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Mossbauer investigation of Fe 57 in Linde L Zeolite Wedd, Robert William James 1969

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MOSSBAUER INVESTIGATION OF Fe IN LINDE L ZEOLITE by ^ ROBERT WILLIAM JAMES WEDD B . S c , U n i v e r s i t y o f V i c t o r i a , 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE^ OF MASTER! OF SCIENCE i n t h e Department o f C h e m i s t r y 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 UNIVERSITY OF BRITISH COLUMBIA May, 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t 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 , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e Head o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f C L±~^> t<H~iA i^ The 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 V a n c o u v e r 8, Canada D a t e , j U t A l 1*6? ABSTRACT 3+ Two Independent Fe s p e c i e s have been s i m u l t a n e o u s l y i n t r o d u c e d i n t o L i n d e L z e o l i t e . One i s an exchanged Fe"^ + s p e c i e s w h i l e t h e o t h e r i s t h e m o l e c u l a r s p e c i e s F e C I ^ , By simpIe, o u t g a s s i n g a t 573°K, t h e 3+ 2+ exchanged Fe i s reduced t o Fe . By sweeping t h e system a t 523°K w i t h r ^ , b o t h s p e c i e s a r e c o n v e r t e d t o f e r r o u s and, a t 573°K, t h e exchanged - s p e c i e s i s reduced t o Fe^ and t h e F e C I ^ i s reduced t o F e C ^ . O u t g a s s i n g t h e l a t t e r s y s t e m a t 573°K o x i d i z e s t h e Fe^ t o Fe^O^ w h i l e t h e F e C ^ re m a i n s i n t a c t . These v a l e n c y changes were examined u s i n g MCssbauer S p e c t r o s c o p y and, u s i n g t h i s t e c h n i q u e , a M o r i n t r a n s i t i o n a p p e a r s t o have been d e t e c t e d i n t h e Fe^ s p e c i e s . - i i ACKNOWLEDGEMENTS I would s i n c e r e l y l i k e t o t h a n k Dr. J.R. Sams f o r h i s i n v a l u a b l e a s s i s t a n c e , a d v i c e , p a t i e n c e and encouragement d u r i n g t h e t i m e I was c o n d u c t i n g t h i s i n v e s t i g a t i o n , a l s o t o t h a n k Mr. J.C. S c o t t f o r t h e use of h i s computer program and h e l p f u l d i s c u s s i o n . M l TABLE OF CONTENTS Page ABSTRACT 1 ACKNOWLEDGEMENTS j l L I S T OF FIGURES i v L f S T OF TABLES v INTRODUCTION I MOSSBAUER SPECTROSCOPY 8 EXPERIMENTAL 18 RESULTS AND DISCUSSION 33 BIBLIOGRAPHY 57 iv L I S T OF FIGURES F i g u r e P a g e 1. E n e r g y l e v e l d i a g r a m s f o r Isomer S h i f t , 12 Q u a d r u p o l e S p l i t t i n g and Zeeman S p l i t t i n g , 2. . M O s s b a u e r C e l I . 23 3, M O s s b a u e r S p e c t r o m e t e r ( S c h e m a t i c ) , 25 57 4, D e c a y Scheme f o r Co , 28 5, T r a n s d u c e r - A n a l y s e r C y c l e , 3 0 6. S p e c t r a o f o r i g i n a l and o u t g a s s e d s a m p l e s , 42 7. S p e c t r a o f r e d u c e d s a m p l e and s u b s e q u e n t 49 o x i d a t i o n , 8, S p e c t r a o f s a m p l e s r e d u c e d u n d e r v a r y i n g 51 c o n d i t i o n s . V L I S T OF TABLES T a b l e P a ge 1, D - s p a c i n g s o f z e o l i t e s u n d e r v a r i o u s 35 t r e a t m e n t s , 2, M O s s b a u e r P a r a m e t e r s o f F e C I ^ - E t h e r 37 e x c h a n g e d z e o l i t e . 3, M O s s b a u e r p a r a m e t e r s f o r o u t g a s s e d 39 p e l l e t i z e d s a m p l e s 4, M O s s b a u e r p a r a m e t e r s f o r p e l l e t i z e d 45 s a m p l e s t r e a t e d w i t h and o u t g a s s e d 5, M O s s b a u e r p a r a m e t e r s f o r p e l l e t i z e d 47 s a m p l e s r e d u c e d w i t h H^. 6, Q u a d r u p o l e i n t e r a c t i o n s i n Z e e m a n - s p l i t 55 s p e c t r a . I. INTRODUCTI ON Z e o l i t e s a r e a c l a s s o f a l u m i n o s i I i c a t e s which e x h i b i t a number o f i n t e r e s t i n g f e a t u r e s . The m o l e c u l a r s i e v e a c t i o n o f z e o l i t e s has been known f o r some t i m e . T h i s p r o c e s s i n v o l v e s t h e b l o c k i n g o f c e r t a i n g u e s t m o l e c u l e s from e n t e r i n g t h e c h a n n e l s o f t h e z e o l i t e s w h i l e o t h e r m o l e c u l e s o f t h e r i g h t shape and s i z e a r e f r e e l y and c o p i o u s l y t r a n s m i t t e d t h r o u g h t h e porous c r y s t a l . T h i s " s i e v i n g " a b i ' l i t y o f z e o l i t e s i s i n h i b i t e d by t h e c a t i o n s which a r e l o c a t e d on t h e w a l l s o f t h e z e o l i t e c h a n n e l s and t h u s t h e s i z e o f m o l e c u l e s imbibed by t h e z e o l i t e s i s a f f e c t e d by s i z e and v a l e n c y o f t h e c a t i o n s . By i o n - e x c h a n g i n g t h e c a t i o n s f o r o t h e r i o n s , t h e m o l e c u l a r s i e v i n g a b i l i t y o f z e o l i t e s can be a l t e r e d . A p r o c e s s has been d e v i s e d ' f o r d e c a t i o n i z i n g t h e z e o l i t e s w h i c h e n l a r g e s t h e z e o l i t e c a v i t i e s and makes them much more a c c e s s i b l e t o g u e s t m o l e c u l e s . A p a r t from t h e i r m o l e c u l a r s i e v i n g , z e o l i t e s have been more r e c e n t l y s t u d i e d as v e r y u s e f u l and v a l u a b l e c a t a l y s t s . I t has been shown t h a t a c i d i c z e o l i t e s a r e v e r y a c t i v e c a t a l y s t s i n 2 3 4 t h e a l k y l a t i o n o f a r o m a t i c s ' ' . The z e o l i t e s employed a r e u s u a l l y t h o s e o f t h e f a u j a s i t e f a m i l y w hich p o s s e s s v e r y open s t r u c t u r e , g r e a t r i g i d i t y and a c r y s t a I l o g r a p h i c a I l y w e l l - d e f i n e d s u r f a c e . Unusual o p p o r t u n i t i e s f o r c a t a l y s i s a r e found t o e x i s t when t h e z e o l i t e s a r e base-exchanged t o remove t h e a l k a l i metal c o n t e n t . The Beckman re a r r a n g e m e n t 5 has a l s o been c a t a l y z e d by a c i d i c z e o l i t e s , S m a l l - p o r e z e o l i t e s have been used i n m o l e c u l a r shape s e l e c t i v e c r a c k i n g , a l c o h o l d e h y d r a t i o n and h y d r o h a l o g e n r e a c t i o n s ^ . The r e a c t i o n o f e t h a n o l and HCI t o form e t h y l c h l o r i d e has been c a t a l y z e d by z e o l i t e s . M o s t o f t h e s e r e a c t i o n s u s e a r a r e e a r t h e x c h a n g e d z e o l i t e a s t h e c a t a l y s t ^ . O t h e r c a t a l y t i c u s e s o f z e o l i t e s a r e i n o r t h o - a n d p a r a - ht, c o n v e r s i o n , t h e e x c h a n g e r e a c t i o n H 2 + D 2 t 2 H D , 8 9 - o x i d a t i o n o f r ^ S t o g i v e s u l f u r , a n d d i s p r o p o r t i o n a t i o n o f NO t o f^O a n d - - 2 ^ 3 ' ^ * V e r y o p e n z e o l i t e s s u c h a s t h o s e o f t h e f a u j a s i t e f a m i l y may - a l s o s e r v e a s c a t a l y s t c a r r i e r s f o r e f f e c t i n g c r o s s - I i n k i n g i n t h e • v u l c a n i z a t i o n o f r u b b e r , o r i n c u r i n g o f s i l i c o n e , n e o p r e n e o r s t y r e n e -b u t a d i e n e e I a s t a m e r s ' ' ' ' ^ . T h e s t r u c t u r e o f z e o l i t e s c a n be b r i e f l y d e s c r i b e d a s c r y s t a l l i n e m e t a l a I u m i n o s i I i c a t e s c o n s i s t i n g o f a r r a y s o f A10^ a n d S i 0^ t e t r a h e d r a c o n n e c t e d t h r o u g h common o x y g e n atoms'"^. P r i o r t o d e h y d r a t i o n , t h e s p a c e s b e t w e e n t h e t e t r a h e d r a a r e o c c u p i e d by w a t e r m o l e c u l e s . D e h y d r a t i o n r e s u l t s i n l a r g e c a v i t i e s o f m o l e c u l a r d i m e n s i o n s w h i c h a r e o b v i o u s s i t e s f o r a d s o r p t i o n o f g a s e s a s l o n g a s t h e c r y s t a l s t r u c t u r e r e m a i n s i n t a c t . T h e v a l e n c y o f . t h e a l u m i n u m i n t h e s t r u c t u r e i s b a l a n c e d by t h e i n c l u s i o n o f t h e c a t i o n . In s y n t h e t i c z e o l i t e s , t h i s c a t i o n i s u s u a l l y s o d i u m o r p o t a s s i u m - t h e s e c a t i o n s c a n be e x c h a n g e d f o r o t h e r c a t i o n s o f t h e same o r d i f f e r e n t v a l e n c y p r o v i d i n g t h e s t r u c t u r e r e m a i n s e l e c t r i c a l l y n e u t r a l . L i n d e z e o l i t e L, t h e t y p e o f z e o l i t e u s e d i n t h i s i n v e s t i g a t i o n i s