i n equation (1.6) may be omitted. The r e s u l t i s (1.8) It must be borne i n mind that when the g - s h i f t i s small, contributions from higher excited states may reverse the sign of the g - s h i f t predicted by this s i m p l i f i e d expression. 17 N u c l e a r H y p e r f i n e S t r u c t u r e N u c l e a r h y p e r f i n e s t r u c t u r e a r i s e s f r o m i n t e r a c t i o n o f t h e m a g n e t i c moments o f t h e u n p a i r e d e l e c t r o n and o f a n y m a g n e t i c n u c l e i w i t h i n i t s o r b i t a l . The i n f o r m a t i o n d e r i v e d f r o m i t i s most u s e f u l . I t i s o f t e n d i a g n o s t i c o f t h e p a r a m a g n e t i c s p e c i e s a n d h e l p s t o d e t e r m i n e t h e o r i e n t a t i o n o f t h e r a d i c a l i n a c r y s t a l . F r o m t h e h y p e r f i n e d a t a t h e s p i n p o p u l a t i o n o f s- a n d p - o r b i t a l s o n m a g n e t i c n u c l e i may be e s t i m a t e d a n d h e n c e t h e e l e c t r o n d i s t r i b u t i o n a n d o f t e n b o n d a n g l e o f t h e m o l e c u l e d e t e r m i n e d . I t i s r e l a t i v e l y s t r a i g h t - f o r w a r d t o r e l a t e t h e number a n d r e l a t i v e i n t e n s i t y o f t h e h y p e r f i n e l i n e s t o t h e m a g n e t i c n u c l e u s ( o r n u c l e i ) i n v o l v e d f o r d i a g n o s t i c p u r p o s e s t h o u g h t h i s may n o t be a l w a y s u n a m b i g u o u s . I t i s how t h e o t h e r i n f o r m a t i o n may be d e r i v e d t h a t r e q u i r e s a c l o s e r e x a m i n a t i o n o f t h e t h e o r y o f n u c l e a r h y p e r f i n e s t r u c t u r e , w h i c h w i l l now be v e r y b r i e f l y r e v i e w e d . The s p i n H a m i l t o n i a n d e s c r i b i n g t h e h y p e r f i n e i n t e r a c t i o n b e t w e e n e l e c t r o n a n d n u c l e a r s p i n a n g u l a r momenta S a n d I may be w r i t t e n as h f s = S • A • I ( 1 . 9 ) w h e r e A i s t h e h y p e r f i n e c o u p l i n g t e n s o r w i t h p r i n c i p a l v a l u e s A , A^ a n d A . T h i s t e n s o r c o n t a i n s a n i s o t r o p i c p a r t a i g o a n d a n a n i s o t r o p i c p a r t B. 18 " A x a i s o = a i s o + B y A z I s o — _ — . . . (1.10) and a l s Q = 3 ( A x + Ay + A z ) ( I . H ) s i n c e B i s a t r a c e l e s s t e n s o r , i . e . , B ' + B + B = 0 x y z The I n t e r a c t i o n r e p r e s e n t e d by t h e i s o t r o p i c h y p e r f i n e c o u p l i n g c o n s t a n t a ^ S Q i s known as t h e F e r m i o r c o n t a c t i n t e r a c t i o n . I t depends on a f i n i t e p r o b a b i l i t y d e n s i t y o f t h e u n p a i r e d e l e c t r o n a t t h e n u c l e u s and i s g i v e n by a i s o = A o • C n s 2 = * f & ^ j \ t n s ( 0 ) | 2 • c n s 2 . . . . ( i . i 2 ) where g-j. and y3^ a r e the n u c l e a r g - f a c t o r and magneton d e f i n e d so as t o c o r r e s p o n d t o t h e d e f i n i t i o n s o f t h e e l e c t r o n i c g - f a c t o r and Bohr magneton. A i s t h e t h e o r e t i c a l i s o t r o p i c o h y p e r f i n e s p l i t t i n g f o r an atom w i t h an u n p a i r e d e l e c t r o n i n i t s n s - o r b i t a l , 'f'ns(o) i s t h e e l e c t r o n i c wave f u n c t i o n o f t h e 2 n s - o r b i t a l a t t h e n u c l e u s and c n g r e p r e s e n t s t h e s p i n d e n s i t y on t h e n s - o r b i t a l o f t h i s atom. F o r an u n p a i r e d e l e c t r o n i n an a t o m i c p - o r b i t a l o r i n a II-type m o l e c u l a r o r b i t a l c o n s t r u c t e d f r o m a t o m i c p - o r b i t a l s , no i s o t r o p i c h y p e r f i n e s p l i t t i n g s h o u l d be o b s e r v e d because ^p(o) i s z e r o . However, two mechanisms may be o p e r a t i n g t h a t g i v e r i s e t o i s o t r o p i c 19 i n t e r a c t i o n . The f i r s t one i s c a l l e d configuration i n t e r a c t i o n , which i s the admixture of excited e l e c t r o n i c states with the ground state to produce a more accurate de s c r i p t i o n of the wave function of the unpaired electron. So, although the ground state wave function may be constructed purely from atomic p - o r b i t a l s , the excited state may have some s-character. The second mechanism i s c a l l e d c o r e - p o l a r i s a t i o n . This i s the magnetic p o l a r i s a t i o n of an atomic or i o n i c core of closed s h e l l s (the inner s-shells) by an u n f i l l e d external s h e l l . It should be noted that spin density on a neighbouring atom may also p o l a r i s e the ©"'-bonding electrons, and according to the Pauli Exclusion P r i n c i p l e the r e s u l t i n g spin " f e l t " by the atom under consideration w i l l be a n t i - p a r a l l e l to the o r i g i n a l spin. A negative contribution to the i s o t r o p i c hyperfine s p l i t t i n g i s thus produced. Both these mechanisms give r i s e to anomalous i s o t r o p i c hyperfine s p l i t t i n g s . They may be regarded as al t e r n a t i v e approaches to the same problem and i t i s i n fact d i f f i c u l t to d i f f e r e n t i a t e between them. The anisotropic i n t e r a c t i o n represented by the tensor quantity B i s e s s e n t i a l l y a c l a s s i c a l i n t e r a c t i o n of two magnetic dipoles JULs and juit separated by a distance r. The corresponding Hamiltonian may be written as ft an!* = g ^ g I % < 3 - C ° S 2 | " 1 > £ ' S (1.13) .1 Where 9 i s the angle between the axes of the dipoles (assumed 20 t o be b o t h q u a n t i s e d i n the d i r e c t i o n o f the e x t e r n a l magnetic f i e l d ) and the l i n e j o i n i n g them. T h i s i n t e r a c t i o n i s seen t o be d i r e c t i o n a l owing t o the te r m c°s 20 - i ^ w h i c h i s an average o v e r the s p a t i a l d i s t r i b u t i o n o f the u n p a i r e d e l e c t r o n . C o n s i d e r a s i n g l e e l e c t r o n i n an n p z - o r b i t a l . When the magnetic f i e l d i s a p p l i e d a l o n g the z - a x i s , 0 i s c l o s e t o z e r o and (3 c o s 2 0 - l ) i s p o s i t i v e and n e a r a maximum i n t h e r e g i o n o f maximum e l e c t r o n d e n s i t y . So the o b s e r v e d h y p e r f i n e s p l i t t i n g B Q w i l l be l a r g e and p o s i t i v e . C a l c u l a t i o n u s i n g e q u a t i o n (1.13) and the wave f u n c t i o n o f a p - o r b i t a l shows t h a t B 0 = | g / r g l f t ( r - 3 ) N P (I.1A) When the magnetic f i e l d l i e s a l o n g t h e x- o r y - a x i s , 0 i s c l o s e t o — and (3 cos 6 - 1) i s n e g a t i v e i n the r e g i o n o f maximum e l e c t r o n d e n s i t y . So the o b s e r v e d h y p e r f i n e s p l i t t i n g w i l l be s m a l l e r and n e g a t i v e . C a l c u l a t i o n shows t h i s s p l i t t i n g t o be - i B Q . A n a l o g o u s l y t o i s o t r o p i c h y p e r f i n e s p l i t t i n g , t h e u n p a i r e d s p i n d e n s i t y on an n p - o r b i t a l c n p 2 i s r e l a t e d t o the o b s e r v e d a n i s o t r o p i c h y p e r f i n e s p l i t t i n g by the e q u a t i o n s B l l = B 0 • C N P2 (1.15) Bj. = -4B 0C n p 2 . . . (1.16) I t s h o u l d be n o t e d t h a t e q u a t i o n (1.13) i s v a l i d o n l y t o the e x t e n t t h a t the n u c l e u s may be assumed t o be q u a n t i s e d a l o n g the e x t e r n a l f i e l d , a s i t u a t i o n a l m o s t r e a l i s e d when t h e r e i s a l a r g e i s o t r o p i c h y p e r f i n e component p r e s e n t . When t h e r e i s l i t t l e o r no s - c o n t r i b u t i o n , the a n g u l a r dependence f o l l o w s (3 cos 20 + 1)^ i n s t e a d o f (3 c o s 2 0 - 1), as p o i n t e d out by Z e l d e s and coworkers ^ 21 A l l the expressions f o r nuclear hyperfine s p l i t t i n g s presented here are i n energy units. But since spectra are almost always recorded at constant frequency and variable magnetic f i e l d , hyperfine s p l i t t i n g s are more conveniently expressed i n gauss. In that case A D = ^ j - Sl / W n s < ° ) | 2 (1.17) B o " | S A ^ " 3 ) n p (1-18) I f the molecular o r b i t a l containing the unpaired electron includes an sp n-hybrid on a c e n t r a l atom that i s magnetic, the bond angle may be determined from the hyb r i d i s a t i o n p r a t i o X . X i s the r a t i o of spin densities on the p- and s - o r b i t a l s of the c e n t r a l atom. - £ (1.19) G s For a C^v molecule the bond angle <t> i s given by <*> = 2 c o s " 1 ( X 2 + 2)~* (1-20) For a C^ v molecule the bond angle <f> i s given by -1 f 1.5 1~| <*> = cos 2\d +3 2 i 1 - 2 1 ) A number of approximations and assumptions are Involved i n c a l c u l a t i n g the spin density by equations (1.12), (1.15) and (I.l6). The major ones include the neglect of contribution 22 to s-character from inner s h e l l p o l a r i s a t i o n and the uncertainty i n published values of \tns(o)\l and <r" >„ p due to the approximations involved i n constructing the wave functions. Further assumptions are involved i n c a l c u l a t i n g the bond angles from the h y b r i d i s a t i o n r a t i o . These include the assumptions that the hybrid o r b i t a l s are directed along the bonds, and that there i s no p a r t i c i p a t i o n of d and of higher o r b i t a l s (or more generally, that the LCAO molecular o r b i t a l f o r the unpaired e l e c t r o n i s v a l i d ) . The bond angle calculated t h i s way i s therefore very approximate and also, unfortunately, not very s e n s i t i v e to the value of X i n the regions of importance. Nevertheless the method gives a f a i r l y good idea as to how the bond angle may vary with the matrices i n which the r a d i c a l i s trapped. Radicals are often trapped i n a s o l i d with preferred ori e n t a t i o n s . The ESR spectra are thus complicated by anisotropics i n the g- and hyperfine tensors. These complications, when unravelled, f u r n i s h important information on the or i e n t a t i o n of the r a d i c a l i n a s o l i d . Q u a l i t a t i v e l y the idea may be i l l u s t r a t e d by an example, the V^-centres i n a l k a l i halides found by Kanzig and coworkers ( 2 ^ ) . The V k-centre i n K C 1 i s e s s e n t i a l l y a C l ^ " molecule-ion i d e n t i f i e d by i t s basic hyperfine pattern of seven l i n e s with i n t e n s i t y r a t i o 1 : 2 : 3 : 4 : 3 : 2 : 1 (both 3 5 C 1 and 3 7 C 1 have nuclear spins I = — ) . The usual treatment of the molecular o r b i t a l begins with a l i n e a r combination of atomic o r b i t a l s of the two chlorine atoms. The unpaired e l e c t r o n should 23 occupy the 3?°^ l e v e l i n i t s ground s t a t e , formed f r o m the at o m i c p - o r b i t a l s o f t h e c h l o r i n e atoms. The m o l e c u l a r z o r b i t a l scheme i s shown i n F i g . 1.3, w i t h t h e e x c i t e d s t a t e s w h i c h c o n t r i b u t e t o t h e g - f a c t o r s h i f t s a l o n g t h e p r i n c i p a l m o l e c u l a r axes. The o r b i t a l a n g u l a r momentum a s s o c i a t e d w i t h t h e m o l e c u l a r a x i s ( z - a x i s ) i s g r e a t l y s u p p r e s s e d o r quenched by f o r m a t i o n o f t h e m o l e c u l a r bond. U s i n g t h e p r i n c i p l e s o u t l i n e d e a r l i e r on g - f a c t o r s , one may p r e d i c t t h a t A g « 0 z A g x « A g y > 0 E M.O, 3 f n , 3 ypU a Atomic p - f u n c t i o n s AE. AE, - P, 1 2 p y + p y ^1 + ^ 2 3 fCT3 -ft- K1 + P 2 z z F i g u r e 1.3 M.O. Scheme o f CI ~ M o l e c u l e - i o n ( a f t e r K a n z l g ) 24 I n f r e e space t h e m o l e c u l e - i o n i s a x i a l l y s y m m e t r i c , w i t h t h e 3pn l e v e l s b o t h d o u b l y d e g e n e r a t e . The degeneracy i s l i f t e d i n t h e p r e s e n c e o f a c r y s t a l l i n e f i e l d h a v i n g o r t h o r h o m b i c symmetry about t h e z - a x i s . The s m a l l d i f f e r e n c e i n A E X and A E y t h e n g i v e s r i s e t o t h e s l i g h t d i f f e r e n c e i n A g Y and A g v . S i n c e t h e ground s t a t e o f t h e " h o l e " i s e s s e n t i a l l y a p - f u n c t i o n , t h e h y p e r f i n e t e n s o r i s p r e d i c t e d z t o be a x i a l l y symmetric about t h e z - a x i s t o a f i r s t a p p r o x i m a t i o n . Maximum s p l i t t i n g s h o u l d be o b s e r v e d when t h e magnetic f i e l d i s a p p l i e d a l o n g the z - a x i s . I n f a c t when the c r y s t a l was o r i e n t e d w i t h i t s [llo] - a x i s p a r a l l e l t o t h e magnetic f i e l d , one s e t o f h y p e r f i n e l i n e s showed a maximum s p l i t t i n g o f about 101 gauss c e n t r e d a t g = 2.0010. T h i s i n d i c a t e s t h a t t h e m o l e c u l a r a x i s o f t h e C l g " i o n l i e s i n t h e [llo] - d i r e c t i o n o f t h e c r y s t a l . More d e t a i l e d a n a l y s i s o f the o r i e n t a t i o n dependence and i n t e n s i t y o f t h e ESR s p e c t r a showed t h a t a l l s i x e q u i v a l e n t [llo] -axes o f t h e c u b i c KC1 c r y s t a l were e q u a l l y p o p u l a t e d by the C±2~ m o l e c u l e - i o n s . Powder S p e c t r a A l t h o u g h r a d i c a l s a r e o f t e n t r a p p e d i n a s o l i d a t t h e same o r i e n t a t i o n o r a t a s m a l l number o f r e l a t e d o r i e n t a t i o n s , d i f f i c u l t i e s a r e sometimes e n c o u n t e r e d i n p r e p a r i n g s i n g l e c r y s t a l s o f t h e h o s t m a t e r i a l f o r ESR s t u d i e s A p o l y c r y s t a l l i n e medium has t o be used. A l s o t h e p a r a m a g n e t i 25 s p e c i e s may be adsorbed o n l y on the s u r f a c e o f the c r y s t a l . I n any o f t h e s e c a s e s , t h e ESR s p e c t r u m o b s e r v e d w i l l be a c o m p l i c a t e d s u p e r p o s i t i o n o f l i n e s due t o a l l o r i e n t a t i o n s o f t h e randomly o r i e n t e d r a d i c a l s . D e s p i t e i t s c o m p l e x i t y , s u c h a s p e c t r u m i s s t i l l o f t e n i n t e r p r e t a b l e . Components o f t h e g- and h y p e r f i n e t e n s o r s may y e t be e x t r a c t e d a l t h o u g h one i m p o r t a n t a s p e c t o f i n f o r m a t i o n , t h e o r i e n t a t i o n o f t h e r a d i c a l i n t h e c r y s t a l , i s l o s t . Methods f o r a n a l y s i n g powder ESR s p e c t r a have been d e v e l o p e d by a number o f workers The s u b j e c t has been r e v i e w e d by A d r i a n ( 2 ^ ) # To show how ESR parameters were e x t r a c t e d f r o m some o f t h e s p e c t r a f o u n d i n t h i s work, t h e q u a l i t a t i v e r e s u l t s are p r e s e n t e d h e r e . I t w i l l be assumed t h a t t h e o r i e n t a t i o n o f t h e r a d i c a l i n t h e c r y s t a l i s c o m p l e t e l y randomised w i t h r e s p e c t t o t h e e x t e r n a l magnetic f i e l d on a m a c r o s c o p i c s c a l e . C o n s i d e r f i r s t an S = j s y s t e m where i n d i v i d u a l ESR l i n e s are broadened by g - a n i s o t r o p y o n l y . The o v e r a l l l i n e shape o f a f i r s t d e r i v a t i v e s p e c t r u m r e s u l t i n g f r o m p i l i n g up o f r e s o n a n c e s due t o t h e s e randomly o r i e n t e d r a d i c a l s t h e n c o n t a i n s t h r e e s h a r p r e a d i l y o b s e r v a b l e peaks ( F i g . I. 4 b ) . These peaks c o r r e s p o n d t o m o l e c u l e s w h i c h are o r i e n t e d so t h a t t h e magnetic f i e l d l i e s a l o n g one o f t h e p r i n c i p a l axes o f t h e g - t e n s o r . The number o f peaks i s r e d u c e d t o two i n case o f a x i a l symmetry. These s h a r p peaks are rounded o f f i f o t h e r s o u r c e s o f b r o a d e n i n g are t a k e n i n t o c o n s i d e r a t i o n , e.g., b r o a d e n i n g due t o u n r e s o l v e d h y p e r f i n e s p l i t t i n g s , f i e l d 26 i n h o m o g e n e i t y , s a t u r a t i o n and r e l a x a t i o n e f f e c t s . The a b s o r p t i o n and t h e f i r s t d e r i v a t i v e c u r v e s o f t h e s e l i n e s e x p e c t e d f o r an a x i a l l y s y mmetric s y s t e m and f o r a non-a x i a l l y symmetric s y s t e m a r e shown I n P i g s . 1 .4a and 1.4b. The p r i n c i p a l g - f a c t o r s can be e s t i m a t e d i n b o t h c a s e s . g|l J gj_ J and g^ a r e t o be t a k e n a t t h e maxima o r minima and gg i s t o be t a k e n f r o m t h e c r o s s - o v e r p o i n t o f t h e f i r s t d e r i v a t i v e c u r v e . W i t h t h e p r e s e n c e o f a mag n e t i c n u c l e u s , h y p e r f i n e s t r u c t u r e f u r t h e r c o m p l i c a t e s t h e s p e c t r u m and a m b i g u i t i e s may be i n t r o d u c e d . F i g . 1.5 shows a t y p i c a l a x i a l l y s y mmetric powder s p e c t r u m f o r r a d i c a l s w i t h one mag n e t i c n u c l e u s o f s p i n 1 = 1 , t o g e t h e r w i t h t h e i n t e r p r e t a t i o n . I t i s assumed here t h a t t h e p r i n c i p a l d i r e c t i o n s o f t h e g- and h y p e r f i n e t e n s o r s a r e c o i n c i d e n t . The s p e c t r u m becomes more and more c o m p l i c a t e d w i t h more magnetic n u c l e i p r e s e n t o r w i t h i s o t o p i c e f f e c t s p r e s e n t . P a r t i c u l a r l y when t h e r e i s more t h a n one r a d i c a l p r e s e n t , t h e s p e c t r u m may be t o o complex f o r an unambiguous i n t e r p r e t a t i o n . But when i t can be i n t e r p r e t e d , t h e r e s u l t s compare f a v o u r a b l y i n g e n e r a l w i t h t h o s e f r o m s i n g l e c r y s t a l s p e c t r a . 