a new f a u j a s i t e s y n t h e t i c z e o l i t e w h i c h c o n s i s t s o f a t e t r a h e d r a l 14 a r r a n g e m e n t o f t r u n c a t e d o c t a h e d r a . The c a t i o n s i n t h i s z e o l i t e o c c u p y t h r e e s i t e s . S i t e I i s i n t h e c e n t r e o f t h e h e x a g o n a l p r i s m s j o i n i n g t h e c e n t r a l o c t a h e d r a t o t h e t r u n c a t e d o n e . S i t e II i s o n t h e hexagonal faces of the t runca ted octahedra and S i t e III i s on the w a l l s of the channels between the cube-octahedra , The large c a v i t i e s which are f i l l e d w i th water p r i o r t o dehydrat ion are e l l i p t i c a l l y shaped, approx imate ly I2A° in l eng th , entered by aper tures of d i s t o r t e d c h a i r shaped, 12 membered r i n g s which have a diameter of approx imate ly 8A° ' . V a r i a t i o n s in the Si/AI r a t i o may occur from z e o l i t e t o z e o l i t e as the AI and Si atoms can rep lace each o ther isomorphousIy on the l a t t i c e 3 + 4 + s i t e s . However, i f Al rep laces Si then a d d i t i o n a l c a t i o n s are 15 in t roduced s u f f i c i e n t t o balance the framework change : Na + + A l 3 + Z S i 4 + . Such double s u b s t i t u t i o n s cannot be a f f e c t e d d i r e c t l y s i n ce i t i s the r e s u l t of d i f f e r i n g chemical environments dur ing the process of z e o l i t e f o r m a t i o n . If the process cou ld go e n t i r e l y t o the r i g h t , no A l ^ + and no exchangeable c a t i o n would remain and a n e u t r a l , porous, hydrated c r y s t a l l i n e s i l i c a would r e s u l t . No such product has yet been prepared. However, i t has been shown tha t the most s i l i c a r i c h z e o l i t e s are those where a c i d s t a b i l i t y i s g r e a t e s t ' 6 . L inde z e o l i t e L, which has a Si/AI 17 r a t i o of approx imate ly 4-5/1 i s the most a c i d s t a b l e of the s y n t h e t i c z e o l i t e s ( Z e o l i t e Y has an Si/AI r a t i o of approx imate ly 1-3/1) and thus i t was chosen f o r t h i s work. The chemical compos i t ion of z e o l i t e L has been r e p o r t e d ' t o be l . 0 + 0 . I M o . 0 :A l o0,:6.4±0 .5S!0 o:yrL0 2/n 2 3 2 ' / where M des igna tes a t l e a s t one exchangeable c a t i o n , n represents the va lence of t h a t . c a t ion and y may have a va lue ranging between 0 and 7 depending upon the degree of dehyd ra t i on . The c r y s t a l s t r u c t u r e of z e o l i t e L u n f o r t u n a t e l y has not been pub l i shed but w i l l be a v a i l a b l e 18 soon , which w i l l make s t r u c t u r a l p r e d i c t i o n s much more r i g o r o u s . There are seve ra l methods f o r s tudy ing the su r f aces and s t r u c t u r a l groupings w i t h i n the z e o l i t e . In f ra red s t u d i e s have been made of the r e s i dua l OH groups which can 19 be seen a f t e r the enc losed water has been d r i v e n out by dehydrat ion The r e s u l t s show three types of r e s i d u a l OH groups, two of which are a s s o c i a t e d w i th the a I um inos i I i c a t e s t r u c t u r e and the t h i r d coupled t o the z e o l i t i c c a t i o n . P r i o r t o dehyd ra t i on , the i n f r a r e d spectrum of 20 the absorbed water in s y n t h e t i c z e o l i t e s has been i n ves t i ga t ed . The spec t r a show c o n s i d e r a b l e d i f f e r e n c e s in the s t a t e of the hydroxy I groups as the exchangeable c a t i o n i s v a r i e d and from one z e o l i t e t o another . Adso rp t i on of gases such as N^, 0 2 , CH^, A r , and Kr on. the inner su r f a ces of d e c a t i o n i z e d z e o l i t e s w i l l r e s u l t in i n f r a r e d s h i f t s in the OH s t r e t c h i n g f requenc ies which can be a t t r i b u t e d t o s l i g h t changes in the s t r u c t u r a l f ea tu res of the z e o l i t e s and s h i f t s in e l e c t r o n d e n s i t i e s at the OH s i t e . A l s o , from adso rp t i on s t u d i e s , one can o b t a i n an es t imate of the i n t e r n a l and ex te rna l su r face areas of the z e o l i t e s and thus c a v i t y volumes can be determined. However, the accuracy of these measurements may be l i m i t e d by pore condensa t ion . Low Energy E l e c t r o n D i f f r a c t i o n (LEED) i s a l s o a technique which 21 might be used to study e x t e r i o r su r f aces . One problem wi th t h i s techn ique i s t ha t i t i s sometimes d i f f i c u l t to assess whether the r e s u l t a n t d i f f r a c t i o n pa t t e rn i s due to adsorbed gases on the sur face s t r u c t u r e o r the ac tua l su r face of the absorbent . As an example, i t has been shown t h a t in the H„-Ni system, some rearrangement of the 5 su r f a ce Ni atom takes p lace on adso rp t ion of \\^ . A s i m i l a r study on a z e o l i t e su r f a ce would probably invo l ve cons ide r ab l e exper imenta l d i f f i c u l t i e s due t o the problem of e l i m i n a t i n g the absorbed mate r i a l p r i o r t o examinat ion and t o the very porous s t r u c t u r e of the z e o l i t e . 22 E l e c t r o n Microscopy i s another t oo l which could be used to study z e o l i t e e x t e r i o r s u r f a c e s . In p r i n c i p l e , us ing t h i s t e chn ique , one cou ld examine p a r t i c l e s of mo lecu la r dimensions which would, in the case of su r f a ce adso rp t i on in z e o l i t e s , lead t o an i n s i g h t i n to the packing arrangement of an adsorbed monolayer. However, the exper imenta l d i f f i c u l t i e s in look ing at a monolayer w i th an e l e c t r o n microscope might l i m i t i t s use fu lness as p ressures of the order of 10 '^ mm Hg are r e q u i r e d . E l e c t r o n microscopy probably would, however, y i e l d va luab l e in fo rmat ion about aggregates on the z e o l i t e s u r f a c e . Both LEED and e l e c t r o n microscopy , a l though very use fu l f o r s tudy ing the e x t e r i o r su r f aces of z e o l i t e s , p rov ide no in fo rmat ion about the i n t e r n a l c a v i t i e s and the changes which occur t h e r e i n wi th a d s o r p t i o n . X-ray and e l e c t r o n d i f f r a c t i o n s t u d i e s w i l l e l u c i d a t e the s t r u c t u r e of the z e o l i t e - pore s i z e s , cage d imens ions , and the l o c a t i o n of the d i f f e r e n t atoms - but a g a i n , no knowledge regard ing the changes incur red w i t h i n the z e o l i t e w i th a d s o r p t i o n , both e x t e r i o r and i n t e r i o r , can be gained us ing d i f f r a c t i o n d a t a . It appeared t ha t Mossbauer spectroscopy might prov ide a va luab l e t oo l f o r s tudy ing the nature of the c a t i o n s i t e s and changes which occur when va r i ous gases are adsorbed. By s u i t a b l y exchanging the z e o l i t e so t ha t t h i s techn ique can be employed, one can determine smal l changes in f i e l d g r a d i e n t s w i t h i n the cages as molecules are adsorbed. A l s o , one can look at s h i f t s in e l e c t r o n dens i t y around the Mossbauer n u c l i d e , which i f embedded in the a I um inos i I i c a f e framework, can y i e l d va luab l e i n fo rmat ion about smal l changes in bonding fo r ces and s t r u c t u r a l f ea tu res which cou ld a l t e r the c a t a l y t i c and molecu la r s i e v i n g a b i l i t y of the z e o l i t e . The purpose of t h i s work was t o dev ise a method f o r p u t t i n g a Mossbauer n u c l i d e i n t o z e o l i t e s , and to c h a r a c t e r i s e the system o b t a i n e d . In o rde r t o make subsequent adso rp t i on s t u d i e s (not a pa r t of t h i s t h e s i s ) mean ing fu l , one would need to have in fo rmat ion regard ing the o x i d a t i o n s t a t e and chemical environment of the Mossbauer n u c l i d e , and i f p o s s i b l e i t s l o c a t i o n in the z e o l i t e . Moreover, i t i s important to determine whether the Mossbauer element r e s i d e s on the e x t e r i o r su r faces or w i t h i n the z e o l i t e , and the e f f e c t s of such fundamental procedures as ou tgass ing and dehyd ra t i on . The f i r s t s tep was t o develop a technique f o r exchanging the o r i g i n a l c a t i o n s in the z e o l i t e f o r a s u i t a b l e Mossbauer c a t i o n . The 57 n u c l i d e chosen f o r t h i s work was Fe , p r i m a r i l y because i t i s the most e a s i l y s tud i ed of a l l Mossbauer elements and shows a r e a d i l y measurable room temperature resonance. A f t e r adopt ing a method f o r i n s e r t i n g i ron i n to the a n i o n i c framework of the z e o l i t e , i t was necessary t o determine the type of i r on spec i es p resen t : whether i t i s a mo l e cu l a r l y neutra l spec i e s merely e n t e r i n g the large c a v i t i e s arid not r e p l a c i n g any c a t i o n s ; a mo lecu la r ion which has to rep lace an ion in the z e o l i t e in order f o r the system t o remain e l e c t r i c a l l y n e u t r a l ; or as severa l d i f f e r e n t spec ies Fur thermore, the o x i d a t i o n s t a t e of the i r on spec ies present must be determined - t h i s i s r e a d i l y o b t a i n a b l e from the Mossbauer parameters . 7 F i n a l l y , t h e e f f e c t s o f o u t g a s s i n g , d e h y d r a t i o n , r e h y d r a t i o n , and t h e p o s s i b i l i t y o f o x i d i z i n g a n d / o r r e d u c i n g t h e i r o n s p e c i e s had t o be i n v e s t i g a t e d . ' B e f o r e d i s c u s s i n g t h e e x p e r i m e n t a l d e t a i l s and r e s u l t s o b t a i n e d , • i t - m i g h t be h e l p f u l t o r e v i e w t h e M O s s b a u e r e f f e c t and t h e t y p e s o f . I n f o r m a t i o n w h i c h c a n be o b t a i n e d f r o m a M o s s b a u e r s p e c t r u m . I I . MOSSBAUER SPECTROSCOPY M O s s b a u e r S p e c t r o s c o p y o r n u c l e a r gamma r a y a b s o r p t i o n , c o n s i s t s of n u c l e a r gamma r a y e m i s s i o n and r e s o n a n t a b s o r p t i o n . In o r d e r f o r t h i s phenomenon t o o c c u r , t h e gamma r a y e m i s s i o n must be a c c o m p a n i e d by no r e c o i I o f t h e e m i t t i n g o r t h e a b s o r b i n g a t o m s . If o n e c o n s i d e r s an i s o l a t e d , e x c i t e d n u c l e u s w i t h e n e r g y E , p r i o r t o gamma r a y e m i s s i o n , t h e n when t h e gamma r a y i s e m i t t e d , t h e n u c l e u s w i l l r e c o i l due t o t h e c o n s e r v a t i o n o f momentum and t h e e n e r g y o f t h e gamma r a y w i l l be l e s s t h a n t h a t n e e d e d t o e x c i t e a n o t h e r s i m i l a r n u c l e u s o f e n e r g y E by t h e e n e r g y o f r e c o i I , E r E 2 E r = 2Mc w h e r e E ^ i s t h e y - r a y . e n e r g y , M t h e mass o f t h e n u c l e u s (atom) and c t h e v e l o c i t y o f l i g h t . A l s o , when a n u c l e u s o f e n e r g y E i s bombarded by a y - r a y o f e n e r g y E , t h e n some o f t h e y - r a y e n e r g y , n a mely E r , w i l l go i n t o r e c o i l i n g t h a t n u c l e u s and t h e r e f o r e t h e e n e r g y a c t u a l l y a v a i l a b l e f o r e x c i t a t i o n i s n o t s u f f i c i e n t and t h e n u c l e u s w i l l s t a y i n i t s g r o u n d s t a t e . T h e l i n e w i d t h o f t h e e m i s s i o n ( a n d a b s o r p t i o n ) l i n e i s r e l a t e d t o t h e h a l f l i f e o f t h e n u c l e u s i n i t s e x c i t e d s t a t e by t h e r e l a t i o n s h i p 0.693tf r = where r i s t h e l i n e w i d t h , H P l a n c k s c o n s t a n t and t h e h a l f - l i f e o f t h e e x c i t e d s t a t e . T h e l i n e w i d t h f o r a t y p i c a l M o s s b a u e r n u c l i d e i s o f t h e _9 o r d e r o f 10 eV w h i c h i s v e r y much s m a l l e r t h a n t h e r e c o i l e n e r g y o f t h e - 3 - 2 n u c l e u s , E r , t y p i c a l l y 10 -10 eV, T h e r e f o r e , t h e r e w i l l be no o v e r l a p o f t h e e m i s s i o n and a b s o r p t i o n l i n e s whose s e p a r a t i o n i s t h e e n e r g y 2 E r 9 and thus no MOssbauer e f f e c t w i l l be observed f o r t h i s model . However, i f the e m i t t i n g and absorb ing nuc l e i are each par t of a l a t t i c e and the r e c o i l energy i s less than the c h a r a c t e r i s t i c energy of the l a t t i c e v i b r a t i o n , the s i t u a t i o n becomes q u i t e d i f f e r e n t . S ince the l a t t i c e i s a quant ized system, the r e c o i l energy i s not s u f f i c i e n t to . exc i te the l a t t i c e phonon f requenc ies and thus there w i l l be no nuc lea r -reco i-l but-mere ly-a-sl-ight-"heating of the - l a t t i c e as a-whole. Over lap of the emiss ion and abso rp t i on l i n e s now occurs about the c e n t r o i d of --energy E q and the MOssbauer e f f e c t i s observed. If the source and the absorber of the y - r a d i a t i o n are in the same chemical env i ronment , the energy of the emiss ion and abso rp t ion w i l l be the same, E . However, i f the immediate environments of the source and ' o ' absorber are d i f f e r e n t , as in d i f f e r e n t chemical compounds, the nuc lear energy l e v e l s w i l l not be e x a c t l y the same. The re fo re , the y r a d i a t i o n from the source w i l l not e x c i t e the absorber s ince the nuc lear t r a n s i t i o n s from which the gamma rays o r i g i n a t e w i l l have s l i g h t l y d i f f e r e n t e n e r g i e s . However, s i n ce the y-ray l i new id th i s about 10 of v e l o c i t y of l i g h t , the energy can be modulated by moving the source r e l a t i v e to the absorber at a few mm/sec. Th i s small range in energy which i s imparted to the source w i l l be wide enough to e f f e c t an abso rp t i on in the absorber . The re fo r e , by measuring the amount of e x t r a energy, p o s i t i v e or nega t i v e , r equ i r ed t o o b t a i n a nuc lea r t r a n s i t i o n in the absorber , one can determine a c c u r a t e l y the change in nuc lear energy l e v e l s in the absorber r e l a t i v e t o the source . The minimum observab le l i new id th tha t can be obta ined in a MOssbauer exper iment i s tw ice the l i new id th of the garnma ray e m i s s i o n . 10 Th is f o l lows geometr i ca My from emiss ion and abso rp t ion l i n e o v e r l a p . However, t h i s l i n ew id th can be broadened cons ide rab l y by sample t h i c k n e s s , source p r epa r a t i on and instrumenta l e f f e c t s . The th ree parameters of p r i n c i p a l i n t e r e s t ob t a i nab l e from MGssbauer Spect ra are the Isomer S h i f t , 6, Quadrupole S p l i t t i n g , A, and Nuc lear Zeeman or Magnet ic Hyper f ine S p l i t t i n g . The energy leve l diagrams f o r 57 Isomer S h i f t and Quadrupole S p l i t t i n g f o r Fe are shown in F i g . l a wh i l e the Nuc lear Zeeman S p l i t t i n g i s i l l u s t r a t e d in F i g . l b . The Isomer S h i f t a r i s e s when the source of the y r a d i a t i o n and absorber are chem i ca l l y d i f f e r e n t . Th i s i s due t o the f a c t t ha t the i n t e r a c t i o n between the nuc lear charge and the s-e l e c t ron dens i t y at the nucleus i s d i f f e r e n t fo r d i f f e r e n t compounds. The s - e l e c t r o n dens i t y has a f i n i t e p r o b a b i l i t y of being in the v i c i n i t y of the nucleus whereas the p- o r d-e l ec t ron dens i t y f u n c t i o n s have e s s e n t i a l l y no p r o b a b i l i t y of being a t the nucleus and t h e r e f o r e the Isomer S h i f t w i l l not change w i th changing p- o r d-e lec t ron dens i t y (except by s h i e l d i n g e f f e c t s ) . The Isomer S h i f t depends upon the f a c t t ha t the e x c i t e d and ground r a d i i are s l i g h t l y d i f f e r e n t and t h e r e f o r e t h e i r e l e c t r o s t a t i c energ ies w i l l d i f f e r . The change in y-ray energy of an e m i t t i n g nucleus may be expressed as : 3 E Y = 2rr Z e2|^ ( o )|2 ( 2 _ R2 1 5 ex gd 2 where Z i s the change on the nuc l eus , e . the charge of an e l e c t r o n , |^(o)| the p r o b a b i l i t y of f i n d i n g an e l e c t r o n at the nucleus ( R = 0 ) and R and R ^ the e x c i t e d and ground s t a t e r a d i i r e s p e c t i v e l y . The energy c f the source and absorber a r e : FIGURE I E n e r g y l e v e l d i a g r a m s h o w i n g : a. Isomer S h i f t and C u a d r u p o l e S p l i t t i n g . b . N u c l e a r Zeeman S p l i t t i n g and t h e e f f e c t t h e r e o n o f Q u a d r u p o l e C o u p l i n g , I S O M E R S H I F T M A G N E T I C M A G N E T I C H Y P E R F I N E H Y P E R F I N E S P L I T T I N G 4 - Q U A D R U -S P L I T T I N G P O L E S P L I T T I N G 13 E = E + fjL Z e 2 |y ( o ) | 2 [ R 2 - R 2 ] s o 5 1 y s 1 ex gd - 1 E = E + |1 Z e 2 U ( o ) | 2 CR2 - R 2 J a o 5 | r a 1 ex gd The Isomer S h i f t (6) i s de f ined as the energy d i f