27 g. -> H F i g u r e 1 .4 T y p i c a l ESR Powder S p e c t r a (a) A x i a l l y Symmetric System w i t h No M a g n e t i c Nucleus (b) N o n - a x i a l l y Symmetric System w i t h No M a g n e t i c N u c l e u s Upper c u r v e : a b s o r p t i o n Lower c u r v e : f i r s t d e r i v a t i v e D o t t e d c u r v e : l i n e shapes due t o g - a n i s o t r o p y o n l y s o l i d c u r v e : o t h e r s o u r c e s o f l i n e b r o a d e n i n g i n c l u d e d T y p i c a l F i r s t D e r i v a t i v e Powder Spectrum f o r an A x i a l l y Symmetric R a d i c a l w i t h One Nucleus o f S p i n 1 = 1 CHAPTER II PARAMAGNETIC BY-PRODUCTS OF REACTIONS OF ALKALI HALIDES AND STRONTIUM CHLORIDE WITH FLUORINE 28 I I . (A) APPARATUS AND PROCEDURE a) M a t e r i a l s S i n g l e c r y s t a l s o f NaC l and KBr were c u t f r o m o p t i c a l c r y s t a l s Jj" i n d i a m e t e r and 1 mm. i n t h i c k n e s s o b t a i n e d f r o m Harshaw Company. S i n g l e c r y s t a l s o f S r C l ^ were p r e p a r e d by t h e z o n e - r e f i n i n g method. The z o n e - r e f i n e r was a h o r i z o n t a l t y p e c o n s t r u c t e d by R.W. B u r t o n . Embedded i n a s b e s t o s s u r r o u n d i n g a h o l l o w c o r e a re two s e p a r a t e w i n d i n g s o f Chrome1 A h e a t i n g w i r e , i n d i v i d u a l l y c o n t r o l l e d by a V a r i a c . The o p e r a t i n g t e m p e r a t u r e was about 890°C a t t h e c e n t r e and 830°C at t h e ends. The s t a r t i n g m a t e r i a l was B a k e r A n a l y s e d S r C l 2 * 6 H 2 0 w h i c h was d e h y d r a t e d by h e a t i n g a t 200°C f o r 24 h o u r s . I t was t h e n p l a c e d i n a g r a p h i t e boat and put i n a s i l i c a t u b i n g t h r o u g h w h i c h d r y n i t r o g e n was p a s s e d . The f u r n a c e was d r i v e n by an e l e c t r i c motor g e a r e d t o g i v e a speed o f a p p r o x i m a t e l y 1 i n c h p e r hour. A f t e r a zone pass t h e f u r n a c e was r e t u r n e d t o i t s o r i g i n a l p o s i t i o n by a l e a d w e i g h t a t t a c h e d t o one end of the f u r n a c e v i a a p u l l e y . The r e c r y s t a l l i s e d S r C l g was t r a n s p a r e n t and sample c r y s t a l s were c u t fr o m the c e n t r a l p o r t i o n . I n some runs P b C l 2 was added t o h e l p e l i m i n a t i n g w a t e r . The l e a d o x i d e and h y d r o x i d e formed by r e a c t i n g w i t h w a t e r and t h e exces s P b C l 2 are more v o l a t i l e t h a n S r C l ^ and t h e r e f o r e e v a p o r a t e d o f f . However subsequent e x p e r i m e n t a l r e s u l t s o b t a i n e d w i t h t h e s e 29 c r y s t a l s do not d i f f e r from the others c r y s t a l l i s e d without PbCl^, Vacuum-evaporated films of NaCl and KC1 were prepared i n an apparatus described by Adams (29) and shown i n F i g . II.1. The main vessel A, side-arm B and the water-cooled condenser C were made of quartz. The platinum scraper blade D was spot-welded to a tungsten rod E sealed into pyrex B-10 cone F. The side-arm B was wound with Chromel A wires G, H and J encased i n alumina and glass wool. An operating current of about 4 amps., co n t r o l l e d by a Variac, was allowed to flow into the heating system. The apparatus was connected to a mercury d i f f u s i o n pump v i a the B-10 j o i n t K. The sample to be evaporated was placed i n side-arm B and outgassed for about 12 hours at a temperature below the evaporation temperature i n a vacuum of less than 10 t o r r . The temperature was then increased so that the s o l i d evaporated and was condensed onto the water-cooled surface of C. The condensed f i l m was scraped o f f and allowed to f a l l down to a c o l l e c t o r connected v i a a B-10 j o i n t L. The c o l l e c t o r was usually an ESR probe which may be vacuum-sealed and used also as a reaction vessel (see F i g . II.3a). A l l the j o i n t s were lub r i c a t e d with Apiezon T high temperature grease and were cooled by a stream of a i r while the heaters were operating. A l l other powder samples were ground from t h e i r single c r y s t a l s . Samples of S r C l ^ were handled i n a dry box since the material i s deliquescent. Figure II.1  Evaporated Film Apparatus 31 Handling of Fluorine Fluorine was handled i n quartz and pyrex systems which are attacked only very slowly i f precautions are taken to exclude moisture. Fluorine from a cyli n d e r quoted as 98$ pure was supplied by Matheson Company. The gas was delivered to the pyrex system v i a a series of monel expansion valves and copper tubing Kovar-sealed to pyrex. The p r i n c i p a l impurity, hydrogen f l u o r i d e , was removed by passing the gas through a NaF trap. A diagrammatic outline of the system i s shown i n F i g . II.2. The f l u o r i n e was stored i n a 5 - l i t r e bulb A. The pressure of f l u o r i n e was usually 290 t o r r , obtained by freezing the gas at f i n g e r B with l i q u i d nitrogen. The pressure was measured by a mercury diaphragm manometer. The gas was disposed of by passing i t slowly through a column of 80-mesh soda-lime that forms part of the disposal l i n e . A l l stopcocks that may come into contact with f l u o r i n e were lubr i c a t e d with Kel-F grease, others with Apiezon N high vacuum grease. The system was evacuated by a Welch Duo Seal pump, backed by a Balzer o i l d i f f u s i o n pump employing Dow Corning S i l i c o n 703 f l u i d . The pressure of the system was measured by a Veeco i o n i s a t i o n gauge. A mobile pyrex unit was also constructed to permit i n s i t u spectroscopic studies. The unit consists of a mercury d i f f u s i o n pump backed by a rotary pump, and a column of soda-lime f o r f l u o r i n e disposal. It may be attached v i a a b a l l - j o i n t and a B-10 j o i n t to any of the reaction vessels described below. Figure I I . 2 Outline of Fluorine Handling System 33 Reaction Vessels and Procedure F i g . II.3 a shows the apparatus used both as c o l l e c t o r f o r evaporated films and as reaction vessel. It may be attached v i a a B-10 socket A to the evaporated f i l m apparatus. A c o n s t r i c t i o n at B allowed flame s e a l - o f f of the sample i n vacuum to avoid i t s contact with a i r . The vessel was then attached to the f l u o r i n e supply system at C. The upper h a l f of the ESR probe D was made of quartz. It was graded-sealed to a 3 mm. S u p r a s i l tube which may be inserted into a l i q u i d nitrogen dewar placed i n the cavi t y of the ESR spectrometer. F i g . I I . 3 c shows the pyrex apparatus with a 100 c.c. f l u o r i n e storage bulb. The f l u o r i n e stored here may be allowed to diff u s e through a stopcock into a pre-evacuated probe to react with the sample f o r i n s i t u spectroscopic studies. For o p t i c a l studies the vessel shown i n F i g . II.3b was used. It was a quartz c e l l of 10 cm. long with o p t i c a l quartz windows of 3 cm. diameter on both ends. It may be connected to the vacuum system and f l u o r i n e supply v i a a B-10 j o i n t . The apparatus used f o r single c r y s t a l ESR studies w i l l be described i n Section d ) ( i i i ) . The usual reaction procedure began with evacuating the whole system to a pressure below 10"^ t o r r . The reaction vessel was then closed to the main vacuum l i n e and f l u o r i n e gas was introduced to the vessel from the storage bulb, usually at a pressure of 290 t o r r . Reaction generally took place a f t e r an induction period that ranged from a few seconds to more than 10 minutes, varying from one sample to another. 34 Reaction Vessels 35 S p e c t r o s c o p i c T e c h n i q u e s i . ESR S p e c t r o m e t e r A V a r i a n E-3 X-band s p e c t r o m e t e r was used t h r o u g h o u t t h e work of t h i s t h e s i s . I t c o n s i s t s o f t h e f o l l o w i n g major s y s t e m components: 1. A C o n s o l Power S u p p l y w h i c h p r o v i d e s power f o r a l l o f t h e components of t h e s p e c t r o m e t e r s y s t e m w i t h t h e e x c e p t i o n o f t h e magnet system. 2 . A F o u r - i n c h Magnet System w i t h a magnet power s u p p l y , w h i c h p r o v i d e s a maximum magnetic f i e l d o f 6 K i l o g a u s s . 3. A 100 Kc M o d u l a t i o n U n i t w h i c h p r o v i d e s t h e f i e l d m o d u l a t i o n f r e q u e n c y , and i s i n c o r p o r a t e d w i t h an a m p l i f i e r , a h i g h - g a i n r e c e i v e r and p h a s e - s e n s i t i v e d e t e c t o r f o r d e t e c t i o n o f t h e ESR s i g n a l . h. A M a g n e t i c F i e l d R e g u l a t o r w h i c h r e g u l a t e s the magnetic f l u x i n t e n s i t y i n t h e a i r gap o f t h e magnet and p r o v i d e s an a c c u r a t e d i a l i n g o f t h e d e s i r e d f i e l d v a l u e s . 5. A 9 . 5 GHz (X-band) Microwave B r i d g e w h i c h c o n s i s t s o f a microwave s y s t e m f o r e x c i t a t i o n and o b s e r v a t i o n o f t h e ESR s i g n a l , and an a u t o m a t i c f r e q u e n c y c o n t r o l u n i t f o r t h e c o n t r o l and c o r r e c t i o n o f t h e microwave f r e q u e n c y . 6. A M u l t i - p u r p o s e r e c t a n g u l a r C a v i t y o f Mode T E 1 0 2 capable o f a c c e p t i n g sample tubes up t o 11 .5 mm. i n d i a m e t e r . 7. An O s c i l l o s c o p e f o r r a p i d i n i t i a l s e t u p and a d j u stment o f t h e s y s t e m and f o r s i g n a l p r e s e n t a t i o n . 36 8. A Re c o r d e r w h i c h p r e s e n t s s p e c t r a l i n f o r m a t i o n i n the f o r m o f f i r s t d e r i v a t i v e s o f t h e a b s o r p t i o n l i n e s on c h a r t s t h a t a r e p r e - c a l i b r a t e d a l o n g t h e x - a x i s i n magnetic f i e l d u n i t s . i i . DPPH S t a n d a r d A p o l y c r y s t a l l i n e sample o f d i p h e n y l p i c r y l h y d r a z y l (DPPH) i n K C 1 was used as a s t a n d a r d f o r measuring ESR l i n e p o s i t i o n s . L i n e p o s i t i o n s were c a l c u l a t e d by measuring t h e f i e l d d i s t a n c e between a b s o r p t i o n l i n e s and t h e l i n e o f DPPH ( g = 2.0036). The g - v a l u e o f a l i n e measured t h i s way i s e s t i m a t e d t o be a c c u r a t e t o + 0.0005 under n o r m a l c o n d i t i o n s The amount o f DPPH i n t h i s s t a n d a r d sample c o n t a i n e d i n a p y r e x c a p i l l a r y tube i s known, so t h a t r a d i c a l c o n c e n t r a t i o n o f an unknown sample may be e s t i m a t e d by comparing the i n t e g r a t e d i n t e n s i t i e s o f t h e sample and t h e s t a n d a r d s i g n a l s . i i i . M ounting o f S i n g l e C r y s t a l s S i n g l e c r y s t a l s o f KBr, Na C l and S r C l g were mounted i n vacuo f o r r e a c t i o n s w i t h f l u o r i n e and ESR s t u d i e s . The app a r a t u s used i s shown i n F i g . I I . 4 a . A p o i n t e r P was a t t a c h e d t o a m e t a l r o d A K o v a r - s e a l e d t o a p y r e x r o d B. T h i s p y r e x r o d B was i n t u r n g r a d e d s e a l e d t o a q u a r t z r o d C. A m a g n i f i e d view o f t h e l o w e r end o f t h i s q u a r t z r o d i s shown i n F i g . I I . 4 b . The s i n g l e c r y s t a l t o be s t u d i e d was a t t a c h e d t o t h e r o d end w i t h P l i o b o n d a d h e s i v e . T h i s a d h e s i v e has 37 F i g u r e I I . 4 Mounting of S i n g l e C r y s t a l f o r ESR S t u d i e s 38 been found to give no ESR s i g n a l before and a f t e r being exposed to f l u o r i n e . The whole rod assembly may be inserted in t o the ESR probe D through the B-10 j o i n t E and was capable of being rotated because of another B-10 j o i n t F. The angle of rotation was indicated by a protractor held i n f i x e d p o s i t i o n by the p l a s t i c support G. The probe may be evacuated and introduced with F g through stopcock H. The c r y s t a l s were mounted v i s u a l l y by recognising t h e i r prominent cleavage faces. These are the (100) faces of NaCl and KBr and the (111) faces of S r C l 2 . i v . UV/Visible Spectrophotometer A Cary 14 Spectrophotometer was used i n o p t i c a l absorption studies. A high i n t e n s i t y hydrogen lamp and a tungsten lamp are used as l i g h t sources i n the u l t r a v i o l e t and v i s i b l e regions respectively. Preparation of Chlorine Dioxide ClOg was prepared by the reaction of KCIO^ with o x a l i c acid according to the equation 2KC10 + 2 H 2 C 2 ° 4 * 2C10 2 + K 2 C 2 0 4 + 2H 20 +2C02 The preparation was c a r r i e d out i n the apparatus shown i n F i g . I I . 5 . About eight parts by weight of o x a l i c acid and two parts of KCIO-^ were mixed i n a round-bottom f l a s k A, and about one part of water was placed i n the side-arm B. The Pump 4 F i g u r e I I . 5 40 whole system was f i r s t evacuated with the U-tube C empty and the water frozen i n l i q u i d nitrogen. Stopcock D was then closed and the empty U-tube was replaced with another one containing P p ^ - T n e system was evacuated again with stopcock D s t i l l closed while the water i n side-arm B was being defrosted. The system was then closed to the pump and the whole apparatus tipped so that the water flowed slowly into the f l a s k . The f l a s k was immersed i n a hot-water bath at 70°C and reaction took place with evolution of CT0 2. The rate of reaction may be c o n t r o l l e d by p e r i o d i c a l l y lowering the temperature of the water bath. Stopcocks D and E were then opened and the ClO^ d i f f u s e d into the ESR probe F after being dried over P 0 C v . The probe, being immersed i n an acetone-dry-ice bath, contained the appropriate solvent f o r CTC»2 (10 M sulphuric acid or CCl^), or powder SrF^ on which the C10 2 was to be adsorbed. f) Heating of S r C l ^ i n Vacuo or Dry Nitrogen ' r The apparatus used f o r dehydrating S r C l ^ i s shown i n F i g . I I . 6. A powder sample of SrCl 2'6H 20 was heated i n a i r at 150°C for 24 hours. The lumpy material a f t e r t h i s preliminary heating was ground to a f i n e powder, loaded into a graphite boat A and placed i n a quartz tubing B. The tube was inserted into the furnace C. The operating temperature, which ranged from 200°C to 850°C, was con t r o l l e d by a Variac supplying current to the furnace. The temperature was 41 measured by a thermocouple using Pt/Pt-10$-Rh. The hot junction of the thermocouple was i n contact with the outer wall of the quartz tube and was connected to a potentiometer through a small "peep-hole" D. A flow of nitrogen supplied from a cylinder and regulated by a needle valve was passed through cone. H 2S0^ and then over Pg 0^ into the quartz tubing. A by-pass and a trap were provided to prevent suck-back of the acid. The whole system may be evacuated through stopcock E by a mercury d i f f u s i o n pump. The sample was heated f o r 24 hours at a f i x e d temperature i n a steady flow of dry nitrogen. For some runs the samples were heated i n vacuo. The current to the furnace was reduced i n steps so that the sample could be cooled to room temperature over a period of 16 hours. It was then removed from the furnace and transferred immediately to a dry box. In the case of heating i n vacuo the pressure of the system was equalised to 1 atomosphere by dry N 2 before the sample could be removed. The top portion of the sample a f t e r treatment was always discarded i n the dry box f o r fear of ad d i t i o n a l moisture picked up during i t s b r i e f contact with a i r . Figure II.6 SrCl2 Drying System 43 I I . (B) REACTION OF KBr WITH FLUORINE: RESULTS AND DISCUSSION a) ESR Spectra KBr was r e a d i l y attacked by f l u o r i n e at room temperature. When f l u o r i n e was introduced at low pressure (290 t o r r or less) and the reaction vessel was immersed i n l i q u i d nitrogen as soon as reaction appeared to have started, ESR s i g n a l was observed. The s i g n a l was obtainable with both single c r y s t a l and powder samples of KBr, but more r e a d i l y with the l a t t e r , and was observable only at low temperature (77°K). Evacuating the probe at 77°K had l i t t l e e f f e c t on the s i g n a l i n t e n s i t y . The spectrum, shown i n F i g . II.7 was always powder-like even with single c r y s t a l KBr. It i s b a s i c a l l y a two-line spectrum with a x i a l symmetry. The following parameters were obtained: g„ = 2.0097 ± 0.0010 g ± = 2.0036 ± 0.0010 A,1 '= 100 + 2 gauss Aj_ = 2 4 + 2 gauss This two-line spectrum suggests a r a d i c a l involving a single nucleus of nuclear spin The most probable nucleus i s ^ F . The small hyperfine s p l i t t i n g s indicate that the unpaired spin density on the f l u o r i n e atom may be small. The r a d i c a l i s therefore u n l i k e l y to be a -p-*5 single f l u o r i n e atom where the unpaired electron i s expected to occupy a 2p-orbital and a large A|| should be observed. If there were appreciable de l o c a l i s a t i o n of the unpaired spin onto neighbouring Br or K atoms, a l l with nuclear spins 3/2, hyperfine s p l i t t i n g s due to these magnetic n u c l e i would be expected to appear. Hence the ESR spectrum i s more l i k e l y to originate from an oxy-fluoride r a d i c a l with i t s unpaired electron residing mainly on the non-magnetic "*"^0 nucleus ( e i ) . This species i s i d e n t i f i e d as the POO* r a d i c a l . The POO* r a d i c a l (30-32) has been studied by ESR by a number of workers v Table II.1 shows the parameters f o r t h i s r a d i c a l . Table II.1 ESR Parameters of the F00- Radical g-tensor A-tensor (gauss) Mat r i x g ? g3 A i ^2. A3 R e f • 0 2F 2 2.0013 2.0013 2.0072 24.0 24.0 98.8 30 Ar 2.0008 2.0022 2.0080 50.4 14 103 31 °3F2 2- 0 0°5 2.0005 2.0074 25 25 100 32 KBr 2.0036 2.0036 2.0097 24 24 100 This work 46 I f f l u o r i n e was allowed to come into contact with the KBr sample at room temperature f o r a longer period (usually 30 minutes or more) and then pumped out, a complicated ESR spectrum was obtained at room temperature ( F i g . II.8). The s i g n a l started to grow as soon as evacuation of the probe began, reached a maximum and then decayed as evacuation continued. The rate of growth and decay could not be reproduced. The s i g n a l i n t e n s i t y usually reached a maximum i n a few minutes, and i f evacuation was discontinued at t h i s point the s i g n a l remained stable and decayed slowly over a period ranging from a few hours to a day. It was found that t h i s s i g n a l did not originate from the reacted s o l i d sample, but from the gaseous products of reaction. The fact that t h i s spectrum covers a wide range of magnetic f i e l d of a few thousand gauss i s i n accord with the presence of a stable and paramagnetic gaseous species. 3 The species i s i d e n t i f i e d as molecular oxygen 0^ i n i t s £ ground state. The gas-phase electron resonance spectrum of 0 2 i n the ground state has been studied and analysed i n d e t a i l by Tinkham and Strandberg (33)^ Comparison with t h e i r data unequivocally i d e n t i f i e s the spectrum of F i g . II.8 as that of molecular oxygen. Formation of F00« The t o t a l absence of o r i e n t a t i o n dependence i n the ESR spectra of F00' indicates that the r a d i c a l s may be present F i g u r e II.8 ESR Spectrum a t 9*43 GHz o f Gaseous  P r o d u c t of R e a c t i o n KBr / F o — i — 4 5 0 0 ji J 5500 60<D0 6 5 0 0 gauss 48 e n t i r e l y on the surface. The p r i n c i p a l impurities here may be surface hydroxyl groups rather than entrapped water. Fluorine attacks hydroxide to form oxygen f l u o r i d e s , with the lower members of the family OF^ and C^Fg predominant. In any case, both 0 2F 2 and O^F^ have been shown to be paramagnetic due to the presence of the free r a d i c a l F00- formed as a re s u l t of the respective decomposition process (3^* 32)^ same r a d i c a l was also found to be present i n l i q u i d 0F 2 upon ( 46 47) photolysis v 3 . The oxygen f l u o r i d e present i n the reactions of KBr and KC1 with F 2 i s more l i k e l y to be O F . This compound freezes at 113°K while 0F2 freezes at 50°K. The ESR spectrum observed at 77°K c l e a r l y shows the anisotropy of the r a d i c a l , which would have been averaged out by tumbling of the molecules i f i t were i n the form of a l i q u i d . ° 2 P 2 i s known to undergo thermal decomposition above -100°C to a mixture of oxygen and f l u o r i n e (32, 48 )^ T h e s e q U e n c e Q f decomposition was suggested by Kasai and Kirshenbaum (32) to be: F-0-0-F > F00- + F- ?