f e r e n c e between the source and absorbe r : Z e 2 C k a ( o ) l 2 - l * s <o) | 2 D CR^ X -o r more c o n c i s e l y a s : « . « = B ^ n « a ( o ) i 2 - ks(oi2] where A R i s the change in the nuc lear r ad ius f o l l o w i n g y-ray e m i s s i o n , R the average nuc lea r charge r ad ius and B a constant determined by the gamma r a d i a t i o n of the p a r t i c u l a r t r a n s i t i o n . The r ad ius of the f i r s t 57 23 e x c i t e d s t a t e of Fe i s less than t h a t of the ground s t a t e and thus a p o s i t i v e Isomer S h i f t i n d i c a t e s a r educ t i on of e l e c t r o n i c charge dens i t y a t the nucleus of the absorbe r . Quadrupole S p l i t t i n g i s a phenomenon dependent upon the i n t e r a c t i o n between the nuc lea r quadrupole moment and the g rad i en t of the e l e c t r i c f i e l d ac ross the nuc l eus . The e l e c t r i c f i e l d g r ad i en t (EFG) i s se t up by charges in the immediate v i c i n i t y of the nuc l eus . Nuc le i w i th sp in 1= 0 , V2 have no quadrupole moment as i s the case in the ground s t a t e of 5*7 57 Fe . However, the f i r s t e x c i t e d s t a t e of Fe has I = V2 and thus has 24 a quadrupole moment. The hami l ton ian f o r the p e r t i n e n t i n t e r a c t i o n i s & = v f / o r ^ 1 x LV (31 2 - K I + I)) + (V - V ) ( I 2 - I ' 2 ) ] 0 4X(2I — I) zz z xx yy x y where eQ i s the quadrupole moment of the nuc leus , I i s the nuc lear s p i n , I. a re the p r o j e c t i o n s of I on the r e s p e c t i v e axes , V . . are the d iagonal 1 4 components of the e l e c t r i c f i e l d g r a d i e n t t enso r in the r e s p e c t i v e d i r e c t i o n s . The EFG can be expressed in terms of two independent parameters s i n ce the Lap l ac i an c o n d i t i o n of van i sh ing charge dens i t y i s s a t i s f i e d a t the nuc l eus : V + V + V = 0 xx yy zz .these two independent components can be conven ien t l y expressed : eq = ..V V - V n = xx yy (Asymmetry parameter) V zz The e igenva lues of the hami l ton ian can now be w r i t t e n a s : V 4 T T ^ T ^ - KI + "3 t! +4>'/2 ,where m^  i s the magnetic quantum number = I, I - I, . . . , - I 57 For Fe , the Quadrupole S p l i t t i n g i s the d i f f e r e n c e in energy l e v e l s between the m j = * ^ a n ^ "^ i e r n j = ± s t a t e s , the degeneracy of I = ^  be i ng p a r t i a I Iy I i f t e d by the EFG. Thus the QuadrupoIe S p I i t t i ng , AE Q i s A E q = E (|) - E 4> = i e 2 q Q [ . + . H | j 1 / 2 The t r a n s i t ion probab i I i t i e s ± ± and i -> * — are equa I, but the angu la r dependence of the two l i n e s i s not always the same due to c r y s t a l l i n e o r i e n t a t i o n e f f e c t s . However, in powdered samples, a l l angles should average out on i n t e g r a t i o n lead ing t o two l i n e s of equal i n t e n s i t y . 15 The c r y s t a l symmetry of the molecule in ques t ion can lead one t o p r e d i c t i o n s about the asymmetry parameter, n, and thus to the Quadrupole S p l i t t i n g , and v i c e - v e r s a . If a molecule has cub i c symmetry, the Quadrupole S p l i t t i n g w i l l be zero s i n ce there w i l l be no EFG across the nuc l eus . For an a x i a l l y d i s t o r t e d cube, such as the t e t ragona l d i s t o r t i o n of an oc tahedron , V = V and thus n = 0 so the Quadrupole ' xx yy " S p l i t t i n g reduces t o : n t Q 2 Other types of d i s t o r t i o n s and symmetries have been t r ea t ed mathemat i ca l l y .and the r e s u l t s compared w i th the expe r imen ta l l y determined Quadrupole 25 5 S p l i t t i n g s . As an example, f o r f e r r i c high sp in compounds, 3d , the h a l f - f i l l e d d s h e l l i s s p h e r i c a l l y symmetric and thus the EFG i s se t up by the arrangement and e l e c t r o n e g a t i v i t i e s of the va r ious I igands and thus i f a l l the I igands are i d e n t i c a l and in a cub i c a r r a y , there w i l l be no Quadrupole S p l i t t i n g . However, i f the I igands are d i f f e r e n t , o r do 3+ not have cub i c symmetry, and EFG w i l l be se t up across the Fe nucleus and a smal l s p l i t t i n g w i l l r e s u l t . However, i f the c en t r a l metal ion is not surrounded by a s p h e r i c a l s h e l l of va lence e l e c t r o n s , a d i f f e r e n t s i t u a t i o n o c c u r s . For high sp in f e r r o u s , 3d^ , f o r example the s i x t h d e l e c t r o n c o n t r i b u t e s much more t o the EFG than do the surrounding I igands and the large Quadrupole S p l i t t i n g tha t r e s u l t s i s due p r i m a r i l y t o t ha t e x t r a e l e c t r o n . The Magnetic Hyper f ine I n t e r a c t i on or Zeeman S p l i t t i n g i s the r e s u l t of the i n t e r a c t i o n between the magnetic d i p o l e moment, u, and the magnetic f i e l d , H, The magnetic f i e l d r e s u l t s from the magnetic p r o p e r t i e s 16 of the ma te r i a l under i n v e s t i g a t i o n . As an example, i ron metal forms c l u s t e r s which se t up a magnetic f i e l d , H, which leads t o the observed s p l i t t i n g . The hami l ton ian f o r the i n t e r a c t i o n i s : = -u«H m ' and the energy l e v e l s obta ined a r e : E = -uHrrv/I = -qu Hm r m l a n I where g i s gyromagnet ic r a t i o , u n the nuc lea r magneton and the o the r •.symbols have been de f ined p r e v i o u s l y . In the absence of any quadrupole i n t e r a c t i o n , the re are 21 + I e q u a l l y spaced l e v e l s , the s p l i t t i n g between ad jacent l e v e l s being gvnH> and the s p l i t t i n g between the lowest and h ighes t l e v e l s 2gu HI . Th is g i ves r i s e t o s i x l i n e s in the MOssbauer spectrum as shown in F i g . l b . The f a c t tha t there are on ly s i x l i n e s i s d i c t a t e d by the s e l e c t i o n r u l e Am .^ = 0, * I. When there i s quadrupole i n t e r a c t i o n a s h i f t in the energ ies o f- the d i f f e r e n t m^ . s t a t e s w i l l be observed . If the magnetic f i e l d i s p a r a l l e l t o the e l e c t r i c I 2 f i e l d g r a d i e n t , then the quadrupole s p l i t t i n g i s e Qq p r o v i d i n g the EFG i s a x i a l ly s y r n m e t r i c ^ ' 2 6 . |f |_| ; s pe rpend i cu l a r t o an a x i a l ly 1 2 symmetric EFG, the Quadrupole S p l i t t i n g i s e qQ, which i s due to s l i g h t l y d i f f e r e n t s h i f t s in the s t a t e s . The Morin t r a n s i t i o n po in t i s the temperature a t which the magnetic f i e l d changes from p a r a l l e l t o pe rpend i cu l a r a l ignment w i th the EFG, These two cases are s imple examples of a wide range of p o s s i b l e combinat ions of magnetic f i e l d s and EFG's which lead t o a la rge assortment of d i f f e r e n t s o l u t i o n s . However, i t i s q u i t e s imple to determine whether or not a hyper f ine pa t t e rn has any quadrupole c o u p l i n g a s soc i a t ed w i th i t . If the s p l i t t i n g of the 17 f i r s t two peaks i s s l i g h t l y d i f f e r e n t from the l a s t two, then there i s a quadrupole i n t e r a c t i o n . A Morin t r a n s i t i o n temperature can e a s i l y be determined s i n c e , a t t ha t temperature , the magnitudes of the s p l i t t i n g s of peaks I, 2 and 5, 6 wiI I r e ve r se . It i s sometimes d i f f i c u l t t o e x p e r i m e n t a l l y determine whether one i s look ing at a quadrupole i n t e r a c t i o n o r exper imenta l e r r o r as the va lues of the quadrupole i n t e r a c t i o n may be very smaI I . Thus, us ing MOssbauer Spect roscopy , the o x i d a t i o n s t a t e , e l e c t r i c a l environment and magnetic p r o p e r t i e s of an i ron spec ies w i t h i n the a I u m i n o s i I i c a t e framework of the z e o l i t e can be a c cu r a t e l y s t u d i e d . M l . EXPERIMENTAL Ion Exchanges - The s o l u t i o n t o the problem of exchanging the potass ium in z e o l i t e L f o r i r on proved to be r a the r d i f f i c u l t . An aqueous s o l u t i o n of f e r r i c c h l o r i d e was f i r s t used whose pH was about 2. The a c i d s t a b i l i t y of z e o l i t e L i s not a c c u r a t e l y known but i t i s thought t o decompose a t pH-2 as i t i s known tha t Y z e o l i t e decomposes at pH's of --the o rder of 3. A s tock s o l u t i o n of 400g/1 of FeC lyCh^O in water was made up and the exchanges were accomplished by s t i r r i n g t h i s s o l u t i o n w i th the z e o l i t e u n t i l exchange e q u i l i b r i u m had been e s t a b l i s h e d , u s u a l l y 3-4 hours . In o rder t o put the maximum amount of i ron i n to the z e o l i t e , m u l t i p l e exchanges were done where each exchanged product was t r e a t e d aga in w i th a f r e sh batch of the s tock s o l u t i o n . Th i s process was repeated severa l t imes and the f i n a l product was f i l t e r e d , thoroughly washed w i th water to a negat ive KCNS t e s t f o r Fe^ + , and d r i ed at I05°C. X-ray powder photographs i nd i c a t ed t ha t the product had l o s t i t s z e o l i t e s t r u c t u r e and thus the amorphous s i l i c a res idue was probably mixed wi th some f e r r i c s p e c i e s . However, MOssbauer spec t ra were determined f o r these p roduc t s . Next , an anhydrous FeCI^ exchange was attempted us ing d i s t i l l e d methanol as the s o l v e n t . The L z e o l i t e was added t o t h i s s o l u t i o n and the r e s u l t i n g s l u r r e y was a g i t a t e d us ing a t e f l o n coated magnetic s t i r r i n g ba r . The product obta ined a f t e r t h i s mate r i a l had been mixed f o r a s u f f i c i e n t l y long t ime f o r exchange e q u i l i b r i u m to be ach ieved , was 3+ washed thorough ly w i th d i s t i l l e d methanol us ing the KCNS t e s t f o r Fe t o a s c e r t a i n when washing should be t e rm ina t ed . The X-ray powder photographs 19 •i ' • . • , • . on the product i nd i c a t ed t ha t the s t r u c t u r a l p r o p e r t i e s of the z e o l i t e had been r e t a i ned a f t e r t rea tment . However, the MGssbauer spec t ra i nd i c a t ed the i ron was a complexed su r face spec ies and not enc losed w i t h i n the z e o l i t e . The c o n d i t i o n s adopted f i n a l l y f o r i n t r oduc ing Fe i n to the z e o l i t e were mix ing the L z e o l i t e w i th a s o l u t i o n of FeCI^»6H20 in e thy l e t h e r . The mechanics of the exchange were the same as desc r ibed p r e v i o u s l y except t h a t any f o r e i g n ma te r i a l was f i l t e r e d o f f from the 33% weight t o volume FeC Iy6H20-ether mix ture before the z e o l i t e was i n t r o d u c e d . The exchange was a l lowed to proceed f o r one hour s i nce i t was found t h a t exchanges which proceeded f o r a longer pe r iod of t ime d id not a p p r e c i a b l y inc rease the amount of i ron in the z e o l i t e . A l s o , Washing the p r e c i p i t a t e w i th la rge volumes of e ther d id not seem t o remove any i r o n , t h e r e f o r e the i ron was probably i n s ide the z e o l i t e and not on the s u r f a c e . X-ray powder photographs on the products showed tha t no loss of c r y s t a I I i n i t y had o c c u r r e d . X-ray Powder Photographs - The samples f o r powder photos were mounted in 0.5 mm g l a s s c a p i l l a r i e s , A sample of length 5 mm in the c a p i l l a r y was s u f f i c i e n t , so powder photographs requ i red on ly very small amounts of m a t e r i a l . The X-ray powder photographs were obta ined us ing a General E l e c t r i c powder camera of 14.32 cm d iameter , w i th Straumanis l o a d i n g . Cu Ka, X = 1.54050 A ° , X - i r r a d i a t i o n was used, w i th a Ni f i l t e r t o reduce KB r a d i a t i o n . The f i l m used was I I fo rd "I I fex sa f e t y base" cut i n to s t r i p s 4 cm x 43.2 cm f o r use in the powder camera. For f i l m s t o be used f o r i d e n t i f i c a t i o n purposes o n l y , when a shor t exposure t ime was d e s i r e d , a s l i t c o l l i m a t o r was used on the camera, but t o o b t a i n 20 photographs f o r measurements of l i n e p o s i t i o n s , a p i nho l e c o l l i m a t o r was used, g i v i n g photographs w i th sharp , low angle l i n e s . The l i n e s on the photographs were indexed on a l i g h t box prov ided w i th a meter s t i c k , t o which was at tached a measuring s l i d e assembly. . . conta in ing a v e r n i e r and a magni f ied c r o s s - h a i r f o r l o c a t i o n of the d i f f r a c t i o n l i n e . ( " F i lm I l l u m i n a t o r and measuring d e v i c e " , type no. 52022/1, P h i l i p s E l e c t r o n i c s I n c . ) . The d-spacings were obta ined "from these measurements by a s imple c a l c u l a t i o n , MOssbauer CeI Is - For powder samples, the z e o l i t e was evenly packed t o a t h i c k n e s s l ess than I mm in a brass c e l l 1.25 cm in diameter having mylar windows. For room temperature runs , the c e l l was clamped in the c o r r e c t l y a l i g n e d p o s i t i o n d i r e c t l y in the y-ray path us ing an aluminum Ho lder . For l i q u i d n i t rogen runs , i t was i nse r t ed in the top of a copper c o l d - f i n g e r which was immersed in a dewar of l i q u i d n i t r o g e n . The n i t r ogen leve l was mainta ined by a Super io r A i r Products Company l i q u i d n i t r ogen leve l c o n t r o l l e r . Styrofoam i n s u l a t i o n was p laced around the, absorber t o min imize heat t r a n s f e r t o the sample wh i l e a l l o w i n g t r a n s m i s s i o n of gamma r a d i a t i o n . .With t h i s arrangement, the sample temperature cou ld be mainta ined at 80 ± | °K , The MOssbauer spec t r a of p e l l e t i z e d samples were run in a r a the r novel type of c e l l a l l o w i n g o u t g a s s i n g , sweeping wi th d i f f e r e n t gases and a d s o r p t i o n t o take p lace p r i o r t o Mossbauer work. For o u t g a s s i n g , the p e l l e t was heated t o temperatures up to 350°C, wh i l e ma in ta in ing a - 4 - 5 vacuum of 10 - 10 mm Hg. Th i s pressure was accomplished us ing a combinat ion of r o t a r y and mercury d i f f u s i o n pumps w i th the pressures being es t imated on a MacLeod gauge. The temperature was c o n t r o l l e d at the 21 r equ i r ed p o i n t w i th a Co le Parmer Proport ioNuI I 1300 s e r i e s c o n t r o l Ier connected t o a furnace cons t ruc ted in t h i s department. Th i s arrangement was a ccu ra te t o 4 I0 °C. The temperature was measured us ing an i r on c o n s t a n t i n e S im-Ply-Tro l pyrometer . For " sweep ing " , o r pass ing gases •'such as 0^ o r F^ over the z e o l i t e , the gas was f i r s t d r i e d by bubb l ing through two v e s s e l s c o n t a i n i n g 36N ^SO^ and subsequent ly in t roduced . i n t o the MCssbauer c e l l , the temperature being mainta ined at any ^required p o i n t us ing the se t up desc r ibed above. Al though adso rp t i on -stud ies were not done in t h i s work, the c e l l and vacuum system cou ld --easily be adapted t o accompl ish such measurements as the equipment inc luded a mercury manometer f o r measuring h igher gas p r e s s u r e s . A schemat ic diagram of the c e l l used i s i l l u s t r a t e d in F i g . 2 . When out-gass ing o r " sweep ing " , the p e l l e t ho lder and p e l l e t were jn the p o s i t i o n .shown, the p e l l e t held t o i t s ho lder w i th Pt gauze, and the sidearms of the c e l l being used e i t h e r f o r ou tgass ing o r f o r p e r m i t t i n g a f low of gas over the p e l l e t . When Mossbauer spec t r a were run , the c e l l was inver ted so t h a t the p e l l e t f i t t e d i n to a s l o t between the 0 .010" Be windows, a l l o w i n g t r a n s m i s s i o n of y r a d i a t i o n through the absorber so t ha t the spec t r a cou ld be run w i thout removing the p e l l e t from i t s c e l l . The p e l l e t s were made us ing a Carver Laboratory Press Model B and t y p i c a l p ressures were of the o rder of 15,000 p s i . They were then shaped to f i t the ho lde r us ing a razor b l ade , MBssbauer Spectrometer - The MOssbauer spec t r a were obta ined us ing a spec t romete r , a b lock diagram of which i s shown in F i g , 3 . The elements of the spectrometer c o n s i s t of a v e l o c i t y t r ansducer (TMC Model 305) which i s e s s e n t i a l l y a loud-speaker c o i l t ha t p rov ides Doppler Modu la t ion t o the 22 FIGURE 2 Diagram of the MOssbauer Cel I used f o r p e l l e t i z e d samples . .23 PYREX y STOPCOCK STOPCOCK Pt" GAUGE ZEOLITE P E L L E T 24 FIGURE 3 B lock diagram showing e s sen t i components of the MOssbauer Spectrometer . 25 M O S S B A U E R S P E C T R O M E T E R S C O P E S Y N C . S I G N A L 4 0 0 W O R D M E M O R Y A N A L Y S E R W A V E F O R M G E N E R A T O R T R A N S D U C E R i i—hT T Y P E W R I T E R ( S O U R C E 0 0 I P U L S E I I I I I I 7 ke V (4-4 ke V D E T E C T O R A B S O R B E R 26 ? ' ' - • . . . • ' • -57 y-ray energy emi t ted from the r a d i o a c t i v e Co source . The decay 57 scheme f o r Co i s g i ven in F i g . 