• F 2 + 0 2 . . . . ( H . l ) rather than F-0-0-F > 2F-0- > F2 + °2 . . . . (H.2) 0 The argument was based on the short 0-0 distance (1.217 A) and the rather long 0-F distance (1.575 A) (^9), and the smaller energy (45.7 Kcal.) required to s p l i t the 0-F bond than the 0-0 bond (62.1 Kcal.) . If the pressure of f l u o r i n e i s s u f f i c i e n t l y high, the decomposition process shown by Equation II.1 may be suppressed. This i s consistent with the observation that molecular oxygen was observed by ESR only on evacuating the f l u o r i n e a f t e r i t s contact with the s o l i d s . This observation may however be explained by the blocking of voids and channels i n the product layer by excess f l u o r i n e , evacuation of which enables the oxygen to escape in t o the main body of the gas. 50 I I . (C) REACTIONS OF KC1, NaCl AND S r C l g WITH FLUORINE: RESULTS a) ESR Spectra i . C o r r e l a t i o n with Each Other and with Previous Work A l l three compounds were reacted, both i n powdered form (NaCl and KC1, vacuum-evaporated; SrClg, ground) and as single c r y s t a l s , with f l u o r i n e at 23°C and 290 t o r r i n the reaction vessel shown i n F i g . II.3 a . In the case of KC1, an ESR s i g n a l was present i n the vacuum-evaporated material before reaction. This s i g n a l i s often seen i n vacuum-evaporated KC1 and has been attributed t e n t a t i v e l y by Catton to 0 2 ~ . The s i g n a l p e r s i s t e d through the reaction with f l u o r i n e . The KC1 reaction also produced signals which could be i d e n t i f i e d as F00' ( F i g . II.9a) and 0 2 , as i n the case of KBr. However, when vacuum-sublimed KC1 was kept i n contact with F 2 at room temperature f o r a period exceeding about 2 hours, the spectrum of F00* disappeared, and a new s i g n a l was observed ( F i g . II.9 b ) . Signals very s i m i l a r to that obtained on long contact of vacuum-evaporated KC1 with f l u o r i n e were obtained i n a l l other cases —powdered and single c r y s t a l NaCl, and SrCl^; and i n a l l cases except vacuum-evaporated KC1, these were the only signals obtained. These signals were weak and, i n the cases of NaCl and KC1, almost s t r u c t u r e l e s s , at room temperature; but they were stronger and better resolved i f observed at 77°K ( a f t e r Figure II.9 ESR Spectrum (77°K) of Vacuum-evaporated KC1 Reacted with F2 (a) a f t e r b r i e f contact with F, (central sharp l i n e from unreacted KC1 evaporated f i l m ) 25 GAUSS I 1 Figure II*9 ESR Spectrum (77°K) o f Vacuum-e v a p o r a t e d KC1 R e a c t e d w i t h Fp (b) a f t e r p r o l o n g e d c o n t a c t w i t h F 2 53 room temperature reaction). A l l these results are summarised i n Table II.2. The 77°K spectra are shown i n Figs. II.9-11. There was very l i t t l e difference between spectra from single c r y s t a l and powder samples. In a l l cases, the spectra were of e s s e n t i a l l y powder character, the single c r y s t a l spectra showing only a trace of o r i e n t a t i o n dependence, not enough to be of any use i n i d e n t i f i c a t i o n . These signals c l e a r l y indicate i n t e r a c t i o n of the unpaired electron with a s i n g l e CI nucleus. This i s best seen i n the spectrum of reacted S r C l 2 shown i n F i g . 11.11. A set of four almost equally spaced l i n e s may be recognised, marked as A, B, C, and D on the spectrum. They are interpreted as the hyperfine lines due to a nucleus with spin I = 3/2. The 35 s p l i t t i n g of t h i s hyperfine multiplet i s 74 gauss. Both CI 37 and CI have nuclear spins 3/2 with magnetic moments 0.8209 and 0.6833 nuclear magnetons respectively. This i s o t o p i c e f f e c t i s indeed resolvable f o r the outer peaks. Peaks E and F can be recognised as the outer components of the 3 7C1 hyperfine •se 37 quadruplet. The s p l i t t i n g s due to ->-'Cl and CI are therefore 74 and 62 gauss respectively and t h i s r a t i o i s i n excellent agreement with that of t h e i r nuclear magnetic moments. The i n t e n s i t y r a t i o of doublet A and E and that of D and F i s something of the order of 3 : 1> which i s the r a t i o of natural 35 37 abundance between CI and CI. It may thus be concluded that the spectrum i s due to a r a d i c a l species containing a single CI nucleus, and has the paramaters 54 g„ = 2.0034 ± 0.0010 A|| ( 3 5C1) = 74 ± 2 gauss A|| (^^Cl) = 62 + 2 gauss It i s d i f f i c u l t to say from t h i s simple analysis whether the spectrum has a x i a l symmetry or not. I f a x i a l symmetry i s assumed, the central portion of the spectrum represents the perpendicular features. Due to poor resolution of t h i s part, accurate values of g ± and A x cannot he obtained. It can only be said that g x i s near 2.02 and Aj_ i s small, i n the order of a few gauss. The pattern of i n t e n s i t i e s i n the powder spectrum indicates strongly anisotropic hyperfine s p l i t t i n g , compatible with an o r b i t a l predominantly of p-character f o r the unpaired electron. The magnitude of the s p l i t t i n g suggests that the unpaired electron i s s u b s t a n t i a l l y l o c a l i s e d on the CI atom; and t h i s together with the averaging out of d e t a i l e d structure and of orie n t a t i o n dependence i n the powder spectrum makes i t d i f f i c u l t to i d e n t i f y p r e c i s e l y the environment of the CI atom i n each case. The resemblance between the spectra for the three compounds is s t r i k i n g , and the spectrum obtained by Catton ( 1 1 ) i n the reaction of powdered S r C l 2 with f l u o r i n e was also very s i m i l a r . But t h i s does not ne c e s s a r i l y imply any closer i d e n t i t y between these species than that they a l l f u l f i l the general s p e c i f i c a t i o n which has been set out i n t h i s paragraph. 55 Table I I . 2 Summary of ESR Results for Reactions of Metal Halides with Fg Halide S o l i d Form ESR Signals Remarks KBr Single c r y s t a l & fre s h l y ground powder evaporated f i l m gaseous 0 2 F00* r a d i c a l no s i g n a l observed at room temperature and 77°K a f t e r prolonged contact with F and evacuation observed at 77°K a f t e r b r i e f contact with F r t KC1 Evaporated f i l m Single c r y s t a l & powder gaseous Cv, F00* r a d i c a l s i g n a l assigned as ClOo weak s i g n a l near g = 2 , u n i d e n t i f i a b l e conditions s i m i l a r to KBr conditions s i m i l a r to KBr appeared at 77°K af t e r decay of F00- s i g n a l NaCl Single c r y s t a l & powder evaporated f i l m gaseous Cv, conditions s i m i l a r to KBr s i g n a l associated observable only at with a single CI 77°K, s l i g h t l y nucleus, assigned o r i e n t a t i o n dependent as C10 2 weak s i g n a l near g = 2 u n i d e n t i f i a b l e i n single c r y s t a l Catton's 7 - l i n e room temperature spectrum irreproducible S r C l 2 Single c r y s t a l s i g n a l assigned as C 1 C U observed at 77°K, s l i g h t l y o r i e n t a t i o n dependent i n single c r y s t a l 58 Catton ' reported spectra a r i s i n g i n the reaction of the same three compounds with F , and the o r i g i n a l objective of the present study was to obtain more information on the same centres. However, i n the cases of KC1 and NaCl, the spectra obtained i n the present work are markedly d i f f e r e n t from that observed by Catton, and c l e a r l y represent d i f f e r e n t structures. The spectra which Catton observed were attr i b u t e d to an H-centre, a species which contains two equivalent CI n u c l e i , while the i n t e r a c t i o n with a single spin 3/2 nucleus i s quite c l e a r l y indicated by the present spectra. For SrClg, the question of whether t h i s spectrum represents the same centre as that studied by Catton i s much more obscure. The spectra observed here are very s i m i l a r to those which Catton found i n powdered SrClg. But Catton's results f o r s i n g l e - c r y s t a l SrClg were very d i f f e r e n t . Strong o r i e n t a t i o n dependence was found, and a much more detailed analysis of the spectrum could be made. It w i l l be shown i n the following sections of t h i s account that there i s s u b s t a n t i a l evidence from studies other than ESR to show that the centre observed i n the present work i n SrClg i s CIO2 derived from H 20 i n the s o l i d phase; and i t i s quite probable that the spectrum seen i n NaCl and KC1 was also C10 2. This raises the question whether the centre which Catton i d e n t i f i e d as a CI atom i n t e r a c t i n g with two Sr ions was i n fac t C10 o. But the question remains unresolved, because 59 the strongly orientation-dependent spectra found by Catton were not reproduced i n t h i s work, and because Catton's spectra appeared strongly i n S r C l 2 treated with PbCl^ to remove oxygen, which type of sample gave only very weak signals i n the present work. In the following account, therefore, the study i s regarded as being quite independent of Catton's; but the account ends with a note showing what the s i t u a t i o n of C1C»2 i n the l a t t i c e would be i f Catton's results were interpreted as being from C10 2. i i . I d e n t i f i c a t i o n The basic 4-line spectra at 77°K shown i n Figs. II.9-11.11 a l l undoubtedly involve a single CI nucleus. The detection of molecular oxygen and FOC" radicals i n the reactions of a l k a l i halides with Fg studied i n the present work leads to the l o g i c a l suspicion that the species i n question may be an oxychloride r a d i c a l . In fact i t was found that by reacting a sample of SrClg^HgO with F 2 , the same spectrum as F i g . 11.11 could be obtained with an i n t e n s i t y about 10 times that from an equal amount of "anhydrous" S r C l ^ . Various oxychloride rad i c a l s have been studied by ESR by a number of workers (36-39* 41-43)^ r p h e m o s t stable of a l l i s of course chlorine dioxide. Table II.3 l i s t s the ESR results f o r C10 2 i n various matrices, together with results obtained i n t h i s laboratory f o r the spectra of S r C l 2 and NaCl reacted with F . These l a t t e r results can be seen to f i t C10 o quite well, better than any other known oxy-radicals 6o of chlorine. It has been pointed out by Atkins and coworkers^"^ that the powder spectrum of C102 at 3 cm. wavelength i s e s p e c i a l l y characterised by a strong, symmetrical l i n e with g-value of about 2.022. Indeed both the spectra of treated NaCl and SrC l show t h i s c h a r a c t e r i s t i c sharp lin e at g = 2.022. The 2 exact p o s i t i o n of t h i s sharp l i n e i n the case of KC1 i s less c e r t a i n due to possible interference from the 0 2~(?) s i g n a l i n the vacuum-evaporated s o l i d , but is c e r t a i n l y very close to g = 2.022. Table II.3 Anisotropic ESR Parameters of CIO^ Matrix & Temperature g-tensor 3 D C 1 A-•tensor (gauss) §1 g 2 g 3 A l A2 A3 Ref H 2S0 4(77°K) 2.0015 70.5 36 H2so4(77°K) 2.0025 2.017 2.011 72 - 9.6 -10.0 37 KC103(295°K) 2.0018 2.0167 2.0111 79.9 -13.4 -11.5 37 KC104(330°K) 2.0036 2.0183 2.0088 73 -15.5 -11.5 38 KC10 4(103°K) 2.0016 2.0167 2.0121 74.8 -10.8 -11.5. 39 Zeolite(77°K) 2.0020 2.0187 2.0123 76.5 - 9.2 - 8.0 40 NaCl(77°K) 2.0025 76 * SrCl 2(77°K) 2.0034 74 * SrCl2(300 oK)# 2.0028 2.0194 2.0162 92 -40 -16 11 * This work # I d e n t i f i e d as CI atom 61 It i s also i n t e r e s t i n g to note that these spectra a l l resemble the one obtained by Gardner (3*0 o n ci atom adsorbed on s i l i c a g e l . Bennett and coworkers (^5) i a ^ e r reported the detection of an ad d i t i o n a l l i n e at high f i e l d and consequently re-interpreted Gardner's spectrum as due to C 1 2 " ion, based on the 7 - l i n e spectrum. Careful search i n our spectra of NaCl and S r C l 2 reacted with Fv, at high gain has f a i l e d to reveal any a d d i t i o n a l l i n e s on e i t h e r side. Atomic CI i s considered an u n l i k e l y i d e n t i f i c a t i o n f o r these spectra which show only minimal e f f e c t of matrix perturbation. The ESR linewidth i s most probably due to unresolved hyperfine i n t e r a c t i o n with neighbouring cations, and yet a l l three spectra show only s l i g h t v a r i a t i o n i n t h e i r linewidths (although "linewidth" i n a powder spectrum i s not well-defined). The difference i n linewidth i n the NaCl and S r C l ^ spectra, i f of any s i g n i f i c a n c e at a l l , i s just contrary to expectation. The linewidth of the NaCl spectrum, with a l l the n u c l e i having spin I = 3 / 2 , i s i n fact narrower than that i n SrClgj with only 7 $ of the cation n u c l e i magnetic. Topotactic Relationship i n Reaction of NaCl with Fg Rantamaa ( 1 2 + 1 3 ^  i n his studies of p a r t i a l reaction of SrCl, with F 2 has found evidence of topotaxy (the phenomenon i n which a single c r y s t a l product is formed from the reaction of a single c r y s t a l ) between the product S r F 2 and the s t a r t i n g SrClg. Topotactic r e l a t i o n s h i p between NaF and NaCl was also found i n the present study. F i g . 1 1 . 1 2 shows the zero-layer 6 2 -m \ \ Figure 11.12 Zero-layer Weissenberg Photograph of NaCl P a r t i a l l y Reacted with F 2 63 Weissenberg X-ray photograph of a single c r y s t a l of NaCl p a r t i a l l y reacted with Fg. Two l a t t i c e s may be i d e n t i f i e d i n t h i s photograph, both oriented i n the same sense. The l a t t i c e parameters were calculated to be 5.50 A and 4.40 A, o corresponding s a t i s f a c t o r i l y to NaCl ( a Q = 5.63 A) and NaF ( a Q = 4.6l A) respectively. The d i f f r a c t i o n spots due to NaF are more di f f u s e , suggesting greater disorder i n the atomic arrangement of the NaF c r y s t a l . The s i g n i f i c a n c e of t h i s w i l l be discussed l a t e r . c) SrClg Reaction: I d e n t i f i c a t i o n of ClOp i n Gas Phase The information that could be extracted from the powder spectra i s so meager that i t cannot be r e l i e d upon s o l e l y for a p o s i t i v e i d e n t i f i c a t i o n of the r a d i c a l i n question. Gaseous ClOg i s known to absorb i n the v i s i b l e and i n f r a r e d regions and t h e i r spectra are well known (^^). It was found that the ESR signals of both NaCl and SrClg p a r t i a l l y reacted with F 2 were present i n the product s o l i d layers and not i n the unreacted materials. I f ClOg i s indeed the responsible species and a reaction by-product, i t may f i n d i t s way through cracks and dislocations to escape from the s o l i d product layer to exist as a gas. A study of the gaseous products from the reaction of S r C l ^ with F^ was therefore made i n an o p t i c a l quartz c e l l with a spectrometer 9 i n s i t u . The o p t i c a l spectrum from 2500 to 5000 A obtained with a f a i r l y dry sample of S r C l 9 i s shown i n F i g . II.13a. 64 The spectrum shows a strong and continuous absorption that s t a r t s from about 5000 A and ris e s to a maximum around 3300 A. This i s c l e a r l y the absorption of gaseous chlorine. Superposed on t h i s continuous absorption i s a very weak banded spectrum. The r e l a t i v e i n t e n s i t y of t h i s banded spectrum could be improved a great deal ( F i g . II.13b) by reacting a sample of Sr C l ^ that had been exposed to moist a i r f o r a few days, followed by evacuating the reaction vessel f o r a few minutes while i t was immersed i n an ice/water bath. The banded a spectrum can be seen to s t a r t from about 4850 A, reach a o maximum at 3450 A and become gradually weaker and more diffuse at shorter wavelength to disappear at about 2800 A. The bands are degraded to the redj at least four progressions are recognisable from the spectrum. This i s undoubtedly the absorption of ClO^, and constitutes p o s i t i v e evidence f o r i t s existence, at least i n the gaseous phase. Being a s o l i d at 0°C, i t could be retained more r e a d i l y i n the reaction vessel than C l 2 at that temperature. SrClg reacted with F^ was also dissolved i n 10 M HgSO^ and CCl^. Both the ESR and o p t i c a l spectra of the f i l t r a t e s confirmed the presence of C102. As a f i n a l confirmation, ClOg was prepared and condensed on the surface of powder S r F 2 . The ESR spectrum obtained was almost i d e n t i c a l to that from the reaction of S r C l 0 and F . 5 0 0 0 A ON 2500 3 0 0 0 3500 Figure II»13 Opt i c a l Absorption Spectra of flaaeous Product of SrClo/Fo Reaction (b) Moist S r C l 2 ON ON 67 SrClg Reaction: Correlation with Water Retention It was also observed that the i n t e n s i t y of the ESR s i g n a l was very much enhanced by reacting hydrated strontium chloride with f l u o r i n e , and the i n t e n s i t y of the banded o p t i c a l spectrum was s i m i l a r l y increased by reacting a sample of SrClg exposed to moist a i r . Since the s t a r t i n g material i n our experiments was SrCl 2*6H 20 and S r C l ^ i s i t s e l f hygroscopic, a cert a i n amount of water may s t i l l be retained i n the s o l i d a f t e r dehydration. This entrapped water i s an obvious source of oxygen impurity which may have led to the formation of C10 2. To evaluate the role of water more qua n t i t a t i v e l y , powder samples of S r C l 2 were dehydrated at temperatures from 250°C to 850°C i n dry nitrogen atmosphere and then reacted with F 2 . The i n t e n s i t y of the strong, symmetrical ESR l i n e at g = 2.022 was selected as a measure of the amount of C10 2 formed. A plot of the peak height of t h i s l i n e against the temperature at which the SrClg had been treated i s shown i n F i g . 1 1 . 1 4 . With a powder sample ground from S r C l ^ that had been r e c r y s t a l l i s e d with PbClg added, the s i g n a l i n t e n s i t y was very small (point P on F i g . II.14). The plot c l e a r l y demonstrates a gradual decrease i n the amount of ClOg formed with an increase i n the temperature of treatment. As the amount of water retained i n the s o l i d S r C l 2 i s expected to decrease by r a i s i n g the temperature of dehydration, the 68 amount of CIO formed may be taken as a measure of the water 2 retention. By comparing the integrated i n t e n s i t y of the C10 2 s i g n a l and that of a DPPH sample of known concentration, the actual amount of ClOg produced i n reacting a weighed sample of S r C l ^ with F 2 may be estimated. Assuming that two moles of water give r i s e to one mole of C10 2, the amount of water retained i n SrClg has been estimated at about 400 ppm (mole) f o r a sample previously treated at 300°C, down to 20 ppm (mole) f o r a sample treated at 850°C. A sample of SrCl 2-6H 20 that had been heated i n a i r at 200°C for 24 hours was found to r e t a i n up to 4000 ppm of water as estimated by t h i s method. Gravimetric method involving the measurement of weight loss on heating also placed the amount of water retained i n the same order of magnitude. 300 70 II . (D) REACTIONS OF NaCl, KCl AND S r C l g WITH FLUORINE: DISCUSSION a) Origin of Impurity In view of the common occurrence of oxygen-containing impurities i n the reactions of these metal halides with f l u o r i n e , one may be tempted to conclude that the oxygen has come from the f l u o r i n e supply. No molecular oxygen or F 0 0 - could be detected i n the f l u o r i n e used over a wide range of pressures. The fa c t that a l l the oxygen impurities detected occur only as reaction by-products and the r e l a t i o n s h i p between the amount of C 1 0 2 formed and water retained i n S r C l ^ suggest that the oxygen comes from the s o l i d halides and not from the f l u o r i n e supply. Oxygen impurities commonly occur i n a l k a l i halides i n the form of entrapped water, OH", Og" or 0". No detectable amount of 0^  was found i n the s o l i d halides used, and although the ESR s i g n a l found i n vacuum-sublimed KCl may be due to Og"* there was no s i g n i f i c a n t change i n i t s i n t e n s i t y throughout the reaction of the s o l i d with Fg. On the other hand, the i n f r a r e d spectra of a l l the s o l i d halides show traces of water with absorp-t i o n bands near 1630 and 3^ 50 cm \ This water absorption i s e s p e c i a l l y pronounced i n SrClg, the most deliquescent of a l l . 71 The p r i n c i p a l impurity i n SrClg i s water retained i n the s o l i d through dehydration processes, which has been shown to be responsible for the formation of ClO^. Presumably t h i s i s also true f o r NaCl. It was observed by Adams that the r e a c t i v i t y of a NaCl single c r y s t a l towards f l u o r i n e was grea t l y enhanced by scratching i t s surface. Rantamaa ^  also observed that n u c l e i were i n i t i a l l y formed on edges of a SrClg c r y s t a l or at a prominent scratch on i t s face and the growth rate was accelerated along a crack. He also observed that new n u c l e i were formed near the end of the nucleation period and the growth rate changed sharply. These phenomena may be explained by the presence of impurities, water i n these cases, which are normally present i n higher concentrations along cracks, scratches and at d i s l o c a t i o n s . The reactions of these s o l i d s with F 2 may occur i n several steps, beginning with a p r e f e r e n t i a l attack on the impurity s i t e s by F^. Gaseous f l u o r i n e i s known to attack water quite v i o l e n t l y with evolution of oxygen: F g + H O > 2H4" 4- 2F" + ^0 2 (II.3) The oxygen released from t h i s reaction further reacts with the chlorine, probably i n i t s atomic form which has Just been oxidised by f l u o r i n e . F a i l u r e to observe the ESR spectrum of atomic CI of course does not rule out i t s existence as a short-l i v e d intermediate. 72 The formation of FOO i n KC1 suggests that the o r i g i n of oxygen impurity i n KC1 i s probably surface hydroxyl, as i n the case of KBr (Section I I . ( B ) ) . It i s not known whether the formation of ClO^ i n the reaction of KC1 i s d i r e c t l y r e l a t e d to the decay of FOO'. It may be speculated that the ClOg i s formed i n a s i m i l a r manner as i n the cases of the NaCl and S r C l 2 , by the reaction of 0^ with a chlorine atom. In t h i s case 0^ comes from the decomposition of probably °2 F2 ( E c l u a ' t i o n I I . 1). This may explain why CK>2 i s not formed i n the i n i t i a l stage of the reaction of KC1 with Fg. b) R e c o n c i l i a t i o n of Anomalies Some experimental features observed i n the S r C l 2 / F 2 reaction were considered to be "anomalous" by previous 114 a) workers i n t h i s laboratory v . They could not be e a s i l y explained by the model of a CI atom f o r the defect. They can however, be explained quite s a t i s f a c t o r i l y by the model of C10 2 i n S r C l 2 as follows. i . The ESR spectrum of the defect shows no hyperfine i n t e r -action a t t r i b u t a b l e to nearby f l u o r i n e atoms, which would have been expected i n view of the s t a b i l i t y and common occurrence of the Xg'-type centres (V^ or H-centres) i n a l k a l i and a lkaline earth halides. Since the defect i n question i s i n fact C102> the unpaired spin density unaccounted f o r by the CI atom i s located on the oxygen 73 atoms. The defect should not be expected to come close to a neighbouring f l u o r i d e ion to form a molecular ion F C 1 ~ , a V k-centre l i k e species which i s the stable form of r a d i c a l ions i n S r C l ^ , at least at low temperatures. i i . The defect i s absent from the bulk of the unreacted material. It i s d i f f i c u l t f o r a large molecule l i k e ClO^ to migrate i n a r e l a t i v e l y ordered c r y s t a l of unreacted SrClg. The molecule i s more l i k e l y to d i f f u s e along cracks i n the product SrF^ l a t t i c e with a Yf% contraction from the o r i g i n a l l a t t i c e parameter. i i i . The k i n e t i c s of development of the ESR s i g n a l obeys a parabolic law, i . e . , the r e l a t i o n s h i p between the extent of reaction a(taken as peak height at time t / f i n a l value) and the square root of time i s l i n e a r . It was also shown i n previous work that f o r one-dimensional d i f f u s i o n - c o n t r o l l e d reaction as i t i s the case i n our experiments, the amount M of transported to the reacting s o l i d by time t i s proportional to t 2 . M i n turn i s proportional to the extent of reaction a . This implies that the defect responsible f o r the s i g n a l is. produced i n amount proportional to the extent of reaction. This i s a l o g i c a l r e s u l t since, according to the reaction scheme proposed i n section a) above, the production of ClOg is also c o n t r o l l e d by the amount of Fg reaching the s o l i d by one-dimensional d i f f u s i o n along a narrow .tube (the reaction v e s s e l ) . 74 i v . The defect exhibits a strong anisotropy, and there i s some powder character i n the single c r y s t a l ESR spectra. These features were explained by a "vernier delocalised" model for the defect, a CI atom, along a [lOCT] d i r e c t i o n of the SrClg c r y s t a l ( F i g . 1.2). But spectra obtained i n the present work show very l i t t l e o r i e n t a t i o n dependence i n si n g l e c r y s t a l s of NaCl and SrClg. To interprete these features a more detailed picture of the s i t u a t i o n of ClO^ i n the host c r y s t a l i s necessary, as w i l l be discussed next. i 75 Orientation of the Radical The lack of useful single c r y s t a l ESR data at 77°K makes i t impossible to determine the o r i e n t a t i o n of the C10 2 molecule i n the NaCl and S r C l 2 l a t t i c e s . I f Catton's room temperature re s u l t s i n SrClg were interpreted as being from ClOg (which, unfortunately, cannot be established i n the present study), some information may be obtained as to the s i t u a t i o n of C10 2 i n the S r C l 2 l a t t i c e . The following ESR parameters were reported by Catton. g x - 2.0028 ± 0.0010 s y = 2.0194 ± 0.0020 2.0162 0.0010 \ = 92 + 1 gauss A y " 40 + 2 gauss 16 1 gauss with x || [ l i o ] , y || [HO] and z || [oOl] . This analysis i s agree upon i n a general way by the present author. The molecule C10 2 belongs to the symmetry group C 2 y . The molecule-fixed coordinate system used here i s shown i n F i g . 11.15, with the a-axis perpendicular to the molecular 2 plane. The ground state of the molecule i s a state with the e l e c t r o n i c structure (la2) 2( 4a-L)2( 3b g) 2( 2b ] [) 1 (51). The b^-molecular o r b i t a l occupied by the unpaired electron i s constructed from out-of-plane p - o r b i t a l s of the atoms. These p - o r b i t a l s have zero component of angular momentum 3. 76 about the a-axis and according to the theory of g-factors outlined i n the Introduction, A g i s expected to be small. 3. P a r t i c i p a t i o n from the d-orbitals w i l l be i n s i g n i f i c a n t due to t h e i r smaller s p i n - o r b i t a l coupling than the more penetrating p - o r b i t a l s . The excited states that can be mixed with the ground state by the angular momenta about the b- and c-axes are the and Bg states respectively. These excited states involve the promotion of an electron from the lower a^-and bg- o r b i t a l s and so Ag^ and Ag Q are expected (51b) (231 to be p o s i t i v e . Both Curl and Symons v 1 predicted that Ag^ should be greater than Ag c. Since the b-^-orbital is constructed e s s e n t i a l l y from the p a - o r b i t a l s , i s o t r o p i c hyperfine s p l i t t i n g from the c e n t r a l CI atom w i l l be small, and a maximum value of the anisotropic hfs should be observed when the magnetic f i e l d l i e s along the a-axis. Prom t h i s r e l a t i v e l y simple argument the x, y and z-axes used i n analysing the ESR spectra may be i d e n t i f i e d with the a, b, and c-axes of the molecule respectively. Figure 11.15 Molecule-fixed Axes Used f o r CIO, 77 This leads to the proposed o r i e n t a t i o n of the ClO^ molecule i n the S r F 2 l a t t i c e , which has the f l u o r i t e structure with a 0 l a t t i c e parameter a Q = 5.80 A. The molecular plane l i e s on a (110)-type plane and the molecular C 2-axis i s i n a [oOl] d i r e c t i o n of the c r y s t a l as shown i n F i g . 11.16. the G102 molecule i s suggested to occupy an anion s i t e f o r the following reasons: i . The p o s s i b i l i t y that C10 2 occupies an i n t e r s t i t i a l p o s i t i o n l i k e the one marked X i n F i g . 11.16 may be ruled out by the 19 s i z e of the molecule and the lack of hfs from F. i i . The predominant i n t r i n s i c disorder i n the f l u o r i t e - t y p e c r y s t a l s i s the Anti-Frenkel p a i r s , i . e . , anion vacancies plus i n t e r s t i t i a l anions (^3)^ These "holes" or anion vacancies form convenient traps f o r the impurity molecule C10 2. This i s the trapping s i t e proposed f o r the V^-centres i n SrF^ (12, 13) K 2 and S r C l 2 i i i . There i s no apparent reason why the ClO^ molecule should move to a less symmetric p o s i t i o n by say, a t r a n s l a t i o n a l movement along the [ooij 1 d i r e c t i o n . The absence of hfs due to also argues against a closer approach of the C10 2 towards i t s F neighbour. The experimental r e s u l t does not d i f f e r e n t i a t e the model shown i n F i g . 11.16 from the one that i s related to i t by rotating the G10 2 molecule by 180 about the x-axis, i . e . , with the Cl-0 bonds pointing towards the two neighbouring cations on the same 78 plane. The choice comes from packing considerations. The distance between an anion and i t s nearest cation neighbour i n the S r F 2 l a t t i c e i s 2.51 A. For the free molecule ClOg the Cl-0 distance i s 1.5 A and the apex angle i s 118° i f ±t can be assumed that the ClOg trapped i n SrFg retains the same bond length and bond angle as i n the free molecule (but see further o discussion below), the sum of the Cl-0 distance (1.5 A) and the i o n i c radius of Sr (1.12 A) i s then 2.62 A, being greater than the Sr-F distance. This makes the model of F i g . II.16 a more favourable choice. It has to be noted however, that the Sr-F-Sr angle i n the c r y s t a l i s only 109° while the apex angle of free ClOg I s about 118°. The two oxygen atoms w i l l no longer be directed towards the two Sr neighbours i f the apex angle i s increased, and the second model f o r the o r i e n t a t i o n of the ClOg i n SrF^ may not be rejected so r e a d i l y from packing considerations. The assumption that ClOg trapped i n a s o l i d retains i t s apex angle as i n the free molecule i s perhaps dubious. In fact the apex angle of ClO^ calculated from hyper-fi n e data of 35QI ^ n t h i s case i s much greater than the free molecule value. Following the treatment of Atkins et a l . (^ 6) one may decompose the anisotropic hyperfine tensor into two components, one a x i a l l y symmetric about the x-axis and the • about the z -axis j "80 88 -8 -52 - -44 4- -8 -28 -44 16 — gauss 79 F i g u r e II.16 T r a p p i n g Model o f C10 g i n S r F ^ L a t t i c e Shown i n (110) P l a n e ( I o n s denoted by d o t t e d c i r c l e s l i e above and below p l a n e ) 80 From the l a t t e r component the spin density i n the CI 3 p z - o r b i t a l C__2 has been calculated to be 0.16 from Equations (1.15) and (I.18), pz p and the 3 s-character on CI C g has also been calculated by using Equation 1.12 to be 0.007 (these have also been calculated by Catton and the values appear i n Table I of Ref. (11) i n the form of percentage). The values of ^ " ^ ^ p a n d l^3s(°)| 2 n a v e been taken from Morton, Rowlands and Whiff en g y U S j . n g Equation (I. 19) and Equation (l.2o) the 0-C1-0 angle was calculated to be 156°, which i s unusually large compared to the free molecule value of 118°. This method of c a l c u l a t i n g the apex angle from spin populations i s a very crude one and not too much confidence may be attached to i t . With no other independent information on the bond angle of ClOg i n SrFg, the trapping model of F i g . II.16 i s p r e f e r r e d . The unpaired electron occupies a molecular o r b i t a l of ClOg and, unlike a CI atom, i t s energy i s perturbed only s l i g h t l y by the c r y s t a l f i e l d i n the SrFg l a t t i c e . A displacement of the ClOg molecule along a [lOO] d i r e c t i o n i s considered i n s u f f i c i e n t to account f o r the predominant powder lineshape of the ESR spectra o at 77 K. Powder character i s also dominant i n the spectra of NaCl reacted with Fg although evidence has been presented (Section I I . (C) b)) f o r topotactic r e l a t i o n s h i p between the product NaF and the s t a r t i n g NaCl. In view of the d i f f e r e n t c r y s t a l structures and d i f f e r e n t i n t r i n s i c disorders present i n SrFg and NaF, i t seems l i k e l y that the powder character of the ESR spectra originates from other common causes than a t r a n s l a t i o n a l 81 movement of the ClO^ molecule along c e r t a i n c r y s t a l l o g r a p h i c d i r e c t i o n . It has been shown that although the product SrF^ and NaF were e s s e n t i a l l y single c r y s t a l s , disorder i n atomic arrangement was apparent from the diffuse X-ray d i f f r a c t i o n spots (Section I . (C) b ) ) . It was also argued that some reaction by-product C10 2 m a y manage to escape in t o the main body of the gas through cracks and channels i n the product layer to become detected by o p t i c a l absorption i n the gaseous phase. These cracks and channels are u n l i k e l y to be r i g i d l y oriented though d i s l o c a t i o n l i n e s have often been found to have ce r t a i n preferred orientations i n s o l i d s . The o r i e n t a t i o n of ClO^ molecules trapped i n these r e l a t i v e l y i r r e g u l a r channels and cracks w i l l be l a r g e l y randomised. It i s therefore proposed that the c r y s t a l disorders are responsible for the p a r t i a l randomisation of the C10 2 molecules, which leads to the powder lineshapes of the observed ESR spectra. CHAPTER III 0 o" SUPEROXIDE ION IN MELT-RECRYSTALLISED STRONTIUM CHLORIDE 82 I I I . (A) APPARATUS AND PROCEDURE The ESR s p e c t r o m e t e r and the p r o c e d u r e f o r measuring t h e l i n e p o s i t i o n s have been d e s c r i b e d i n C h a p t e r I I . a) Sample P r e p a r a t i o n The s t a r t i n g m a t e r i a l was B a k e r A n a l y t i c a l Reagent SrClg'SHgO. The sample was f i r s t d e h y d r a t e d a t 200°C f o r 12 hours i n a i r . I t was t h e n g r o u n d t o a f i n e powder and t r a n s f e r r e d t o a g r a p h i t e c r u c i b l e i n a d r y box. The sample was t h e n t r e a t e d i n t h e a p p a r a t u s d e s c r i b e d i n C h a p t e r I I f o r w a t e r r e t e n t i o n s t u d i e s ( F i g . I I . 6 ) . I n s t e a d o f p a s s i n g d r y n i t r o g e n , d r y oxygen was p a s s e d t h r o u g h t h e system. A f t e r t h e powder m a t e r i a l was m e l t e d i n t h e c r u c i b l e i t was k e p t j u s t below t h e m e l t i n g p o i n t a t about 850°C f o r 24 hours i n an oxygen atmosphere, and t h e n c o o l e d t o room t e m p e r a t u r e o v e r a p e r i o d o f 12 h o u r s . C r y s t a l s o f S r C l g t h a t p r o d u c e d an ESR s i g n a l c o u l d a l s o be p r e p a r e d by m e l t i n g t h e powder i n a p l a t i n u m c r u c i b l e i n a i r . I n b o t h cases t h e r e s u l t i n g r a d i c a l c o n c e n t r a t i o n was i r r e p r o d u c i b l e . I n some runs no r a d i c a l s were d e t e c t e d i n t h e S r C l ^ c r y s t a l s a f t e r t h i s t r e a t m e n t i n a i r . 