4 . The sou r ce , which i s embedded in a Pd m a t r i x i s mounted on the t r ansduce r which i s d r i v en at constant a c c e l e r a t i o n by a TMC Model 306 wave form genera tor (WFG). Once per c y c l e the WFG supp I i es an 8-vo l t s y n c h r o n i z a t i o n s i gna l f o r phase l o ck i ng the RIDL Model 24-2 400 word mul t i channe l ana l yze r (MCA). Th i s c o n s i s t s •of t r i g g e r i n g the MCA a t a s p e c i f i c po in t t o s t a r t r e c e i v i n g counts from •the s i n g l e channel ana l yze r (Nuclear Chicago Model 33-15) , which ^ d i s c r i m i n a t e s pu l ses from the p r o p o r t i o n a l counter (Reuter-Stokes Model -RSG-30A) a l l o w i n g on l y those f a l l i n g w i t h i n a c e r t a i n energy range t o be 57 passed on to the MCA. In t h i s case , the 14.4 KeV y r a y of Fe was s e l e c t e d . Thus in each of i t s 400 channe l s , the MCA s t o r e s the number of counts cor respond ing t o a p a r t i c u l a r v e l o c i t y increment from the motion of the t r a n s d u c e r . Th i s in fo rmat ion can be tapped from the memory us ing a t y p e w r i t e r (IBM Model 44-16) and the progress of the spectrum can be viewed v i a an o s c i l l o s c o p e (Hewlett-Packard Model I20B). The ra te a t which the MCA sweeps through each of the 400 channels i s determined by a Nuc lear Chicago Model 54-6 t ime base generator (TBG). F i g . 5 i l l u s t r a t e s the c y c l e g i ven the t ransducer by the WFG. The t ime of the complete c y c l e can be a l t e r e d , and by a s u i t a b l e cho i ce of c a p a c i t o r s in the WFG, one can set the per iod t o be s l i g h t l y longer then the t ime t h a t the MCA takes t o sweep through i t s 400 channels wh ich , i f the TBG i s se t f o r 200 usec/channeI , i s 82.5 msec. If the TBG i s se t to count a t 100 usec/channeI , then the MCA w i l l on ly s to r e counts f o r a l i t t l e l ess than ha l f the wave form g iven by the WFG. It w i l l then wa i t f o r the next t r i g g e r i n g before i t begins t o s t o r e a g a i n . However, i f the TBG i s 27 FIGURE 4 Diagram showing the decay scheme 57 f o r Co and the a ssoc i a t ed parameters . 28 7/2 C O 5 7 2 7 0 d E.C. 9 9 . 8 4 % I 5 / 2 137 keV 9 % 5/2 1/2 123 k e V 91 % ! 4 . 4 k e V M O S S B A U E R y - R A Y S T A B L E Fe 5 7 8 H A L F - L I F E (t I/2) O F F I R S T E X C I T E D S T A T E = 9 . 7 7 x IO S E C . I SOTOPIC A B U N D A N C E ( I A ) = L I N E W I D T H ( D = 2 . 19 % 12, 4 . 6 6 9 7 x IO ke V M IN IMUM O B S E R V A B L E L INEW IDTH (Wo ) = O. I9427 M M / S E C E r = 1.9567 x I O - 3 e V 29 FIGURE 5 D i a g r a m s h o w i n g how t h e MCA sweeps t h r o u g h I t s c h a n n e l s a s d i c t a t e d by t h e t r i g g e r i n g f r o m t h e WFG. 30 VELOCITY O SYNC. SIGNAL MEMORY CHANNEL NUMBER COUNTS T I M E O + v 4-V O -v >-CHANNEL N6.= VELOCITY 31 se t f o r 200 usec/channeI , then near l y the whole waveform i s used f o r s to rage of data and two spec t r a per waveform are o b t a i n e d , each a m i r r o r image of the o t h e r . Each spectrum w i l l thus c o n s i s t of 200 ra the r than 400 channels^and thus the r e s o l u t i o n w i l l be cut in h a l f but the count ing r a t e i s increased c o n s i d e r a b l y . The instrument was operated in the l a t t e r f a sh ion dur ing the course of t h i s i n v e s t i g a t i o n . • Nuc lear y-ray count ing i s a s t a t i s t i c a l process and thus a l l peaks observed should have normal d i s t r i b u t i o n s and l o r enz i an l i n e shapes. Thus c o n s i d e r a b l e t ime has t o be a l lowed to b u i l d up s u f f i c i e n t s t a t i s t i c s in o rde r t o o b t a i n accura te va lues f o r the peak p o s i t i o n s . In t h i s work, the spec t r a r equ i r ed 6-12 .hrs, t o become we l l r e s o l v e d . Using a l e as t-squares computer program w r i t t e n by J , C , S c o t t , the data were f i t t e d on an IBM 7044 or 360/67 computer t o l o r enz i an cu r ves , and the peak p o s i t i o n s , isomer s h i f t s and quadrupole s p I i f t i n g s g iven in terms of channel numbers. In o rde r t o get energy e q u i v a l e n t s f o r the channel numbers, the v e l o c i t y s c a l e used has t o be c a l i b r a t e d at f requent i n t e r v a l s between runs . The s c a l e was c a l i b r a t e d aga ins t the quadrupole s p l i t t i n g of an N.B.S. standard sodium n i t r o p r u s s i d e s i n g l e c r y s t a l absorber which has the va lue of 1.726 mm/sec. T h i s g i ves a c a l i b r a t i o n f a c t o r in terms of mm/sec ch which when m u l t i p l i e d by channel numbers g i ves the energy which can , i f necessary , 57 be converted t o eV us ing a convers ion f a c t o r , which f o r Fe , i s : -7 I mm/sec = 4.3 x 10 eV E r r o r s - The e r r o r s incur red in a Mossbauer experiment are p r i m a r i l y s t a t i s t i c a l in na tu re . Thus i t i s d i f f i c u l t t o quote abso lu te e r r o r s as such and i t i s found much mere convenient t o se t up conf idence l i m i t s . These are es t imated on the bas i s of the standard d e v i a t i o n s of the 32 computer f i t and the r e p r o d u c i b i l i t y of the spectrum. The conf idence l i m i t in these exper iments i s ± 0,02 mm/sec f o r the isomer s h i f t and quadrupole s p l i t t i n g and a s l i g h t l y l a rge r v a r i a t i o n in the l i n e w i d t h s . IV. RESULTS AND DISCUSSION The breakdown in s t r u c t u r e of the aqueous f e r r i c exchange was undoubtedly due t o the low pH of the exchange s o l u t i o n . Th i s l a t t i c e . breakdown has a l s o been observed f o r attempted f e r rous and prolonged 27 f e r r i c exchanges by o the r workers . S o l u t i o n s of low pH wiI I remove A l 3 + i n t t h e f o l l o w i n g way'^: r i 0 H \ 1 - / + \ / 3+ _ S i 0 — Al 0 S i — + 4 h L 0 -^Si OH HO S i - +AI +4H„0 / I \ 3 / \ 2 0 H [ 9 so t ha t the a I um inos i I i c a t e framework i s e s s e n t i a l l y des t royed . The MGssbauer spec t r a on the r e s u l t i n g product show a magnetic hype r f i ne pa t t e rn a t l i q u i d n i t rogen temperature and both the method of p repa ra t i on and the MQssbauer spec t r a obta ined are i n d i c a t i v e of 3 - FeOOH which is 28 probably mixed w i th the s i l i c a r es idue . The method of i d e n t i f y i n g t h i s g - FeOOH from i t s MOssbauer hype r f i ne p a t t e r s i s by making a comparison w i th tha t of a known s p e c i e s , s i n ce both the o rde r i ng temperature and magnitude of the Zeeman s p l i t t i n g are c h a r a c t e r i s t i c of a g iven compound. For the exchanges which were attempted in methanol , a l though the X-ray powder photographs i nd i c a t ed no s t r u c t u r a l breakdown, the Mbssbauer r e s u l t s were not encourag ing . A hype r f i ne pa t t e rn of s i x l i n e s or more was observed a t both 80°K and 298°K. Th i s was i n d i c a t i v e c f large c l u s t e r s 34 (approx imate ly IO 3 atoms or more) of some spec i es of i ron which was magne t i c a l l y o rde r ed . From an examinat ion of the hyper f ine p a t t e r n , the spec i e s appeared to be Fe20^. P a r t i c l e s i z e s of the dimensions requ i red t o o b t a i n a hype r f i ne pa t t e rn are too large t o be enveloped by the i n t e r n a l c a v i t i e s of the z e o l i t e s i n c e the maximum c a v i t y s i z e i s approx-14 imate ly I3A° and thus the on ly p o s s i b i l i t y i s tha t the mate r i a l was depos i ted on the e x t e r i o r s u r f a c e s . A t e s t was made of t h i s hypothes is by evapora t ing a FeCI^-methanoI s o l u t i o n t o dryness and running the MOssbauer spectrum on the r e s u l t i n g s o l i d r e s i d u e . The spectrum showed t h a t , indeed, t h i s was the case and the sur face mate r i a l was some form of Fe^Oj or a mix ture of the d i f f e r e n t v a r i e t i e s . The e ther exchanges were much more i n t e r e s t i n g as the r e s u l t s i nd i c a t ed t ha t the i ron had gone in to the z e o l i t e wi thout r up tu r i ng the l a t t i c e . The r e s u l t s of X-ray powder photography are g iven in Table I a long w i th the va lues obta ined f o r the z e o l i t e p r i o r t o any t rea tment , those reported f o r z e o l i t e L.J and the va lues f o r a subsequent ly t r e a t ed p e l l e t which w i l l be d i scussed l a t e r . Most of the d-spacings l i e w i t h i n the range of the reported va lues and any d i s c r epa nc i e s or omiss ions can be accounted f o r by the f a c t t ha t the photographs were taken us ing a p in-ho le c o l l i m a t o r and the l i n e s were thus ra the r weak and sometimes i n d i s t i n c t . The Mb'ssbauer r e s u l t s showed double ts a t both 80°K and 295°K w i th Isomer S h i f t s c h a r a c t e r i s t i c of f e r r i c s p e c i e s . (Typ ica l Isomer S h i f t s f o r the three i r on o x i d a t i o n s t a t e s which were r e l evan t t o t h i s work are F e 3 + - 0.6 mm/sec,• F e 2 + - 1.2 mrn/sec, and Fe^ - 0.26 mm/sec, r e l a t i v e t o sodium n i t r o p r u s s i d e ) , The observed l i n e s were non-Iorenzian at both temperatures and very broad at 80°K; much broader than would be 35 L i t e r a t u r e Neat Z e o l i t e VaIues 16.1 ± .3 15.77 7.52 ± .04 7,47 6.00 ± .02 6,01 4.57 .03 4.58 4.35 ± .04 4.38 3.91 ± .02 3.90 3.65 3.47 ± .02 3.45 3.28 ± .02 3.27 3.17 ± .01 -. 3.16 3.07 ± .01 3.06 2.91 ± .01 2.90 2.65 ± .01 2.64 2.46 ± .01 2.48 2.42 ± .01 2.43 2.19 ± ,01 2.19 See Reference i . TABLE I Fe-L-Zeo l i t e Treated L-Zeo l i t e Pel l e t 15.77 15.77 7.42 6,04 5.98 4.58 4.56 4.35 3.90 3.88 3.63 3,63 3 .47 ' 3.46 3.26 3.27 3.17 3.17 3.06 3.05 2.90 2.89 2.65 2.64 2.49 2.40 '2,19 2,18 36 expected merely from v i b r a t i o n s caused by l i q u i d n i t rogen b o i l - o f f . The e x p l a n a t i o n f o r t h i s i s t ha t there are probably two f e r r i c spec ies in the z e o l i t e w i th nea r l y i d e n t i c a l Isomer S h i f t s and Quadrupole S p l i t t i n g s , which makes r e s o l u t i o n of the four peaks imposs ib l e . However, the f a c t t h a t the peaks are much broader at 80°K than a t 295°K suggests t ha t spec t r a run a t even lower temperatures might enable r e s o l u t i o n of the peaks. The MOssbauer parameters f o r th ree independent p repa ra t i ons of the e the r exchanges us ing i d e n t i c a l c o n d i t i o n s are l i s t e d in Table 2. The exper imenta l e r r o r here i s about * 0.04 mm/sec. The l a rge r e r r o r in these spec t r a i s due t o the poor q u a l i t y of the computer f i t caused by non- lo renz ian l i n e shapes. The va lues f o r 6 and A l i e w i t h i n the range of exper imenta l e r r o r but the % e f f e c t , e, i s g r ea t e r a t 295°K than 80°K. Th i s i s r a t h e r d i f f i c u l t t o e x p l a i n as one would expect the reverse on the bas i s of r e c o i l - f r e e f r a c t i o n arguments. Some lowering of c i s expected due t o styrofoam i n s u l a t i o n , d i s t ance of source from absorber and " i c i n g up" which cou ld lead to b a c k - s c a t t e r , but the d im inu t i on here i s g r e a t e r than u s u a l l y observed . It should be noted the v a r i a t i o n in e between d i f f e r e n t samples i s expected s i nce e changes wi th sample t h i c k n e s s as we l l and thus comparison of e from one sample to another is not very s i g n i f i c a n t . In o rder t o determine whether the spec t r a obta ined were due t o FeC I .j* 6H20-ether complexes on the sur face of the z e o l i t e , a p o r t i o n of the 33$ weight t o volume s o l u t i o n was evaporated to dryness and the MOssbauer spectrum was run on the r e s i d u e . The parameters obta ined are a l s o g iven in Table 2. They suggest t ha t the spec ies being observed by MBssbauer 37 TABLE 2 T r i a l No. Temp ( °K ) 6 a 1. 295 0.64 0.76 0.52 8,18 80 0.74 0.92 0.69 7.02 Washed w i th 295. 0.65 0,74 0,47 14.18 20 - 100 ml p o r t i o n s of 80 0,74 0,91 0,69 4.93 e the r 2 . 295 0.65 0,76 0.48 1 1.26 80 0.73 0.90 0.76 9.17 3. 295 0.64 0,77 0.50 10.15 80 0.72 0.85 0.67 10.13 Evap. E ther 295 0.66 0.93 0.46 ' 9.90 r e s idue 80 0.76 0.90 . 0.51 5.80 a AI I vaIues i n mm/sec and r e l a t i v e t o sodium n i t r o p r u s s i d e 38 spect roscopy i s not an i ron c h l o r i d e e ther res idue but by no means r u l e out the p o s s i b i l i t y . A l s o repeated washing t reatments of 20 r- 100 ml p o r t i o n s of e ther d id not seem t o remove any FeCI^ o r change the MCssbauer parameters as shown in Table 2 which a l s o suggests t ha t the spec i es i s not on the s u r f a c e . The computer f i t t o the l i n e s , however, i s much b e t t e r a t both 80°K and 295°K which suggests on ly one r a the r than two f e r r i c s p e c i e s . Fu r the r ev idence which lends i t s e l f t o the proposal of two f e r r i c spec i es i s the a n a l y t i c a l d a t a , performed b y . D r s . F. & E. Pascher , Bonn, Germany, which g i v e an i ron t o c h l o r i n e r a t i o of 2 : 3 . Th i s suggests tha t some of the i ron has entered the z e o l i t e as an i o n , F e 3 + , and some as the molecu la r spec ies FeCI^. No carbon was found which i n d i c a t e s FeCI^ o r FeClyXH^O ra ther than an FeCI^-ether complex in 27 the z e o l i t e . A comparison w i th r e s u l t s obta ined by Mor ice and Rees ( to be d i s cussed more f u l l y l a t e r ) shows t ha t some of the i ron has entered 3+ as Fe , exchanging the c a t i o n . It would t h e r e f o r e appear , on the bas i s of t h i s ev idence t ha t approx imate ly 50% of the Iron in the z e o l i t e i s present as F e 3 + and 50$ as FeCI-^; the p o s i t i o n of the l a t t e r i s thought t o be i n s i d e the pores of the z e o l i t e but a t t h i s s t age , t h i s i s not very impor tant . As t h i s system wi th two f e r r i c spec ies appeared ra the r i n t e r e s t i n g , a l l f u r t h e r work was c a r r i e d out on i t . The sample was f i r s t p e l l e t i z e d by the method p r e v i o u s l y desc r ibed and subsequent ly run in i t s c e l l on the MQssbauer spec t rometer . Typ i ca l r e s u l t s from a large number of experiments are g iven in Table 3. By i n s p e c t i o n of t h i s t a b l e and comparison w i th Table 2, i t i s c l e a r tha t va lues of the isomer s h i f t and' the quadrupole s p l i t t i n g f o r the p e l l e t have not changed app rec i ab l y a t 295°K from those of the powder (assuming 39 TABLE 3 Treatment None 0 / g b @ 423°K 0/g § 498°K 0/g @ I&2 573°K I&3 Exposed t o atmosphere Temp ( °K ) 6 a A a r a e {%) 295 0.63 0,78 0,65 6.0 80 0.66 1.13 0,83 3.6 295 0.59 0,86 0.60 5.5 80 0,69 1 ,01 0,65 8.1 295 0.59 1.07 0.65 5.8 80 0.68 1.17 0.81 6.3 295 0.61 0.99 _ 4.8 295 1.27 2.31 - 1.4 295 0.63 0,84 0.55 7.1 In mm/sec and r e l a t i v e t o sodium n i t r o p r u s s i d e b ' - • Outgassed 40 an exper imenta l e r r o r of ± 0,04 mm/sec), a l though the l i new id ths are s l i g h t l y l a rge r f o r the p e l l e t , However, a t 80°K, both 6 and A va lues are d i f f e r e n t from those f o r the powder. Thermal motions which g i ve r i s e t o the second order Doppler S h i f t cou ld e x p l a i n the change in 6. The inc rease in A and r f o r the p e l l e t cou ld be a t t r i b u t e d t o the f a c t t h a t under p r e s su r e , as in the p e l l e t i z e d samples, the water i s squeezed out thus lowering the loca l symmetry about the i ron which increases A. The r e s u l t s of va r ious ou tgass ing t reatments are a l s o g iven in Table 3. The isomer s h i f t i s e s s e n t i a l l y constant and independent of ou tgass ing temperature , but the quadrupole s p l i t t i n g inc reases as the ou tgass ing temperature i n c r eases . Th is aga in i s probably due to loss of water caus ing a decrease in loca l symmetry about the i r o n . The % e f f e c t , E , f o r outgassed samples i s less f o r spec t ra run at 298°K than those run at 80°K, but the reverse i s t rue f o r powdered'samp Ies, Thus the removal of water.accompanied by an increase in e must be due t o the f a c t t ha t samples w i th l i t t l e or no water cannot " i c e up" i n t e r n a l l y but the untreated samples can " i c e up" thus reduc ing y-ray t r a n s m i s s i o n and i n c r eas i ng b a c k - s c a t t e r i n g . The appearance of three l i n e s when the sample i s outgassed at 573°K as shown in F i g . 