83 Mounting of C r y s t a l S r C l ^ r e c r y s t a l l i s e d from the melt was found to have two cleavage planes. The most prominent cleavage i s the (111) plane and the other, less prominent, i s the (100) plane. X-ray d i f f r a c t i o n was used to determine the nature of a cleavage plane. A c r y s t a l small enough f o r X-ray work was cut o f f from a larger c r y s t a l and mounted on a STOE precession camera with the precession axis perpendicular to the cleavage. The camera was f i t t e d with Polaroid XR-7 Land D i f f r a c t i o n Cassette system. A xero-layer precession photograph taken on a Polaroid 3000 Speed 4 x 5 Land Film was s u f f i c i e n t to determine the nature of the cleavage plane. The larger c r y s t a l was reserved f o r ESR work. Crystals larger than 5 mnr were often found not to be single c r y s t a l s , but to consist of two or three c r y s t a l s s l i g h t l y misoriented with respect to each other. Consequently c r y s t a l s of about 3 x 3 x 1 mm were usually used f o r ESR work. The c r y s t a l was cemented with Pliobond adhesive to a Teflon rod. The angle of ro t a t i o n was given by a pointer attached to the rod and a f i x e d protractor. The c r y s t a l was mounted v i s u a l l y with i t s cleavage perpendicular to the v e r t i c a l axis of r o t a t i o n which i n turn was perpendicular to the magnetic f i e l d . The accuracy of c r y s t a l o r i e n t a t i o n i s about ± 5°. The c r y s t a l and rod were immersed i n l i q u i d nitrogen i n a double-walled dewar which was inserted into the spectrometer cavity. Spectra o o were taken at 5 i n t e r v a l , with an accuracy of ± l 84 II I . (B) EXPERIMENTAL RESULTS At ambient temperature only weak and u n i d e n t i f i a b l e ESR absorption was observed i n m e l t - r e c r y s t a l l i s e d S r C l ^ . At l i q u i d nitrogen temperature well-resolved spectra were recorded with both powder and si n g l e c r y s t a l samples. The c r y s t a l could be stored for months with no measurable decay of the ESR s i g n a l . The s i g n a l decayed s l i g h t l y a f t e r the c r y s t a l was annealed f o r 24 hours at 200°C. a) Analysis of P o l y c r y s t a l l i n e Spectrum F i g . I I I . l shows the p o l y c r y s t a l l i n e spectrum of m e l t - r e c r y s t a l l i s e d SrClg at 77°K. The spectrum i s c h a r a c t e r i s t i c of a species of S = with no hyperfine s p l i t t i n g s due to magnetic n u c l e i . The three " l i n e s " correspond to the magnetic f i e l d oriented along the p r i n c i p a l axes of a f u l l y anisotropic g-tensor. The following p r i n c i p a l g-values were obtained: g-L = 2.053 ± 0.001 g 2 = 2.0093 ± 0.0005 g 3 = 2.0030 + 0.0005 g^ and g^ have been taken from the f i e l d at the peak and trough of the f i r s t derivative curve (which correspond to shoulders on the undi f f e r e n t i a t e d absorption curve), and g 2 from the point where the f i r s t derivative curve crosses the base l i n e (which represents a peak i n the absorption curve). Analysis of Single Crystal Spectra 86 Figs. III.2a and III.2b show plots of magnetic f i e l d at which resonance lines appear for the c r y s t a l being rotated around a f o u r - f o l d axis and a t h r e e - f o l d axis respectively. The spectra are characterised by a species of Laue symmetry 3 (55) mmm i n a c r y s t a l of Laue symmetry m3m ( c u b i c ) v , provided the centre displays no hyperfine s p l i t t i n g . The p r i n c i p a l axes of the g-tens or l i e along the [OOl] , [llo] and the [llo] d i r e c t i o n s . For a general o r i e n t a t i o n of the magnetic f i e l d , s i x species should be observed i n such a case, with d i f f e r e n t d i r e c t i o n cosines between the molecular axes and the d i r e c t i o n of the magnetic f i e l d . However, with the magnetic f i e l d i n an (ImO) type plane or along an [imm] d i r e c t i o n , symmetry consideration predicts that only four species (55) should be observed The problem can also be approached i n the following way. There are three equivalent [OOl] -directions i n a cubic c r y s t a l . I f the r a d i c a l were a x i a l l y symmetric about an [OOl] type axis, three species should be observed f o r a general d i r e c t i o n of the magnetic f i e l d . But i n t h i s case, the species has orthorhombic symmetry and the [llo] and [llo] directions become magnetically inequivalent. A maximum of six species should be observed instead of three. The inequivalence of species A and B both with t h e i r z-axis along the [OOl] d i r e c t i o n can be seen i f the y-axis i s i n the d i r e c t i o n of the l i n e j o i n i n g two nearest neighbouring 87 cations (Pig. III.3 ) . When the magnetic f i e l d H l i e s along [ l i o ] , i t i s i n the d i r e c t i o n of the x-axis of species A, but i n the d i r e c t i o n of the y-axis of species B. Both g x x and g v v can be extracted from the spectrum taken with t h i s o r i e n t a t i o n of H ( F i g . III.4 a ) . For the same o r i e n t a t i o n of H with respect to the c r y s t a l , a l l the other four species with t h e i r z-axis along [loo] and [oio] directions become magnetically equivalent. Therefore a t o t a l of three species should be observed with abundance r a t i o 4 : 1 : 1 . When the magnetic f i e l d H l i e s along a Cz|-axis, say the [oOl] -axis ( F i g . III.4 b ) , species A and B become quivalent, gi v i n g the value of g„_. A l l the other four species should be observed with abundance r a t i o 2 : 4 . These facts are borne out i n Figs. III.2 and III.4 . The i n t e n s i t y r a t i o of the li n e s i n Figs. III.4 a and III.4b does not agree very w e l l with the above p r e d i c t i o n , but t h i s may be due to one or several e f f e c t s , e.g., saturation e f f e c t . The o v e r a l l experimental results are considered to be consistant with the conclusion from t h i s analysis that the r a d i c a l i n melt-r e c r y s t a l l i s e d SrClg i s characterised by a g-tensor: g x x = 2 .0028 i 0 .0005 g y y = 2 .0093 ± 0 .0005 g z z = 2 .053 ± 0 . 0 0 1 with x (or y) || [110] y (or x) || [lio] • II [ooi] [110] [211] [ M l ] [Oil] T3 r-i <U -H •P c bO 2 3270 f 3260 L 3250 3240 3230 j-3220 r 9 angle of rotation Figure III.2b " Road Map" of Resonance F i e l d s f o r Rotation Around C n f l l l l Axis 0 0 90 It should be noted here that the [llo] and the [llo] axes are equivalent i n a cubic c r y s t a l and are c r y s t a l l o g r a p h i c a l l y i n d i s t i n g u i s h a b l e . (See further discussion i n Section I I I . ( C ) c ) ) , The p r i n c i p a l g-values obtained were i n good agreement with those extracted from the powder spectrum. Figure III.3 Magnetic Inequivalence of Radical Species A and B i n SrClg Projected on a (110) Plane Figure III.4 Single C r y s t a l ESR Spectra of Melt-r e c r y s t a l l i s e d SrClp i 3270 gauss 92 I I I . (C) DISCUSSION OF RESULTS a) I d e n t i f i c a t i o n of Spectra Since the ESR spectra show no hyperfine s p l i t t i n g s from magnetic n u c l e i , i d e n t i f i c a t i o n of the paramagnetic species i s d i f f i c u l t , and has to come from chemical considerations and the g-factor. Radicals involving chlorine atom(s) can be ruled out since the spectra i n no way have the c h a r a c t e r i s t i c of species containing atom(s) with nuclear spin ^ . The large g-anisotropy i s not expected f o r a Sr ion l i k e S r + with i t s unpaired electron i n an s - o r b i t a l . ( S r 3 + i s chemically very u n l i k e l y due to the large i o n i s a t l o n p o t e n t i a l f o r the t h i r d e l e c t r o n ) . The r e l a t i v e l y small linewidth and the anisotropy of the spectrum would not be expected f o r an F-centre. The F-centre i n SrF^ has a g-value of I.989 ( 7 ^ # Most probably then, the ESR s i g n a l arises from an impurity centre i n S r C l ^ . The most common impurity atom with no nuclear spin found i n a l k a l i and alk a l i n e earth halides i s oxygen. The presence of water i n Sr C l ^ was discussed i n Chapter II of t h i s t h e s i s . Water and hydroxide ions are commonly found i n commercial a l k a l i halides. Kanzig ^ estimated that commercial KC1, KBr and KI cry s t a l s often contained between l O 1 ^ and 10"^ 0^" 93 3 centres per cm . Thus from chemical considerations alone, the species 0 +, 0", , 0 2", 0^~, OH and H0 2 are a l l possible paramagnetic impurity centres i n SrClg. The expected g - s h i f t s of these species w i l l now be discussed i n terms of the g-factor theory reviewed i n Chapter I and compared with experimental r e s u l t s . The large g - s h i f t ( A g ) observed f o r the r a d i c a l i n SrClg i s quite c h a r a c t e r i s t i c of an unpaired electron i n a II - type o r b i t a l with the degeneracy of the LT-type o r b i t a l s l i f t e d by a c r y s t a l f i e l d . i . Expected g - s h i f t s f o r 0~ 2 The e l e c t r o n i c ground state of the 0 ion i s Pj_ 2 2 2 1 a r i s i n g from the configuration (2p v) (2pT,) (2p ) . I f the x y 2 degeneracy of the p - o r b i t a l s i s l i f t e d so that P_ l i e s w e l l above p and p , the energy l e v e l scheme w i l l be as shown x y i n F i g . III.5a. A l l matrix elements of L , L and L vanish x y z except <Px| Ly|P z> " < py| Lz| px> = < pz| Lx| py> ' 1 < p z N p x > " < p x | L z | p y > ' < p y l L x l p z > = - 1 In the ground state of 0 , the unpaired electron i s i n p This wave function i s mixed with the excited states p„ and p x y by L and L respectively through s p i n - o r b i t coupling. In J X both cases an electron from an inner f i l l e d o r b i t a l i s excited to the o r b i t a l occupied by the unpaired electron. The corresponding g - s h i f t s are therefore p o s i t i v e , with t h e i r 94 magnitude dependent upon the c r y s t a l f i e l d s p l i t t i n g s E.__ and E„„. Since the ground state wave function p can be admixed yz z with neither p nor p by L , the g - s h i f t i n the z d i r e c t i o n X y J Z ° w i l l be zero to a f i r s t approximation. So f o r the 0~ ion i n an orthorhombic c r y s t a l f i e l d , i t i s expected that A g X X * A S y y > 0 A g z z * o In a tetragonal f i e l d p and p may remain degenerate i n which x y case one would expect A g x x = A g y y = A g i > o 2P, 4 K E yz 2 P x 2 P V 4t-2P, Ex: 4" ^ i t -2P. (a) 0" (b) 0 + Figure III.5 Energy Level Scheme of 0" and 0 + with p - o r b i t a l s Degeneracy L i f t e d 95 Some experimental g-factors for 0 i n various matrices are l i s t e d i n Table I I I . l . It can be seen that they agree q u a l i t a t i v e l y with p r e d i c t i o n , but do not f i t the data f o r the r a d i c a l found i n Sr C l ~ . Table I I I . l P r i n c i p a l g-factors ; of 0" i n Ionic Crystals and Glasses Host La t t i c e gxx g yy g zz Ref 3 C a 3 ( P 0 4 ) 2 -CaF 2 2.0516 2.0516 2.0012 56 Nal 2.2931 2.2931 1.9769 57 KC1 2.2217 2.4524 1.9^75 58 Hydroxide glasses 2.07 2.07 2.002 59 Expected g - s h i f t s f o r 0 + + 2 The ground state of the 0 ion i s P^ . with the configuration 2 1 (2p ) (2p ) ( F i g . III.5b). The matrix elements of L between y x x the ground state and the other two p-function excited states are a l l zero and so Ag i s expected to be close to zero. L„ X X j and L connects the ground state with p and p respectively, z z y A g v v should therefore be negative since the e x c i t a t i o n involves promotion of the unpaired electron to an outer vacant p o r b i t a l , while Ag should be p o s i t i v e . So to a f i r s t approximation the expected g - s h i f t s f o r the 0+ ion would be 96 A s - o xx A s y y < o Unlike the 0~ ion which i s quite well characterised, the 0 + ion has not been p o s i t i v e l y i d e n t i f i e d by ESR. i i i . Expected g - s h i f t s f o r 0^" The ozonide ion 0^" i s i s o e l e c t r o n i c with ClOg with nineteen valence electrons. The unpaired electron has been shown to occupy a b-^  molecular o r b i t a l constructed out of the three 2p_ oxygen atomic o r b i t a l s ( . Qne would expect that f o r 0 A g y y * A S Z Z » 0 A g x x « 0 j u s t as f o r ClOg. (The molecular o r b i t a l s and g - s h i f t s of ClOg was discussed i n Chapter II.) T y p i c a l g-values of 0^ are: g x x = 2.0025, g y y = 2.0174 and g z z = 2.0133 i n KCK>3 ^ 3 6 \ More data f o r 0^" are shown i n Table IV.2 i n Chapter IV. i v . Expected g - s h i f t s f o r 0g + and 0g~ Both the 0 2 + and Og" free ions would be i n a 2TT state, which i s s p l i t into 2nj_ and 2 n A states by s p i n - o r b i t a l c o u p l i n g ^ 1 ) 2 2-Og has the e l e c t r o n i c configuration ( 2ng( y)) (2n g^ x^)° f o r which the 2nj_ state l i e s lowest. 0o~ has the configuration 2 e L 2 1 2 (2n_/u.\) (211/ x) and the II A state l i e s lowest. The energy g\ y) g \ x j *• 97 l e v e l scheme f o r 0 2~ i n a c r y s t a l f i e l d which l i f t s the degeneracy of the 0 l e v e l was given by Kanzig and Cohen and i s shown here i n F i g . III.6a together with the l e v e l scheme f o r 0 + ( F i g . I I I . 6b). Since EI transforms l i k e p , n l i k e p x x y y 2 2 P and IT l i k e p , the Hi and Iii cases are not unlike the p. z z 2 z r-y 2 and cases discussed above. Since the energy separation E 2 between the pc? and the 2pTT states is expected to be much g S greater than the s p l i t t i n g A within the 2pII state, one would S expect for 0 A S zz » 0 A g y V > 0 but small Ag - 0 xx and f o r 0, 2 > Ag yy A s Z 2 « 0 - A g x x * 0 2 P c r u 2po: u 4 — \ — % 4 t 4f 4tr 2pTI u 2P01 E 4f Tig TTg 2pTI g 2pTT u 2pcr g (a) 02-Figure III.6 Energy Level Scheme fo r Oxygen Molecular Ions with  TT-level Degeneracy L i f t e d 98 The i d e n t i t y of Cv," has been well-established by ESR i n * 4 • * i (60* 61) . (62, 63) a number of i o n i c c r y s t a l s v molecular sieves s ', and molecular c r y s t a l ^ . Some t y p i c a l g-f actors of Cv,- are shown i n Table III.2. The g-values of the r a d i c a l formed i n m e l t - r e c r y s t a l l i s e d S r C l 2 are also shown for comparison. As a diatomic n-radical, Og i s very susceptible to environmental e f f e c t s ; i t s g-factor varies considerably from one host l a t t i c e to the other. More exact expressions f o r the t h e o r e t i c a l g-factor of the 0^' molecule-ion were derived by Kanzig and Cohen (^0) and w i l l be discussed i n more d e t a i l i n Section III.(C) Table III.2 T y p i c a l g-values of 0 o i n Various Host Crystals Host Lattice gxx g y y gzz# Ref. KCl 1.9512 1.9551 2.4359 60 Na0 2 1.99 1.99 2.175 ' 41 MgO 2.0011 2.0073 2.077 61 ZnO 2.0020 2.0082 2.051 61 Ba-Y z e o l i t e * 2.0046 2.0093 2.057 62 * Sr-Y z e o l i t e 2.0017 2.011 2.049 63 Na-Y z e o l i t e * 2.002 2.007 2.074 63 H 20 2-C0(NH 2) 2 2.001 2.008 2.049 64 S r C l 2 2.0028 2.0093 2.053 This work. * g-valiies shown f o r 0 2~ occupying one of multiple s i t e s . I z i s defined as the internuclear axis, and x the p I I-function of the unpaired electron. 99 0 2 + on the other hand, is poorly characterised. Although Jaccard i d e n t i f i e d one of the radi c a l s found i n KC1 and KBr doped with KNO^ as 0 2 +, a l l three p r i n c i p a l g-values f o r that species were found to be greater than the free spin value. This c e r t a i n l y contradicts p r e d i c t i o n and casts serious doubt on the i d e n t i f i c a t i o n of the r a d i c a l as 0 2 +. The molecule-ion N 2" i s . 2— i s o e l e c t r o n i c with 0 o with the same ground state. The g- s h i f t s f o r both radicals should therefore be q u a l i t a t i v e l y very s i m i l a r ; indeed the r a d i c a l i d e n t i f i e d as N 2~ has g-values that f i t much better with p r e d i c t i o n . The g-values f o r 0^+{?) and N 2" are shown i n Table III.3 f o r comparison. Table III.3  g-factors of 0^ and Ng" Radical C r y s t a l gxx g y y S z z Ref. o 2 + ( ? ) KC1 2.010 2.003 2.042 65 N2~ KC1-KN03 2.000 1.998 1.908 66 N 2" KN 3 2.0008 2.0027 1.983 67 v. Possible I d e n t i f i c a t i o n as OH or H0 2 From consideration of g - s h i f t s alone, the ra d i c a l s OH and HO^ are also possible i d e n t i f i c a t i o n s . The unpaired electron i n the hydroperoxyl r a d i c a l H0 2 occupies an antibonding T l - o r b i t a l constructed from oxygen p - o r b i t a l s directed perpendicular to the molecular plane ( 2 ^ ) . i t i s c l o s e l y related to the 0^ molecule-* ion and the degeneracy of the II levels formed by the oxygen p a i r 100 i s l i f t e d by bonding with the proton. The g - s h i f t s would be expected to be s i m i l a r to Og" with A g z z > 0 and Ag > Ag ^ 0, yy although the s p l i t t i n g of the IT levels A i s expected to be greater and therefore Ag smaller than i n 0 ~ due to the greater Z Z c. (23) e f f e c t of the proton . The unpaired electron i n the hydroxyl r a d i c a l OH also resides mainly i n an oxygen p - o r b i t a l with the configuration OH [(ltf) 2(2tf) 2(3<*) 2] 0 [ ( 2 P y ) 2 ( 2 p x ) 1 ] ^ 6 9 \ The degeneracy of the p and p - o r b i t a l s i s again l i f t e d by the c r y s t a l f i e l d (or hydrogen-bonding i n aqueous medium) so that A g z z > 0 8 1 1 ( 1 A g y y ~ A g x x ~ °* T y p i c a l g-values of OH and HOg are shown i n Table III.4. However, the proton hyperfine s p l i t t i n g (~ 43 gauss f o r OH and ~13 gauss f o r HO ) was not observed i n the ESR spectrum f o r the r a d i c a l i n SrClg. This s p l i t t i n g should be resolvable i f i t existed because of the r e l a t i v e l y narrow linewidth of the spectrum ( ~ 2 gauss). Table III.4 ESR Parameters of OH and H0 o Radicals Radical Matrix g z z f y y gxx Ref. OH ice 2.0585 2.0090 2.0050 69 i c e 2.06 2.009 2.005 70 LiSO^'HgO 2.0667 2.0065 2.0065 71 CaSO^HgO 2.1108 2.0028 2.0028 71 H0 2 HgO 2.034 2.006 2.006 72 H 20 2- HgO 2.0353 2.0086 2.0042 73 argon 2.0393 2.0044 2.0044 74 SrClg'6HgO 2.0355 2.008 2.003 75 HgOg-00(1^2)2 2.049 2.008 2.001 64 I 101 v i . I d e n t i f i c a t i o n as Cv> The radicals 0~, 02+', and 0^~ may be rejected as possible i d e n t i f i c a t i o n s on the basis of the g - s h i f t s , while OH and H0 2 may be rejected because of the absence of proton hyperfine s p l i t t i n g . The only p o s s i b i l i t i e s l e f t are 0 2 and 0 +. The superoxide ion 0 2~ i s considered to be a much more favourable i d e n t i f i c a t i o n than the poorly characterised 0 + ion. The former has been found i n a l k a l i halides grown from the melt and i t s concentration could be enhanced by heating the c r y s t a l s i n an oxygen atmosphere — processes quite s i m i l a r to the production of the r a d i c a l i n SrClg. The thermal s t a b i l i t y and the general requirement of observing the ESR spectrum at low temperatures are also consistent with the ch a r a c t e r i s t i c s of trapped i n i o n i c c r y s t a l s . It i s thus concluded that the ESR spectrum observed i n melt-r e c r y s t a l l i s e d S r C l 2 arises from the superoxide ion 0 2~. Site and Orientation of the Defect From an analysis of the single c r y s t a l spectra i t was concluded that the defect has a g-tensor g x x = 2.0028 g y y - 2.0093 g z z = 2.053 102 w i t h z i n a [OOl] t y p e d i r e c t i o n and x and y i n [ l i o ] t y p e d i r e c t i o n s . T h i s i s c o n s i s t e n t w i t h t h e model o f Cv," i f z i s t a k e n as t h e i n t e r n u c l e a r a x i s and x t h e d i r e c t i o n o f t h e pTT-f u n c t i o n o f the u n p a i r e d e l e c t r o n . However, the [lio] and [ l l O ] axes a r e e q u i v a l e n t i n a c u b i c c r y s t a l l i k e S r C l g . I f t h e 0 2 ~ m o l e c u l e - i o n e x i s t e d i n a n o r m a l l a t t i c e s i t e , s a y a c h l o r i d e vacancy, i t s x and y - d i r e c t i o n s would be e q u i v a l e n t . T h i s i s c o n t r a d i c t o r y t o o u r o b s e r v a t i o n t h a t t h e degeneracy o f t h e TT„/„\ a n d u , . o r b i t a l s has been l i f t e d as d i c t a t e d by t h e s ( x ) g(y) a n i s o t r o p i c g - t e n s o r . A n o r m a l a n i o n s i t e i n S r C l 2 i s i n a c u b i c e n v i r o n m e n t , b e i n g s u r r o u n d e d t e t r a h e d r a l l y by f o u r c a t i o n s S i n c e t h e 0^" f r e e i o n has an o r b i t a l l y d e g e n e r a t e ground s t a t e , t h e J a h n - T e l l e r theorem p r e d i c t s t h a t t h i s "complexed" s y s t e m w i l l be u n s t a b l e and t e n d t o have t h e symmetry l o w e r e d i n o r d e r t o l i f t t h e degeneracy. So t h e 0g i o n may move a l o n g a [OOl] d i r e c t i o n , a d i s p l a c e m e n t s i m i l a r t o t h a t o f t h e 0" i o n (77) i n C a P 2 . Fo u r p o s s i b i l i t i e s w i l l be c o n s i d e r e d . The 0 2 i o n may i . occupy an i n t e r s t i t i a l p o s i t i o n between two l a t t i c e a n i o n s ; i i . occupy the same p o s i t i o n b u t be a s s o c i a t e d w i t h two a d j a c e n t a n i o n v a c a n c i e s ; i i i . s u b s t i t u t e a n o r m a l a n i o n b u t be d i s p l a c e d a l o n g a [001] d i r e c t i o n ; i v . occupy a normal a n i o n s i t e b ut be a s s o c i a t e d w i t h a n o t h e r (nonmagnetic) i m p u r i t y . 103 The distance between two chloride ions i n S r C l ^ i s 3.4-9 A. The i o n i c radius of C l ~ i s 1.81 A and the internuclear distance o of Cv>"" i s 1.32 - 1.35 A from i n t e r p o l a t i o n of the 0-0 distances of 0 2 +, 0 2 and 0 2 2~ (7®). Case ( i ) may be rejected on the grounds of s i z e , e l e c t r o s t a t i c repulsion between C l ~ and 0^", and the absence of CI hyperfine s p l i t t i n g . Unless there i s evidence f o r another impurity associated with 0 2", case ( iv) may be considered less l i k e l y than cases ( i i ) and ( i i i ) . The choice between case ( i i ) and case ( i i i ) i s more d i f f i c u l t , although i t seems more reasonable that the 0 2~ ion i s displaced to the more symmetrical p o s i t i o n of case ( i i ) than to any intermediate p o s i t i o n of case ( i i i ) . The model proposed f o r case ( i i ) projected on a (110) plane i s depicted i n F i g . III.7. T h e o r e t i c a l consideration on the bonding scheme also favours case ( i i ) . To support t h i s proposal simple group t h e o r e t i c a l calculations w i l l be c a r r i e d out and the i n t e r a c t i o n between the oxygen-group o r b i t a l s and the metal o r b i t a l s w i l l be discussed i n the following section. Bonding Scheme of the Defect The model shown i n F i g . III.7 i s analogous to the structure of some dimeric 0 2 - t r a n s i t i o n metal complexes, e.g., the y#-superoxocobalt (III) decammine complex (NH 3) 5Co.0 2.Co(NH 3) 5^ 4' ( 7 9 \ Our treatment of the defect bonding scheme follows c l o s e l y that given by Vlcek (^°) on ^(.-superoxocobalt complexes. 104 a 0 = 6 .977 A 3.488 A • * ~ 1.32 A -[ H o ] || [no] -> z l l f o o i F i g u r e I I I . 7 Model of 0 2~ i n SrCn P r o j e c t e d on a (110) Plane (Ions denoted by broken c i r c l e s l i e x;V2&0 above and below the plane.) 105 The l o c a l symmetry of the ClgSr-0 2~•SrClg "complex" i s best seen i n F i g . III.8. Each Sr atom i s surrounded by four chloride ions i n a square plane and i n addition by two chloride Ions and the oxygen-group i n a plane perpendicular to the f i r s t . This system belongs to the symmetry group D p h. Figure III.8 Local Symmetry of the 0o~ ion i n S r C l 106 The' two Sr atoms taken as a p a i r form 14 cr-bonds with the CI atoms and the Og" group. This set of <J-bonds transforms as the reducible representation R( o~) = 3 A . . + B.. + B_ 4- 2B 0 -f- A n 4- 2B_ + 3 B _ + B 0  v y S r x g 2 g 3g l u l u J 2u 3u The two a-bonds formed by the 0g~ group transform as R( o" ) . = A 4- B 'Og l g 2u These two molecular o r b i t a l s are formed by the atomic o r b i t a l s of the oxygen atoms: A l g - 7 7 ( P . 1 - P . 8 ) T> 1 / 1 2 \ B = —To (p + P ) 2U V 2 V ^ y * y / (The superscripts 1 and 2 denote the two oxygen atoms.) The corresponding A^ g and Bg u molecular o r b i t a l s of the Sr p a i r are formed by the atomic s, p_^  and d x a o r b i t a l s (or t h e i r properly hybridised o r b i t a l s ) of the Sr. Neglecting the chlorine atoms i n the xz-plane, the Sr p a i r may form 12 if -bonding o r b i t a l s i n the yz-plane. They belong to the reducible representation * ( " ) s r = A l g + B l g 4- Bg g 4- B 3 g 4- A l u 4- 3 B l u 4- Bg u 4- 3 8 ^ The Og group has four o r b i t a l s f o r 7T-bonding transforming as R ( ^ ) 0 g = B l u + B2g + B3g + B3u 10? Among these B i s a d v - p r -type molecular o r b i t a l whereas jg Bgg i s a d g — p 5 -type molecular o r b i t a l . F i g . III.9 shows the p i c t o r i a l view of these IT and d bonds. The r e l a t i v e energies of these molecular o r b i t a l s are unknown. But i t may be assumed that the Tg^y) o r b i t a l of the 0^" group i s more s t a b i l i s e d by TT-bonding with the metal atoms, forcing the unpaired electron into the B 2 g molecular o r b i t a l which i s mainly l o c a l i s e d on the 0 2 ~ group. x (a) B 3 g d^ - bonding (b) B 2 g d g - p g bonding Figure III.9 Tf- and 5 -bonding of the Sr-Cg-Sr Group 1 0 8 The g - f a c t o r and O r b i t a l Angular Momentum Reduction Kanzig and Cohen (^) developed the f o l l o w i n g equations f o r the g - f a c t o r of the i o n i n the case where the symmetry around the molecular a x i s i s removed by a c r y s t a l f i e l d : g z z = g e + 2 i (1 + - f r (in.] gyy + - £ ) " 4 " 4 [ < 1 + - # - i " < 1 + $ " * " *] where g i s the f r e e e l e c t r o n g-value, A i s the s p i n - o r b i t e coupl i n g constant, A i s the c r y s t a l f i e l d s p l i t t i n g parameter, E Is the 2p]f - 2p (J s e p a r a t i o n ( F i g . I I I . 10), and I i s an g g e m p i r i c a l parameter which represents a c o r r e c t i o n t o the angular momentum about the molecular a x i s caused by the c r y s t a l f i e l d . 2P0-U ( 2 P 1 - 2p f) =d>. t * 1 o -rr /2 ' X X ' ^ X t* ft s p \ 4* <2py " ^ -*• 7 2 E * j r 2na tt 2 p 0 s - "7? + p« s>' Figure I I I . 1 0 Molecular O r b i t a l Scheme of 0o~ Ion i n a C r y s t a l F i e l d ( D 2 h ) 109 Combining equations (III. 2 ) and (III. 3 ) one obtains g y y - gxx = < 2 "4") [ i - ( i + g y y + gxx = 2 ( g e + _A.) (i + ( i n Equations (III.4) and (III. 5 ) may be solved f o r _A_ and A E by s u b s t i t u t i n g the experimental values of g and g . Using x x yy the value of so obtained and the experimental g„„, the A z z parameter / may i n turn be calculated. The results are l i s t e d i n Table I I I . 5, together with t y p i c a l values of these parameters found f o r Og" i n other host c r y s t a l s . Table I I I . 5 Spectroscopic Parameters A. A and £ of 0 o ~ A — £ & d— Calculated from Experimental g-values C r y s t a l A A A E / Ref. S r C l 2 0 .01 0.0032 2 .5 This work MgO 0 .035 0.0032 1.04 61 ZnO 0.017 0.0032 1.22 61 KCl 0.23 0 .0025 1.04 60 Nal 0.052 0.0004 1.8 81 NaCl 0.249 0.0030 0 .932 81 a-Y z e o l i t e 0.028 0.0038 0.99 62 110 The appropriate s p i n - o r b i t coupling constant A should be that f o r 0" ( . A. for 0" i s not known accurately, but extrapolation from the atomic energies of the i s o e l e c t r o n i c sequence of F, Ne +, Na** etc. (^2) gives the value of A = 0.014 eV. Using t h i s value of A , one obtains A = 1.4 eV E = 4.4 eV The magnitude of E, which should not be changed very much by the environment, i s i n good agreement with the value of 5 eV (81) found from the UV absorption spectra of 0g~ . Comparing the spectroscopic parameters of Og" i n various c r y s t a l s (Tables III.2 and I I I . 5 ) , i t i s apparent that A g i s usually zz smaller f o r crystals with divalent cations than with monovalent cations. This r e f l e c t s the greater c r y s t a l f i e l d s p l i t t i n g A due to the divalent cations surrounding the superoxide ion, which leads to a smaller value of A and consequently smaller A g A z z The parameter I i s c l o s e l y related to the o r b i t a l reduction fa c t o r k i n t r a n s i t i o n metal complexes. An up-to-date review on the subject was given by Gerloch and M i l l e r (^ 3)^  This quantity (84) k was o r i g i n a l l y devised by Stevens to explain the low 4- 2 -ESR g-values f o r NiClg and 1TQ,\^ . The o r b i t a l angular momentum associated with the t ^ metal o r b i t a l s was considered to be 2g "reduced" through the admixture of metal and ligand o r b i t a l s to form molecular o r b i t a l s of the complex. In the case of Og" i n I l l S r C l 2 , one may consider the e f f e c t of mixing the Sr and O^-group o r b i t a l s on the angular momentum along the molecular axis of the 0 2 " ion. The n g ( x ) and n g ^ o r b i t a l s of the 0^' ion w i l l be denoted as q> and d> respectively ( F i g . III.10). The matrix elements under L are z = - i OxkK) <*yKK>= i This i s equivalent to saying that there i s unit o r b i t a l angular momentum associated with the molecular axis (by d e f i n i t i o n of a n - o r b i t a l ) , defined as the z-axis. This o r b i t a l angular momentum i n the z d i r e c t i o n i s modified by the formation of molecular o r b i t a l s between 0 2 ~ and the Sr atoms i n the following manner. It was shown i n Section I I I . (C) c) that the molecular o r b i t a l s of the Sr p a i r with the proper symmetry to overlap with the n g£y^ and n g ^ o r b i t a l s of 0 2 ~ are B ^ r = ( d v z 1 - d v z 2 ) 3g y/2 K yz yz *2g = V ? ^xz 1 " dxz The formation of molecular o r b i t a l s between the metal and oxygen atoms leads to the following wave functions: Tx = N x (<J»X + c ^ 1 - c x d x z 2 ) ( I I I . 6 ) = \ ( * y + c y V a - vW° 112 where N^ i s the normalisation f a c t o r and c^ i s the mixing c o e f f i c i e n t . Neglecting the overlap between metal atoms, N \ = 1 4- 2 c x2 + 2 C x S g - 2 c x S g = 1 4- 2 c x 2 . . . . (III.8] = 1 4- 2 c/ 4- 4 c y S ? r (HI.9) where 4? xz = /4> Id? y yz Now the matrix element </dby | L z | 3>x ^becomes / " T | L | r > = N N / d 3 4- c ri 1 - c d J \ 'y1 z 1 x / x y\ y y yz y yz *x + c x d x z ' X X Z > (III.10) Since \ y I z x x / (^y | L z | ^ xz " ^xz ^ = = i 2iS ^ z 1 ) = ( ^ y z 2 | L z | f e 2 ) = 1 (^yz 1! L z | * x z 2 ) = (4>yz 2| ^ l ^ x z 1 ) = 0 < r y | L z ' r x > = V y ( 2 + 2 C X S 7 r + 2 C Y S 7 R 4- 2 C x C y ) i . . . ( I I I . 11) 113 S i m i l a r l y , <TxlLjTy> - N x N y ( 1 + 2 c x c y ) (-1) ( I I I . 12) Terms involving S§ i n equation (III.12) disappear because the overlap between <±, and d 1 has the opposite sign to the overlap x xz between and d (F i g . I I I . 9 ) . Recalling that the expression f o r ^ g z z involves the product ^  |ALZ j (4y|L z ^x)> » formation of molecular o r b i t a l s between metal and oxygen modifies t h i s expression to < T x | A L z | T y ) < T y | L z | T x > = *N xN y (1 + 2 0 ^ + 2c yS T + 2 c x c y ) (1 + 2 c x c y ) . . ( I I I . 13) This i s equivalent to multiplying the o r i g i n a l expression f o r A 6 Z Z *>y a fa c t o r I where ^ = N x 2 N y 2 ( X + 2 C X S T T + 2 c y S y + 2 c x c y ) ( l + 2 c x c y ) ( I I I . 14) Substituting f o r N and N from equations III. 8 and III. 9 x y one obtains n (1 + 2C-X .S7,- + 2c,pSlr + 2c Yc ) (1 4- 2c c ) t = - X _ Z UL x y' V x r . . . . ( I I I . 15) (1 4- 2c y2 4- 4c yS 7) (1 + 2c x2) Equation (III.15) predicts an o r b i t a l reduction f a c t o r / 7a 1. In most cases I f o r Og" i n an i o n i c c r y s t a l was found to be close to unity but smaller than unity (Table III.5) except i n Nal ( 8 l ) . The large value of I (-2.5) f o r 0 2~ i n SrClg i s unusual. Shuey and Z e l l e r (^ 5) postulated the presence of a 114 dynamical Jahn-Teller e f f e c t to explain the unusually large I (~ 1.8) i n Nal. A s i m i l a r e f f e c t may be operating i n the case of SrClg where the 0 2~ centre Is i n a s i m i l a r environment to that i n Nal, with s i x neighbouring cations arranged octahedrally but d i s t o r t e d along the y-axis ( F i g . III.7). CHAPTER IV DEFECTS AND IMPURITIES IN RECRYSTALLISED AND X-IRRADIATED STRONTIUM COMPOUNDS 115 After Og was I d e n t i f i e d i n m e l t - r e c r y s t a l l i s e d SrClg, an ESR study was made on SrClg doped i n the melt with hydroxide and other oxyanions to see whether 0g~ ion could be formed t h i s way. Other inorganic strontium compounds were also r e c r y s t a l l i s e d e i t h e r from the melts or from aqueous solutions to see what defect or impurity centres may be incorporated. In the preliminary survey, only p o l y c r y s t a l l i n e samples were used. They were X - i r r a d i a t e d and studied by ESR. IV. (A) APPARATUS AND PROCEDURE a) Sample Preparation i . Doping of SrClg The method of preparation of "anhydrous" SrClg powder has been described i n Chapter I I . SrClg was f i r s t melted i n a platinum crucible i n an e l e c t r i c 2- 2-furnace. The dopant (Sr s a l t s of OH , CO^ , SO^ & KCIO^) was slowly added to the melt. The sample was allowed to cool to room temperatures over a period of about 12 hours, i i . P r e c i p i t a t i o n of SrCO^ and SrSO^ from Aqueous Solutions SrSO^ was p r e c i p i t a t e d from aqueous solutions of SrClg and (NH^)gSO^ i n stoichiometric amounts. SrCO^ was p r e c i p i t a t e d by adding dropwise a s o l u t i o n of S r ( N 0 3 ) 2 to a s o l u t i o n of NagCO^, again i n 116 stoichiometric amounts. The p r e c i p i t a t e s were f i l t e r e d , thoroughly washed with d i s t i l l e d water and then dried i n a i r f o r 24 hours at 200°C. i i i . M e l t - r e c r y s t a l l i s a t i o n of Strontium Nitrate The s t a r t i n g material was of a n a l y t i c a l grade from Fisher Company. The sample was melted i n a platinum crucible i n the a i r by induction heating or i n an e l e c t r i c furnace. The temperature was maintained j u s t above the melting point f o r about 10 to 40 minutes. Current to the induction c o i l or furnace was then cut o f f and the sample was allowed to cool to room temperature. This took about 15 minutes i n the case of induction heating and about 4 hours i n the case of furnace heating. The molten s a l t was found to be brownish yellow i n colour and the r e c r y s t a l l i s e d s o l i d acquired a t i n t of yellow colour. Gases evolved from the molten s a l t slowly during heating. I f the sample was heated strongly, say 50 degrees or more above the melting point, a brown gas was evolved, presumably nitrogen dioxide. Infrared Studies A l l i n f r a r e d spectra were taken on a Model 21 Perkin-Elmer Infrared Spectrophotometer with NaCl op t i c s . Both KBr disks and Nujol mulls were used. But since absorptions from Nujol i n t e r f e r e d with sample absorptions i n the region that was of i n t e r e s t , only spectra taken on KBr disks w i l l be displayed. 117 No exchange of any of the carbonates and n i t r a t e s studied with KBr was observed. A t h i n f i l m of polystyrene was used for c a l i b r a t i n g the wave numbers. A l l frequencies were estimated to be accurate to within 2 cm \ X-ray Powder Techniques X-ray powder photographs of s o l i d s were obtained with a General E l e c t r i c powder camera of 14.32 cm, diameter with Straumanis loading. CuKa r a d i a t i o n with a Ni f i l t e r was used from an X-ray tube operated at 40 KV peak, and 20 mA. The sample was loaded i n a 0.5 mm. lead-free glass c a p i l l a r y which was then flame-sealed and mounted on the camera. Fil m s t r i p s of the proper si z e were cut from I l f e x Safety Base 14" x 17" X-ray films and were developed with Agfa Dektol D-19 Developer and f i x e d with Kodak Rapid Fixer. Time of exposure ranged from 16 to 24 hours. X - i r r a d l a t i o n X - i r r a d i a t i o n of a l l samples was c a r r i e d out with an X-ray source from a tungsten target operated at 45-50 KV and 23 niA. Since the X-ray window i s i n a ho r i z o n t a l p o s i t i o n a s p e c i a l apparatus f o r i r r a d i a t i o n at l i q u i d nitrogen temperature was constructed and i s shown i n F i g . IV.1. The powder sample was placed i n a quartz tube which may be inserted i n a dewar containing l i q u i d nitrogen. The lower end of the dewar i s unsilvered. The quartz tube 118 i s graded-sealed to a sidearm made of Sup r a s i l . This sample tube assembly may be closed by a BIO cone with Apiezon grease to prevent condensation inside the tube, or i t may be connected to the vacuum pump through a BIO j o i n t and stopcock when contact of the sample with a i r i s undesirable. After X - i r r a d i a t i o n the whole assembly was taken out of the dewar and the sample transferred to the sidearm as quickly as possible. The sidearm was then put into another dewar f o r ESR studies at l i q u i d nitrogen temperature. ^ To pump Quartz .tube Graded s e a l Dewar -> - / S u p r a s i l sidearm <-Sample 4 Stand X-ray source Figure IV.1  Apparatus f o r X - i r r a d i a t i o n at 77°K 1 1 9 IV. (B) EXPERIMENTAL RESULTS a) Summary of ESR I d e n t i f i c a t i o n of Radicals i n  X - i r r a d i a t e d Strontium Compounds The ESR results of a survey made on S r C l ^ doped with OH and some oxyanions and on some Sr compounds r e c r y s t a l l i s e d from melts or aqueous solutions are summarised i n Table I V . 1 . A l l i r r a d i a t i o n was c a r r i e d out at ambient temperature unless otherwise stated. Powder spectra were observed at both ambient and l i q u i d nitrogen temperatures. In doped SrClg, radicals o r i g i n a t i n g from the dopant materials could be i d e n t i f i e d and the results are not unexpected. However, some in t e r e s t i n g impurity species were found i n SrSO^ and SrCO^j and i n m e I t - r e c r y s t a l l i s e d Sr(N0^) 2. These r e s u l t s , together with those f o r X - i r r a d i a t e d Sr(N0„) , 3 2 warrant further comments and w i l l now be discussed i n greater d e t a i l s . i . H Atom i n SrSO^ The doublet found i n the spectrum of X-i r r a d i a t e d SrSO^ was remarkably stable and was observable both at room and l i q u i d nitrogen temperatures ( F i g . IV.2). It i s centred at g = 2.0025 (calculat e d by the Breit-Rabi equation) and the two li n e s are separated by 500 gauss. The s a t e l l i t e l i n e s on eit h e r side of each component of the doublet are c h a r a c t e r i s t i c of the trapped H atom. They have 1 2 0 been explained as a r e s u l t of a simultaneous spin f l i p of a neighbouring proton ( 8 ^ ) . The g - s h i f t i s very small and perhaps p o s i t i v e , i n contrast to the large p o s i t i v e g - s h i f t s generally observed f o r H atoms trapped on sulphate ions with s i m i l a r thermal s t a b i l i t y ( 8 7 ) . Re-interpretation of the Spectrum of X - i r r a d i a t e d Sr(NC> 3) 2 Zdansky and Sroubek reported the detection of four radi c a l s i n Sr(NOo) X - i r r a d i a t e d at 20°C ( 8 8 ) . Two of them were i d e n t i f i e d as N0 2 and 0^". The i d e n t i f i c a t i o n of the t h i r d one as N0^2~ was l a t e r considered to be erroneous ( 8 9 ) # This and the fourth hence remained u n i d e n t i f i e d . This work was repeated by the present author and the spectra recorded at ambient and l i q u i d nitrogen temperatures are shown i n Pig. IV.3. Results are compared i n Table IV.2 Only three rad i c a l s were detected i n t h i s work. The assignment of r a d i c a l A to NO^ i s considered to be correct by the present author. Its spectrum shows a x i a l symmetry at room temperature, but becomes f u l l y anisotropic at 77°K. The apparent a x i a l symmetry observed at room temperature may be interpreted as a r e s u l t of rapid r e - o r i e n t a t i o n of the molecule about an axis perpendicular to the molecular plane. This l i b r a t i o n a l motion of the molecule i s "frozen-in" at low temperatures. o This i s revealed by the observation that at 77 K 5 ( g y + g z) = 1 . 9975 i ( A Y 4- A Z ) = 57.1 gauss These are j u s t the values of gj_ and AJL of the room temperature spectrum within experimental errors. 121 Table IV, 1 Summary of Sr Compounds Investigated f o r ESR Signals S o l i d Treatment Results SrC doped with OH" X-i r r a d i a t e d and Probably HOg. and other u n i d e n t i f i e d radicals doped with SO4 2" and X - i r r a d i a t e d SO^ -., S 0 ^ ~ , and 0-^" r a d i c a l ions i d e n t i f i e d doped with C 0 o 2 ~ and X - i r r a d i a t e d CO2" ( r o t a t i n g about an axis along 0 - 0 d i r e c t i o n ) , and 0 ^ " r a d i c a l s doped with ClO-^" and X - i r r a d i a t e d decomposition of the chlorate; broad ESR signals u n i d e n t i f i a b l e SrSO, p r e c i p i t a t e d from aq. solu t i o n and X- i r r a d i a t e d trapped H atom, SO3"" and S 0 ^ " r a d i c a l s i d e n t i f i e d SrCO- p r e c i p i t a t e d from aq. s o l u t i o n by S r ( N 0 3 ) 2 + Na^CO^ and X - i r r a d i a t e d t r i p l e t spectrum i d e n t i f i e d as NO32-; and CO^" ion S r ( N 0 3 ) 2 r e c r y s t a l l i s e d from aq. s o l u t i o n and X-i r r a d i a t e d i r r a d i a t i o n at 300 K produced NO, 2> and possibly an asymmetric NO? r a d i c a l ; i r r a d i a t i o n at 77°K produced t r i p l e t at g 2.030, probably the symmetric NO3 r a d i c a l S r ( N 0 o ) 2 r e c r y s t a l l i s e d from melt with no i r r a d i a t i o n t r i p l e t spectrum i d e n t i f i e d as N 0 ^ (see rest of t h i s Chapter) 122 H 5 0 G A U S S I 1 (a) (b) DPPH Figure IV.2 ESR Spectra of H Atom i n SrSO), (a) Room temperature (b) 77°K 123 lO GAUSS I 1 Figure IV.3 •Powder ESR Spectra of X-i r r a d i a t e d SrtNO^K (a) Room temperature (b) 77°K 124 Table IV.2 ESR Results of Sr(N0g) 2 X - i r r a d i a t e d at Ambient Temperature Species Temp.°K g. g-tensor g. A-tensor (gauss) I den. Ref. A 300 2.0054 1.9975 50 59 N0 2 * 300 70 2.0052 2.0060 1.9976 1.9930 2.0020 50.0 48.0 57. 46.4 5 67.8 N0 2 ** ** B 300 300 70 2.0140 & e 2.003 2.0065 2.0133 2.0151 2.0111 -— — V °3" * ** C 300 2.0027 2.0087 1.2 _ 9 * * 300 70 2.0030 2.0033 2.008 2.008 ~1.2 -1.5 ~1. ~2. .6 0 N0 3 ** ** D 300 2.0041 2.0359 * * Ref. C88j ** This work 125 A s i m i l a r phenomenon was observed f o r r a d i c a l B, i d e n t i f i e d as 0o~. The average of g and g from the 77°K o y z spectrum i s 2.0131, which i s very close to the g-value of the feature marked B on the room temperature spectrum (Pig. IV.3a). This feature at g = 2.0133 In fac t corresponds to gj^ although the negative half of the derivative curve i s l o s t i n the much stronger s i g n a l from r a d i c a l C. Reference to Table IV.2 shows that the gj ( and g^ f o r 0^ " given by zSansky & Sroubek should be reversed. 0^ " i n S r ( N 0 3 ) 2 was studied by Kikuchi and coworkers (90) reported the following g-values at 300°K g„ = 2.0033 gj_ = 2.0135 These values are seen to be very close to what was observed i n t h i s work. The small hyperfine t r i p l e t of Radical C suggests that 14 the unpaired electron interacts with one N nucleus and i t probably resides mainly on a non-bonding oxygen o r b i t a l . Of a l l the known oxy-nitrogen r a d i c a l s only NO^ shows such small hyperfine s p l i t t i n g s . The observed g - s h i f t i n the perpendicular d i r e c t i o n i s p o s i t i v e i n accord with expectation f o r the r a d i c a l NO^, but i t i s much smaller than what was observed f o r t h i s r a d i c a l i n other matrices where the g x values ranged from 2.0207 i n (NH 4) 2Ce(N0 3) 6 to 2.0232 i n KN03 (Table IV.3). It i s however much closer to the g - s h i f t f o r a r a d i c a l observed i n KN0_ and NaN0Q and t e n t a t i v e l y i d e n t i f i e d by Cunningham as 126 (92) the peroxy-nitrogen-trloxide r a d i c a l , a s t r u c t u r a l isomer of the symmetric N O ^ with a structure . Radical C i n X - i r r a d i a t e d Sr(N0 3) i s most probably the same species. Table IV.3  ESR Parameters of Radical N O Q Matrix A,i Ax Ref. KN03 2.0032 2.0232 4.3 3.5 93 NaN03 2.0022 2.0217 4.1 3.5 96 Pb(N0 3) 2 1.998 2.029 - - 94 (NH 4) 2Ce(N0 3) 6/ H0N02-HOC103glass 2.0041 2.0207 - - 95 KN03 * 2.001 2.006 - - 91 NaN03 2.0024 2.0085 - - 92 S r ( N 0 3 ) 2 2.003 2.008 -1.2 -1.6 This * the unsymmetric isomer / / ^ 2-The spectrum reported by Zdansky & Sroubek f o r NO^ i n Sr(NO^)^ X - i r r a d i a t e d at 90°K ^^) CQU^^ n o t be reproduced i n t h i s work a f t e r repeated attempts. Instead a t r i p l e t spectrum at g R J 2.030 was found, believed to arise from the symmetric N0 3 r a d i c a l . ) 12? i i i . Spectrum of M e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 This ESR s i g n a l found without i r r a d i a t i o n i s most i n t e r e s t i n g and forms a more det a i l e d i n v e s t i g a t i o n and discussion f o r the rest of t h i s Chapter. Similar work was also done on B a ( N 0 3 ) 2 and the results were compared and used to a s s i s t i n t e r p r e t a t i o n of the S r ( N O Q ) j 2 spectrum. 2-b) The N O g Radical Ion i . I d e n t i f i c a t i o n of ESR Spectrum i n M e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 The material r e c r y s t a l l i s e d from molten S r ( N 0 3 ) 2 was translucent with a very pale t i n t of yellow colour. It has strong ESR signals c h a r a c t e r i s t i c of a powder sample even with a r e l a t i v e l y transparent small c r y s t a l . Attempts to grow c r y s t a l s with s i n g l e - c r y s t a l ESR signals have f a i l e d mainly because slow cooling of the melt over a period of four hours or more i n v a r i a b l y resulted i n almost complete decomposition of the n i t r a t e with a white residue. In some runs K N O - ^ UP to 20% by weight was added to lower the melting temperature of S r ( N 0 3 ) 2 and thus reduce the extent of decomposition. The experimental results did not d i f f e r s i g n i f i c a n t l y from "pure" S r ( N O o ) 128 The ESR spectrum observed at ambient temperature i s shown i n F i g . IV.4b. The s i g n a l i n t e n s i t y was found to increase with the time f o r which the n i t r a t e had been kept i n the molten state before being r e c r y s t a l l i s e d . It i s b a s i c a l l y a three-line spectrum characterised by i t s large g- and hyperfine anisotropics. This powder spectrum has a x i a l symmetry with the following parameters: g|( = 2.0001 ± 0.0005 g ± = 2.0056 ± 0.0005 A|| = 65.O ±1.0 gauss A_j_ = 32.0 ±1.0 gauss The three-line spectrum suggests a r a d i c a l containing 14 a single N nucleus with nuclear spin 1=1. The rad i c a l s N, NO, N0 2, N0 2 2", N0^ and NO^2" are a l l chemically f e a s i b l e species and a l l have been investigated by ESR. Of a l l these 2-only N0 2 and NO^ are known to have comparably large hyperfine s p l i t t i n g s . Some t y p i c a l ESR parameters f o r these two r a d i c a l s are shown i n Table IV.4. The experimental g- and hyperfine tensors i n the present spectrum show a x i a l symmetry and are seen to be closer 2-to those f o r N0^ than f o r NO^. The parameters found i n the present work are, however, s i g n i f i c a n t l y d i f f e r e n t from those of NO^2- as previously reported i n X - i r r a d i a t e d S^NO^)^. This Is discussed i n d e t a i l l a t e r . The apparent a x i a l 129 symmetry of N0 2 i n a variety of Inert and i o n i c matrices has been shown to a r i s e from rapid r e o r i e n t a t i o n of the molecule i n the matrix (94,100, 101)^ ^ temperatures t h i s " r o t a t i o n a l " motion may be "frozen-in" and the spectrum then becomes completely anisotropic. This has been recognised as a f a i r l y common phenomenon among small free radicals and was found f o r NOg i n NaN02 ^ 1 0 2 ^ and N20^ ( 1 0 1 ) . It was also found i n the work of t h i s thesis that the molecule NCv, i n X - i r r a d i a t e d S r ( N 0 3 ) 2 undergoes rapid r e o r i e n t a t i o n about an axis perpendicular to the molecular plane but remains stationary at l i q u i d nitrogen temperature (Section IV.(B) a) ). Observing the spectrum for the r a d i c a l i n m e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 at l i q u i d nitrogen temperature, however, resulted only i n some line-broadening with no apparent loss of i t s a x i a l symmetry (Pig. IV.4a). p -The species NO^ on the other hand, i s expected to (23) have a x i a l symmetry i n i t s g- and hyperfine tensors (103) According to Walsh t h i s AB^ type molecule with 25 valency electrons should be pyramidal and the unpaired electron should occupy a t o t a l l y symmetric a^-type molecular o r b i t a l under Group C 3 v, which has large s and p-characters on the c e n t r a l atom. A large i s o t r o p i c hyperfine component should therefore be observed on nitrogen together with a superimposed anisotropic component of considerable magnitude, the largest being along the molecular C^-axis (B ( (). This i s just what was observed experimentally. 130 Figure IV.4  ESR Spectra of M e l t - r e c r y s t a l l i s e d S r ( N 0 ? ) , (a) 77°K (t>) Room temperature 131 Table IV.4 ESR Parameters of 1 N0o and NO,2" g-tensor " "C j A-tensor (gauss) Radical La t t i c e g l g2 g3 A l *2 A3 Re: N0 2 N 20 4 1.9922 2.0022 2.0061 48.2 67.3 50.3 97 Pb(N0 3) 2 1.995 2.004 57 50 94 S r ( N 0 3 ) 2 1.9975 2.0054 59 50 88 NO/" CaCO 3 2.0066 2.0027 34.2 66.8 98 S r ( N 0 3 ) 2 2.0060 2.0019 37.3 68.8 89 Pb(N0 3) 2 1.9912 1.9857 36.5 66.0 89 Ba(N0 3) 2 2.0057 1.9997 35.3 67.6 99 The experimental hyperfine tensor can be decomposed into an i s o t r o p i c part a . a n and an anisotropic part B. Taking ISO p o s i t i v e signs f o r a l l three p r i n c i p a l values of the hyperfine tensor A one obtains a i s o = sauss B|| = 22.0 gauss Bj_ = ~2-B|| = -11.0 gauss Using these experimental hyperfine coupling constants the 14 ? unpaired spin density on the N 2 s - o r b i t a l c 2 g and that on 14 2 , the N 2 p z - o r b i t a l c 2 p (z being coincident with the C 3 molecular axis) may then be calculated by the method outlined 132 i n Chapter I, provided p o l a r i s a t i o n of the Is- and 2 s-orbitals can be neglected. The values of |V^ 2S(°)I 2 ^ r ~ 3 ) 2 p (54) f o r nitrogen have been taken from Morton, Rowlands and Whiffen J ^ 23(0)1 2 = 4 - 7 7 a.u. < \ r _ 3 ) 2 p = 3.10 a.u. One then obtains = 2 s 2 - 0.078 = 2 p 2 = 0.65 2 and f§p_ = 8 > 3 c 2 c 2 s Since the molecular o r b i t a l occupied by the unpaired electron 2 _ i n NO- i s e s s e n t i a l l y an sp n-hybrid on the N atom, the bond 5 c angle Z.0N0 may be derived from the hy b r i d i s a t i o n r a t i o \ = _p c by using equation (1 .21) . Assuming symmetry s <t> = cos • i r 1.5 _ i i [275+3 2 J 114° It should be noted however, that t h i s c a l c u l a t i o n of bond angle 4> i s not very s e n s i t i v e to the 2p/2s r a t i o i n t h i s 2 2 region. The t o t a l spin density on the nitrogen atom C2p + c 2s 3 the 2p/2s r a t i o and the bond angle derived from i t a l l agree 133 p- (104) c l o s e l y w i t h t h e v a l u e s a c c e p t e d f o r t h e r a d i c a l i o n N 0 ^ These e l e c t r o n i c p a r a m e t e r s d e r i v e d f r o m e x p e r i m e n t a l h y p e r f i n e d a t a f o r NO^ 2 - and N0 2 are summarised i n T a b l e I V . 5 . I t may be c o n c l u d e d t h a t t h e ESR s p e c t r u m i n m e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 a r i s e s f r o m t h e r a d i c a l i o n NO^ . T a b l e IV. 5 p_ E l e c t r o n i c Parameters and /lONO o f N 0 o and NOV R a d i c a l Mat r i x 2 c 2 s 2 c 2 p 2 2 c 2 s + c 2 p c 2 / c 2  c 2 p / c 2 s Z.0N00 Ref N 0 2 N 2 ° 4 0 . 0 9 5 0.350 0 .445 3.8 130.5 97 gas 0.090 0 .381 0.471 4 . 2 132.5 105 S r ( N 0 o ) v 3 y 2 O .098 0.403 0 .501 4 . 1 132 * NO 2 ~ CaCO^ 0.082 0.64 0.722 7 .8 115 98 S r ( N 0 3 ) 2 0 . 0 8 6 0 .62 0 .706 7.2 114.5 89 B a ( N 0 3 ) 2 O .083 O .63 0.713 7 . 6 114 99 m e l t -S r ( N 0 3 ) 2 0.078 O .65 0.728 8.3 114 * m e l t -B a ( N 0 3 ) 2 0.083 0.57 0.652 7 . 1 114 . 5 . * * T h i s work i i . S u p e r h y p e r f i n e S t r u c t u r e f r o m Ba V e r y s i m i l a r r e s u l t s were o b t a i n e d w i t h m e l t - r e c r y s -t a l l i s e d B a ( N 0 3 ) 2 > The ESR s p e c t r u m i s b a s i c a l l y a l s o a 134 t r i p l e t ( F i g . IV.5) with the following parameters g|| g l A|| Ai ( 1 4N) 1.9953 ± 0.0005 2.0042 + 0.0005 64 .6 ±1.0 gauss 35.3 4- 1.0 gauss It d i f f e r s from the spectrum of Sr(NOo) i n the appearance of 2 " s a t e l l i t e " l i n e s about each member of the N t r i p l e t . This i s interpreted as the superhyperfine s p l i t t i n g due to i n t e r -action of the unpaired electron i n N0^ with a single neigh-bouring Ba nucleus. The s i g n i f i c a n c e of t h i s i n r e l a t i o n to c r y s t a l structures and the loc a t i o n of the r a d i c a l w i l l be discussed l a t e r i n Section IV.(C) b). The analysis goes as follows. 82.09$ of the Ba i n natural abundance have zero 13B nuclear spin. There are two magnetic Ba isotopes, -^Ba and ^ T f i a , with nuclear magnetic moments 0.837 and O.936 nuclear magneton and abundance 6.59 and 11.32 per cent respectively. Both have nuclear spin I = 3/2. I f the s l i g h t difference i n t h e i r nuclear magnetic moments i s neglected f o r the moment, in t e r a c t i o n of the unpaired electron with a single Ba nucleus would result i n a f i v e - l i n e spectrum with i n t e n s i t y r a t i o 1 : 1 : 18.5 : 1 : 1 ( F i g . IV.6a). I f the i n t e r a c t i o n involves two equivalent Ba n u c l e i , the r e s u l t i n g spectrum would be more complicated ( F i g . IV.6b). This eleven-line spectrum would have the i n t e n s i t y r a t i o 1 : 2 : 37 : 3 : 37 : 340 : 37 : 3 : 37 : 2 : 1 The i n t e n s i t y of the lines r e s u l t i n g from both Ba being magnetic 135 may be so small that these l i n e s are l i k e l y to be l o s t i n the background. The resu l t would s t i l l be a f i v e - l i n e spectrum with i n t e n s i t y r a t i o 1 : 1 : 9 . 2 : 1 : 1 . Analysis of the powder spectrum f o r the superhyperfine structure i s d i f f i c u l t and measurement of the integrated i n t e n s i t y r a t i o of each 14 component of the N t r i p l e t to i t s next s a t e l l i t e l i n e i s much clo s e r to 18 : 1 than to 9 : 1. The i s o t o p i c e f f e c t due to the two Ba magnetic isotopes i s barely resolved at low f i e l d . This superhyperfine s p l i t t i n g associated with A^ (^N) has the approximate value of 6.2 gauss and 7.0 gauss, the r a t i o being very close to that of the nuclear magnetic moments of and ^^Ba. Thus the superhyperf ine structure may be ascribed to the r e s u l t of i n t e r a c t i o n of the unpaired electron 2-i n NO^ with a single neighbouring Ba nucleus. A s i m i l a r i n t e r a c t i o n with the ^ ^Sr nucleus of 1% abundance and nuclear spin I = 9/2 would have given 10 super-hyperf ine l i n e s each of i n t e n s i t y only 1/133 that of the main l i n e . So the superhyperfine structure associated with Sr would not be observable although i t may be assumed to e x i s t . l O G A U S S I 1 Figure IV.5 ESR Spectrum of Me I t - r e c r y s t a l l i s e d Ba(N0.O, ON 137 (a) Interaction with one Ba nucleus (b) Interaction with two equivalent Ba n u c l e i Figure IV.6 T h e o r e t i c a l Hyperfine Spectrum of Ba (Neglecting difference between Ba and Ba) 138 i i i . Infrared and X-ray D i f f r a c t i o n Studies An i n t e r e s t i n g point that has emerged from the above study and deserves attention i s the discrepancy i n ESR parameters f o r the N0^2~ found i n m e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 and i n X - i r r a d i a t e d S r ( N 0 3 ) 2 reported by Zdansky and Sroubek ( 89)^ T h e < j [ i f f e r e n c e Q f m 0 r e than 5 gauss i n the values of Aj_ cannot be accounted f o r by experimental errors or error from analysis of powder spectrum, s i m i l a r discrepancy was found i n the case of Ba(N0 3) 2. This arouses the suspicion that the NO^2" ion i s i n fact i n an impurity host. The n i t r a t e i s expected to decompose p a r t i a l l y on melting. The most probable s o l i d products of decomposition are the n i t r i t e and the oxide. Attempts were made to characterise the products of decomposition. F i g . IV.7a shows the i n f r a r e d absorption spectrum from 650 to 1850 cm - 1, i n KBr disc, f o r a s o l i d sample r e c r y s t a l l i s e d from S r ( N 0 3 ) 2 a f t e r b r i e f melting. A l l the bands can be associated with the n i t r a t e ion of S r ( N 0 3 ) 2 i n D^n symmetry except the ones at 83^, I269 and 1625 cm - 1. The 1625 cm ^ band i s the well-known water absorption and the other two can be assigned to the O-N-0 bending and the O-N-0 symmetric-stretching vibrations of the n i t r i t e ion i n C 2 v symmetry. —i 1 1 1 1 1 1 1 1 1 1 n — 1800 1700 1600 1500 1400 1300 1200 IIOO IOOO 9 0 0 8 0 0 700Cm Figure IV.7 Infrared Spectra of M e l t - r e c r y s t a l l i s e d Sr(NO^)^ (a) a f t e r b r i e f melting 1800 700cm- 1 Figure IV.7 Infrared Spectra of M e l t - r e c r y s t a l l i s e d Sr(NO^)^ (b) a f t e r heating the melt f o r 30 minutes o 1800 I 1700 1600 I 1 1500 1400 1300 I200 lOO — \ 1 IOOO 900 —I 1 8 0 0 700 cm- 1 Figure IV,7 Infrared Spectra of M e l t - r e c r y s t a l l i s e d Sr(NO^)^ (c) a f t e r prolonged heating of melt 142 I f the sample had been kept i n molten state f o r over h a l f an hour and then cooled, a d d i t i o n a l hands were recorded with the r e c r y s t a l l i s e d s o l i d ( F i g . IV.7b). The bands at 696, 704, 854 and I762 cm""1" can be assigned unequivocally to 2- -1 the CO3 ion i n SrCO^. The bands at 838 and 1070 cm are very weak, but can reasonably be assigned to the same CO^2" -1 as well . The strong band around 1450 cm has now broadened and increased i n r e l a t i v e i n t e n s i t y due to the overlap of the n i t r a t e absorption and the carbonate absorption. Assignment of the bands at 721 and 1052 cm _ 1 i s less straight-forward and requires a closer examination of the v i b r a t i o n a l modes of the n i t r a t e ion. \ The unperturbed planar n i t r a t e ion belongs to the symmetry group D-^. gives r i s e to s i x normal modes of vi b r a t i o n belonging to the i r r e d u c i b l e representations f g = A^ + A 2 + 2E' The A^ mode v i b r a t i o n i s Raman active only, A^ mode i s i n f r a r e d active only and the two E 1 modes are both doubly degenerate and in f r a r e d as well as Raman active. Their fundamental frequencies are l i s t e d i n Table IV. 6. The form of the v i b r a t i o n a l modes also depends on the symmetry of the environment. So the behaviour of the v i b r a t i o n a l states may be modified when the molecule i s placed i n a c r y s t a l 143 where the environmental symmetry or s i t e symmetry i s lower than the molecular symmetry. This type of perturbation may res u l t i n ( i ) breakdown of the s e l e c t i o n rules, and ( i i ) removal of degeneracies i n vibrations belonging to classes E or T. These e f f e c t s may or may not be observable depending on just how strongly the molecule interacts with i t s environment. A c l a s s i c a l example was found i n the two polymorphic forms of P-CaCOo* where these e f f e c t s on the CO ion are demonstrated i n going from a s i t e symmetry of D~ i n c a l c i t e to C i n 5 ° aragonite (106) Table IV. 6 Fundamental Vibrations of the D^n Nitrate Ion Mode Symmetry A" A2 \/ (N-0 sym. s t r . ) i/g (out-of-plane) T/^ ( n _ 0 asym. s t r . ) E' l / ^ (in-plane bending) E 1 Frequency (cm" 1) A c t i v i t y ~1050 Raman ~820 IR 1420 740 IR & Raman IR & Raman Sr(N0 ) belongs to the space group T, with four (107) molecules per unit c e l l . The orthorhombic space group T. ^  contains only two s i t e s with point symmetries C and C_. ^ -^ 6) h 3 31 The f i r s t i s a sub-group of the molecular group D^n of the unperturbed n i t r a t e ion whereas the second i s not. Hence the 144 n i t r a t e ion must be on s i t e and the Sr ion on s i t e Reference to the character table f o r group shows that a l l six normal modes of v i b r a t i o n of the n i t r a t e ion are now both i n f r a r e d and Raman active while and i ) ^ s t i l l remain doubly-degenerate. On the other hand, SrCO c r y s t a l l i s e s i n the 16 aragonite structure and belongs to the space group V^ with four molecules per unit c e l l ^ . The s i t e symmetry of the carbonate ion i s now C_. Reference to the character table f o r s group C g again shows that a l l six normal modes of v i b r a t i o n f o r the ion are both i n f r a r e d and Raman active and are a l l non-degenerate. I f a n i t r a t e ion i s incorporated i n the SrCO-^ l a t t i c e , i t would most l i k e l y replace a CO^2" ion i n i t s normal l a t t i c e s i t e . The 1^-frequency of t h i s n i t r a t e ion now i n s i t e symmetry C i s expected to appear i n the i n f r a r e d s spectrum, while ^ and \}^  are expected to be both s p l i t into doublets. Based on the above analysis the appearance of the 1052 cm"1 band i n m e l t - r e c r y s t a l l i s e d Sr(N0 3)^ may be interpreted as the resu l t of i n f r a r e d a c t i v i t y of the i)^-mode v i b r a t i o n of the n i t r a t e ion, which has been placed i n a low symmetry s i t e of the SrCO^ l a t t i c e . S i m i l a r l y the 721 cm""1 band together with the 738 cm"1 band can be a t t r i b u t e d to s i t e symmetry s p l i t t i n g of the ^-mode. These results and t h e i r assignments are summarised i n Table IV.7. The i j ^ - f requency around 1420 cm"1 should also be s p l i t i n t o two components according to t h i s model, but superimposition of the n i t r a t e , carbonate and possibly n i t r i t e absorptions would make t h i s 145 s p l i t t i n g unobservable. Furthermore, some s p l i t t i n g of t h i s frequency has been observed i n pure Sr(N0~) where a possible 2 t r a n s i t i o n from D^n to C^v symmetry of the n i t r a t e ion has been suggested through covalent bonding with the metal ions ^ Table IV.7 Addition a l Infrared Absorption Bands of Melt-• r e c r y s t a l l i s e d Sr(N0o)o Frequency ( c n f 1 ) Intensity As s ignment 696 704 m m 721 ra 1/)i+-N03" i n s i t e C g 834 m 1VN02~ 838 w p-C 03 854 s l ) 2 - c o 3 2 -1052 w ^1~ N 03~ i n S i t e C s 1070 w 1269 s, broad 1762 w V i + 1V C0 32~ The n i t r a t e s of Sr and Ba are i s o - s t r u c t u r a l , and so are the carbonates. Again carbonate was detected i n melt - r e c r y s t a l -l i s e d Ba(N0 ) . Additio n a l bands were observed at 695, 857, -1 1057 and 1750 cm which can be assigned to frequencies of the CO^ 2 - ion i n BaCO^. However, the expected i n f r a r e d a c t i v i t y of 146 at ~1050 cm""1 and the s p l i t t i n g of l)^ at ~74o cm"1 of the n i t r a t e ion were not observed. This could be the result of weaker perturbation on a ion i n the BaCO^ than i n the SrCO^ l a t t i c e . That t h i s may well be the case i s manifested by the observation that while 1)^  has moderate i n t e n s i t y and l ) 4 i s s p l i t by about 8 cm"1 i n SrCO^, the 1^-band i n BaCO^ i s weak and there i s no observable s p l i t t i n g of 1^. Powder X-ray photographs were taken on samples from the same batches used for i n f r a r e d studies. A l l the li n e s i n F i g . IV.8b could be assigned to Sr(NC> 3) 2 plus SrCO^. The X-ray photographs also show that there i s no measurable change i n l a t t i c e parameters of the carbonate compound due to incorporation of some n i t r a t e ions' and there i s no new c r y s t a l phase formed between them. These results are consistent with those drawn from i n f r a r e d studies. (a) a f t e r b r i e f melting (b) a f t e r heating the melt f o r 30 minutes (c) a f t e r prolonged heating of melt Figure IV.8 Powder X-ray Photographs of M e l t - r e c r y s t a l l i s e d SrCNO^^ 148 IV. (C) DISCUSSION 2-a) The Host L a t t i c e of NO^ In order to arrive at some l o g i c a l conclusion to the problem of host l a t t i c e f o r the r a d i c a l ion NO 2 " i n me It-re c r y s t a l l i s e d Sr(N0 o)^ and Ba(N0 o) , 3 3'2 3 2 the following l i n e s of experimental evidence are summarised. 2-i . The NO^ detected i n X - i r r a d i a t e d S r ( N 0 3 ) 2 by s * / / ^ ( 8Q ) Zdansky and Sroubek v y ' has s i g n i f i c a n t l y d i f f e r e n t ESR parameters from the r a d i c a l i d e n t i f i e d as NO^2" i n t h i s work. They also d i f f e r i n thermal s t a b i l i t y ; the former r a d i c a l disappeared completely a f t e r the sample had been warmed up to room temperature whereas the l a t t e r was found to be remarkably stable at room temperature. It i s obvious that t h i s r a d i c a l ion exists i n d i f f e r e n t environments i n the two cases. i i . S i m i l a r discrepancy was found i n the case of Ba^O^)^, Furthermore, the spectrum of Ba(N0 3)^ X - i r r a d i a t e d at 77°K has no superhyperfine structure that can be associated with a Ba nucleus as i n meIt-recrystal-l i s e d Ba(N0 3) 2 > This again suggests the association of the NO^ 2 - with an impurity l a t t i c e i n the l a t t e r . 14-9 i l l . Although n i t r i t e s and oxides are the most probable products of decomposition of the group IIA n i t r a t e s , very l i t t l e of these materials were found from i n f r a r e d and X-ray studies. Instead carbonates were unequivocally i d e n t i f i e d . Indeed Hester and Krishnan observed two weak i n f r a r e d absorption bands at 865 and ~700 cm"1 i n Sr(NG\^)2 and Ba(N0 3) 2 melts at high temperatures ( 1 0 9 ) # They also found that the r e l a t i v e i n t e n s i t y of these bands increased with increasing temperature. These bands were also observed i n the cooled s o l i d samples of Sr(N0 o)^/KN0_ and Ba(NC> ) /KN0o x 3'2 3 3 2 3 af t e r t h e i r melts had been strongly heated. These workers suggested that they seemed to be connected with some decomposition product of the n i t r a t e s . In fac t they undoubtedly ari s e from the carbonates. i v . Some evidence has been presented f o r the n i t r a t e ion occupying a normal anion s i t e i n the carbonate l a t t i c e i n m e l t - r e c r y s t a l l i s e d S r ( N 0 3 ) 2 . The r e l a t i v e i n t e n s i t y of p _ ESR s i g n a l a t t r i b u t e d to NO^ was found to increase with the amount of carbonate present. These observations a l l strongly suggest the association of the r a d i c a l with a carbonate l a t t i c e . v. As a f i n a l confirmation SrCOo and BaCO were doped with 3 n i t r a t e by c o - p r e c i p i t a t i o n and then X - i r r a d i a t e d at room temperature. The r e s u l t i n g spectra were almost i d e n t i c a l 150 with the corresponding ones obtained from the me I t - r e c r y s t a l -l i s e d materials except f o r a s l i g h t v a r i a t i o n i n linewidth. 2-Formatlon and S t a b i l i t y of NO^ i n Carbonate Although various properties of molten a l k a l i metal n i t r a t e s have been studied quite extensively, r e l a t i v e l y l i t t l e i s known about the behaviour of molten a l k a l i n e earth n i t r a t e s . From i n f r a r e d studies Hester and coworkers suggested that some n i t r a t e ions i n these systems had e f f e c t i v e C 2 v symmetry through complex formation of the type M-O-NO^  whereas others remained undisturbed and retained the D o v % 3h symmetry ("^9)^ Devlin and coworkers on the other hand, considered a perturbed l a t t i c e structure i n the molten state, e.g., vacancies accompanied by stronger anion-distorting forces between r e s i d u a l neighbouring ions ^ -1-10) ^  The group IIA n i t r a t e s decompose on strong heating to form oxides and n i t r i t e s M(N0 3) 2 > M(N0 2) 2 4- 0 2 (IV.l) M ( N 0 2 ) 2 > MO + NO + N0 2 (IV.2) It i s also generally taken f o r granted that the n i t r a t e melt i s s e l f - d i s s o c i a t i n g y i e l d i n g oxide ions 2N03~ 2N0 2 4- j k ) 2 4- 0 2" (IV.3) 151 However Jordan and coworkers have pointed out that the important oxygen anions i n the n i t r a t e melts are superoxide Og and peroxide Og 2" rather than the oxide ( d u e to oxygen tr a n s f e r N03~ + 0 2~ ; * NOg" + Og 2" (IV.4) In the presence of oxygen at appreciable p a r t i a l pressure the peroxide may also be converted to superoxide i n accordance with the equilibrium 0 2 + Og 2" 20 2" (IV. 5) The equilibrium constants f o r reactions IV.4 and IV.5 were estimated to be 3 and 3 x 10 respectively by the same workers. K 4 = L _ J 1 2_ j « 3 (IV. 6) f.o 3- ] [ o - j K. = l°2 ]l » 3 x 105 (IV.7) Ml °2 It may be suggested that atmospheric C0 2 slowly dissolves 2 -i n the n i t r a t e melt to form CO3 by oxygen t r a n s f e r 0 2" + C0 2 > C 0 3 2 " . . . . . . . . . (IV.8) 152 The oxygen species Involved might be peroxide or superoxide rather than 0 . The oxide ion i s not a very stable species, 2 -since the second electron a f f i n i t y of 0 i s negative, and 0 i s known p r i n c i p a l l y i n i o n i c c r y s t a l s i n which i t i s s t a b i l i s e d by e l e c t r o s t a t i c forces of the l a t t i c e . Evidence on i t s presence or absence i n melts i s inconclusive. Removal of the carbonate ions from the melt by p r e c i p i t a t i o n with the cations makes equilibrium (IV.8) favourable i n competing with reaction (IV.4) f o r the oxide ions. Nitrate ions have been found to be very e f f e c t i v e electron scavengers i n frozen aqueous media ( 1 1 2 ) . There has been other evidence pointing to the conclusion that electrons released by high-energy r a d i a t i o n tend to be attached to the n i t r a t e ions ("^3)^ p Q r ^ e i e c - t r 0 n to become l o c a l i s e d on a p a r t i c u l a r ion the conditions at or around t h i s ion must d i f f e r from other neighbouring ions. According to Cunningham, t h i s l o c a l i s a t i o n of electron onto the n i t r a t e ion was s t a b i l i s e d by the intramolecular configuration change when the planar n i t r a t e ion d i s t o r t s towards the pyramidal form of ?- (114) the NO^ ion '. It was pointed out e a r l i e r that i n most group IIA n i t r a t e melts some n i t r a t e ions were found to d i s t o r t to C symmetry e i t h e r through complex formation or by l a t t i c e perturbation. These d i s t o r t e d n i t r a t e ions then become e f f e c t i v e s i t e s f o r electron trapping. Once trapped, the excess electron i s s t a b i l i s e d by intramolecular configuration 2 -change, i . e . , change from a planar n i t r a t e to a pyramidal NO^ 153 This r a d i c a l ion becomes further s t a b i l i s e d when incorporated i n a carbonate l a t t i c e f o r two good reasons. i . Conditions become unfavourable f o r resonant t r a n s f e r of the unpaired electron to a neighbouring NO^- ion since they are now r e l a t i v e l y i s o l a t e d from one another i n the 2-carbonate l a t t i c e . This explains why NO^ found i n a l l pure n i t r a t e c r y s t a l s i s stable at low temperatures only, but remains stable to above room temperature when occupying an otherwise normal anion s i t e i n doped c r y s t a l s , e.g., i n halides ( . Indeed the rad i c a l s found i n meIt-recrystal-l i s e d S r ( N 0 3 ) 2 and Ba(N0 3) 2 were stable f o r weeks at room temperature. An excess electron on a carbonate ion, i . e . , CO^ 2 -, would be r e a d i l y transferred to a neighbouring CO^ 2 -ion or to an impurity NO3"* ion. A doubly charged n i t r a t e ion i n a carbonate l a t t i c e presents no problem of charge compensation and i t s s t a b i l i t y i s e a s i l y understood. i i . There i s some i n d i r e c t evidence f o r intermolecular i n t e r a c t i o n between the r a d i c a l and l a t t i c e ions that further s t a b i l i s e s 2-the NO^ r a d i c a l , at least i n the BaCO^ l a t t i c e . This evidence comes from the a d d i t i o n a l superhyperfine s p l i t t i n g 14 ?-due to Ba i n the ^0^ spectrum. This radical-metal ion i n t e r a c t i o n i s non-existent or unobservable i n the nitr a t e c r y s t a l . Without s i n g l e - c r y s t a l ESR data, l i t t l e can be s a i d about the o r i e n t a t i o n of the N0_ ion i n the carbonate 154 l a t t i c e . Since C0 o and N O " ions are both planar and 3 3 comparable i n size ( t h e i r e f f e c t i v e i o n i c r a d i i being I.85 and I.89 A r e s p e c t i v e l y ) , i t i s l o g i c a l to assume that the n i t r a t e ion replaces a normal carbonate ion i n the l a t t i c e . The aragonite structure, to which both SrCO^ and BaCO^ belong, i s orthorhomblc but the ions have pseudo-hexagonal arrangements when viewed along i t s b-axis ( F i g . IV.9). Planes of carbonate ions and cations are stacked one upon the other along the b-axis. Each c e n t r a l carbon atom i s "sandwiched" between two planes of cations, above and below. These s i x cations however are not equidistant from the C atom; one of them i s nearer than a l l the others. This i s the one marked +4- i n F i g . IV.9 (A perpendicular to the plane of F i g . IV.9 i s not a thre e - f o l d axis; the c r y s t a l has no 2-th r e e - f o l d symmetry). The N O ^ r a d i c a l ion i s pyramidal and not planar. This i s tantamount to a movement of the nitrogen atom along the b-axis. The r e s u l t i s an increase i n i t s distance from three cations on a plane but a decrease i n distance from the other three on another plane. This may constitute the intermolecular i n t e r a c t i o n that helps 2-to s t a b i l i s e the electron-excess NO3 ion. The i n t e r p r e t a t i o n of the superhyperfine structure being connected with a single Ba nucleus i s also consistent with t h i s model. In the c r y s t a l structure of S r ( N O o ) and B a ( N O o ) the NO-" ion i s centred i n a t r i a n g l e of cations, 155 g i v i n g i t three nearest neighbours (a fourth cation, completing a tetrahedron with the other three, i s more di s t a n t ) . NOo2" has been studied ( 89) i n Pb(NO_) , which has the same structure, and, although the three-fold axis of the r a d i c a l is p a r a l l e l to one of the c r y s t a l , the centre of the r a d i c a l i s displaced from the centre of the t r i a n g l e of cations. The superhyperfine s p l i t t i n g was found to be at t r i b u t a b l e to two equivalent cations and one non-equivalent cation ( P b 2 + ) . 156 Figure IV.9 Projection of the Aragonite Structure along b-axls Showing Pseudohexagonal Arrangement of Ions CHAPTER V CONCLUSION AND SUGGESTIONS FOR FUTURE WORK 157 V. (A) CONCLUDING REMARKS The present work has revealed that minute amount of impurities may be incorporated i n many inorganic compounds through ordinary processes of r e c r y s t a l l i s a t i o n and " p u r i f i c a t i o n " . Compounds prepared i n the presence of the atmosphere are p a r t i c u l a r l y susceptible to incorporation of oxygen and carbon dioxide i n various forms. Kinetics results of s o l i d state reactions have to be interpreted with caution as nucleation processes and the rate of reaction are l i k e l y to be affected by the nature and amount of impurities present. The present work has also demonstrated the usefulness of the ESR technique i n detecting impurities i n s o l i d s . Normally diamagnetic impurities may be converted to paramagnetic species by chemical reactions or high-energy i r r a d i a t i o n s . The absolute concentration of an impurity may also be estimated i n d i r e c t l y by measuring the s i g n a l i n t e n s i t y of i t s paramagnetic form. 158 V. (B) SUGGESTIONS FOR FUTURE WORK a) Reaction of SrClg with F g In view of the possible presence of water i n SrClg dehydrated at temperatures w e l l below i t s melting point, i t s reaction with Fg should be re-investigated by using highly p u r i f i e d samples. The i r r e p r o d u c i b i l i t y of some of the previous k i n e t i c s measurements i s l i k e l y to be a consequence of the varying amount of water entrapped i n the s o l i d samples used. One i n t r i g u i n g aspect of t h i s reaction i s the absence of interhalogen compounds i n the gaseous products. C 1 F or CIF^ are expected to form i n t h i s reaction i n small quantities at room temperature, but none has been i d e n t i f i e d i n the o p t i c a l absorption spectra. Two possible causes may be envisaged. It has been pointed out by previous worke rs (14) that the temperature i n the reacting zone may be very high (above 600°Q), since the reaction i s highly exothermic. This condition may not be favourable f o r the formation of C 1 F or CIF3. A l t e r n a t i v e l y , the C 1 F or CIF^ that may be formed i n i t i a l l y could react further with the s o l i d to produce more Clg. This l a t t e r p o s s i b i l i t y may be examined by reacting SrClg with C 1 F and with CIF^ with a spectrophotometer i n s i t u . 159 The controversy over the i d e n t i t y of the single c r y s t a l spectra obtained by Catton i s s t i l l unsettled. Since i t has not been possible to reproduce these spectra i n the present work and there i s no other known way of trapping a CI atom i n the S r C l 2 c r y s t a l , the experimental conditions (temperature and pressure of fluorine) should perhaps be varied over a wider range i n the hope of obtaining better single c r y s t a l spectra. This would shed some l i g h t on the question. The 0 " Ion i n SrClg There is no s a t i s f a c t o r y explanation f o r the anom-alously large angular momentum reduction f a c t o r f o r Cv," i n S r C l . The proposed model f o r t h i s r a d i c a l ion can 2 17 also be checked by using oxygen enriched i n 0. Hyperfine 17 structure from 0 should help to confirm the i d e n t i f i c a t i o n and provide more information about the trapping s i t e . Most sample cry s t a l s used for t h i s study were found to be not single c r y s t a l s . The p o s s i b i l i t y of twinning should be investigated by X-ray d i f f r a c t i o n method more c a r e f u l l y and with greater p r e c i s i o n . The existence of twinning i n the sodium aild e c r y s t a l s has been pointed out by Ge l e r i n t e r and Silsbee ^ 1 15) suggested that twinning may be responsible f o r the t i l t i n g of the g-tensor f o r the N 2~ r a d i c a l ion i n sodium azide. 160 c) The NOg Ion In a Carbonate L a t t i c e In the ESR spectrum of m e l t - r e c r y s t a l l i s e d Ba.(^0^)^ the superhyperfine s p l i t t i n g s due to Ba, when completely unravelled, should provide more rigorous proof f o r the 2-proposed model of NO^ i n the carbonate l a t t i c e . 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