6b i s r a the r i n t e r e s t i n g . The asymmetry of peaks I and 2, read ing from l e f t t o r i g h t , c l e a r l y i n d i c a t e t ha t the o the r ha l f of the quadrupole doub le t a s soc i a t ed wi th peak 3 i s embedded in peak I, Thus, on the bas i s of t h i s , one can a t t r i b u t e peaks I and 2 t o a f e r r i c spec ies and I and 3 t o a f e r rous s p e c i e s , - t h i s i s subs t an t i a t ed by the Mossbauer parameters g i ven in Table 3. The appearance of a f e r rous peak when the 27 system i s outgassed at 573°K is r a the r hard t o e x p l a i n . Mor ice and Rees 57 who have observed s i m i l a r r educ t ions w i th exchanged Fe in Linde X and Y 4 1 FIGURE 6 MOssbauer spec t r a o f : a. O r i g i n a l Fe^ + s p e c i e s . b. Outgassed sample c o n t a i n i n g F e + Fe . 43 z e o l i t e s and C l i n o p t i l o l i t e , have pos tu l a t ed a mechanism; X - 0~ + F e 3 + ( H 2 0 ) -»• X - OH + F e 2 + - OH 2 F e 2 + - 0 H ^ * > e 2 + + H 20 + i ; 0 2 Al though t h i s mechanism seems ra the r i n v o l v e d , i t i s hard t o c r i t i c i z e w i thout a more in t ima te knowledge of the system. The r e - o x i d a t i o n t o f e r r i c r e q u i r e s both oxygen and water which is observed in the present work and in the work done by Mor ice and Rees. As we l l as e l i m i n a t i n g 2+ - • the Fe peak on exposure t o the atmosphere, i t i s seen t ha t the parameters , 6 and A, have gone back to t h e i r p rev ious va lues p r i o r to ou tgass ing - thus the r e - s o r p t i o n of water must c rea te an environment f o r the, i r on s i m i l a r to t ha t present p r i o r t o dehyd ra t i on . The cha rac te r-2+ i s t i c s of the Fe peak in z e o l i t e s has a cons ide r ab l e dependence on the type of z e o l i t e being t r e a t e d . Mor ice and Rees found severa l d i f f e r e n t c o n d i t i o n s were necessary f o r the r e - o x i d a t i o n o f F e 2 + -> F e 3 + f o r d i f f e r e n t z e o l i t e s . However, by compar ison, i t i s c l e a r tha t there i s some exchanged F e 3 + in the z e o l i t e . A l though the appearance of the f e r rous peak was r e v e r s i b l e in the sense t ha t i t was e l i m i n a t e d upon exposure t o the atmosphere, the f e r rous peak cou ld not be produced again by s imple o u t g a s s i n g . Th i s cou ld mean tha t the mechanism of Mor ice and Rees may not be t o t a l l y c o r r e c t . At t h i s po in t i t was f e l t t ha t f u r t h e r ou tgass ing treatment would not y i e l d any more r e v e a l i n g r e s u l t s so a d i f f e r e n t l i n e of a t t a ck was employed, A f r e sh p e l l e t was outgassed at 573 ° K , y i e l d i n g a 3- I ine MSssbauer spectrum as shown in F i g . 6 b , Exposure to the atmosphere 2+ r e s u l t e d once more in d e s t r u c t i o n of the Fe l i n e . The c e l l was then 44 evacuated , 300 t o r r of 0^ admi t t ed , and the sample was heated at 573°K f o r 8 hours . The c e l l was re-evacuated, thoroughly purged w i th 0 2 , heated f o r a f u r t h e r 20 hours a t 573°K in 600 t o r r of 0 2 , and f i n a l l y evacuated a g a i n . As can be seen from Table 4, t h i s t reatment y i e l d s the same f e r r i c MOssbauer parameters as does s imple o u t g a s s i n g , but the oxygen t reatment does not a f f o r d a f e r rous s p e c i e s . These r e s u l t s a l s o make i t appear u n l i k e l y tha t ^©2^3 i s produced in t h i s t rea tment , a l though t h i s cannot be ru l ed out w i th c e r t a i n t y . It should be noted, however, t h a t i f F&2®3 — P r e s e n + must be paramagnetic ' , the re being no ev idence f o r a magnetic hype r f i ne i n t e r a c t i o n . Fo l l ow ing t h i s t rea tment , the same p e l l e t was heated in at 573°K f o r 6 hours and subsequent ly outgassed at the same temperature . The r e s u l t i n g spectrum was most s u r p r i s i n g in t ha t a hype r f i ne pa t t e rn r e s u l t e d w i th a Zeeman s p l i t t i n g cor responding t o F^O^, superimposed on a quadrupo lar d o u b l e t . The parameters of the l a t t e r cou ld not be d e t e r -mined a c c u r a t e l y because of ove r l ap w i th the hype r f i ne spectrum, but i nd i c a t ed the presence of a f e r rous spec ies r a the r than f e r r i c (see Table 4 ) . Th i s seemed a very s t range phenomenon t o occur in a reducing atmosphere. A repeat of the rL, p lus ou tgass ing t reatment was done w i th a new p e l l e t and the same spectrum r e s u l t e d , t ha t due t o F^O-j p lus a c e n t r a l f e r r ous d o u b l e t . These r e s u l t s showed th ree important f e a t u r e s . F i r s t l y , a t l e a s t pa r t of the i r on present was being reduced from f e r r i c t o f e r r o u s , as was t o be expec ted . . Secondly , and q u i t e s u r p r i s i n g l y , there appeared t o be p roduc t ion of F^O^ ' n a hydrogen atmosphere. F i n a l l y , the f a c t t ha t the Fe„0^. was magne t i c a l l y ordered a t room 45 TABLE 4 Treatment None 0/g @ 573°K c Heated i n 0^ 6 573°K 1 Heated in H ':§ 573°K and 0/g @ 573°K F e 2 0 3 (bu lk ) 6° 0.65 0.61 0.61 ' A 0 . 6 0 e 0.62 A 0.76 0.93 0.95 1.9 0.49 0,57 0.62 1.3 Zeeman S p I i t t i ng a , b 15.83 16.70 In mm/sec, r e l a t i v e to sodium n i t r o p r u s s i d e . A l l r e s u l t s obta ined at 295°K b I mm/sec = 30.96 KOe c 3+ Parameters f o r Fe component on ly are quoted . d c 2+ 4 . Fe component e Hyper f ine component f "MSssbauer E f f e c t Data Index" (Ed. A .H . M u i r , K . J . Ando, H.M. Coogan), I n t e r s c i e n c e , New York , 1966. 46 temperature showed c o n c l u s i v e l y t ha t t h i s spec ies was aggregat ing on the e x t e r i o r su r f aces of the z e o l i t e , and was no longer conf ined t o the i n t e r i o r z e o l i t i c c a v i t i e s , A new p e l l e t was made and heated in a stream of hydrogen at 573°K f o r 6 hours , and not subsequent ly ou tgassed , The spectrum of t h i s ma te r i a l was a c e n t r a l f e r rous double t superimposed on a m e t a l l i c i r on 2+ hype r f i ne pa t t e rn as shown in F i g , 7 a . The parameters f o r the Fe doub le t and the Fe^ hype r f i ne s p l i t t i n g appear in Table 5, Outgass ing t h i s pel l e t a t 573°K produced the r ^ O ^ s P e c + r u m superimposed on an 2+ Fe d o u b l e t , whose parameters are in Table 4 and the spectrum i s shown 2+ in F i g . 7 b . The asymmetry of the Fe doub le t in these spec t ra are i n d i c a t i v e of p r e f e r e n t i a l o r i e n t a t i o n . It was now c l e a r t ha t i t was in f a c t the ou tgass ing of the sample which led t o ^ 2 ^ 3 p r o d u c t i o n , and t h a t the hydrogen t reatment r e s u l t e d in complete r educ t i on of one f e r r i c spec i es and p a r t i a l r educ t i on of the o t h e r . To i n v e s t i g a t e in more d e t a i l the r educ t i on process a new p e l l e t was made and t r e a t e d w i th hydrogen under less severe c o n d i t i o n s . The ' p e l l e t was i n i t i a l l y heated in hydrogen at 473°K f o r 7 hours . The spectrum ( F ig .8a ) was the same as f o r an untreated sample ( f e r r i c d o u b l e t ) , 2+ Next , i t was heated f o r 8 hours in at 523°K where on ly a Fe doublet r e s u l t e d as shown in F i g .8b and the assoc i a t ed parameters in Table 5. 0 2+ Heat ing in at 573°K f o r 8 hours gave the Fe + Fe spectrum as shown in F i g , 8 c w i th parameters comparable t o those in Table 5 . . Heat ing f o r a longer t ime in a t 573°K lessened the i n t e n s i t y of the 2+ Fe peak but prolonged treatment at t h i s temperature d id not a l t e r the 47 TABLE 5 Treatment Temp ( °K ) 6 a T a Zeeman S p l i t t i n g 3 ' Heated in H„ @ 523°K 295 1.02 2,08 0.74 Heated i n H .573°K 80 295 1,14 2.30 0.72 1.31, 2.13 0.78 0 . 2 6 d - 10.72 Fe metaI 80 1,44): 2.42 0.29 -295 0.26 0.76 10.93 10.66 In mm/sec, r e l a t i v e t o sodium n i t r o p r u s s i d e . b I mm/sec = 30.96 KOe C F e 2 + component ^ Hyper f ine component e "MOssbauer E f f e c t Data Index" (Ed. D.H. M u i r , K. J . Ando, H.M, Co'ogan) I n t e r s c i e n c e , New-York, 1966, 48 FIGURE 7 MOssbauer S p e c t r a o f : a . Fe + Fe . 2+ b . F ^ O ^ + Fe 49 —i 1 1 1 1 ••—i 1 1 1— - 1 2 - 6 O 6 12: D O P P L E R V E L O C I T Y ( M M S E C " ) 50 FIGURE 8 MOssbauer spec t r a o f : a . O r i g i n a l Fe^ + s p e c i e s . 2+ b. Sample reduced to Fe in h^. c . Sample reduced in f o r 6 hours t o F e 2 + + Fe^. d . Sample reduced in a f u r t h e r 7 hours . e. Sample reduced in H„ t o Fe^. 51 52 0 2+ s p e c t r u m a p p r e c i a b l y f r o m t h a t shown i n F i g t 8 d . H e a t i n g t h e F e / F e 2+ s y s t e m t o a p p r o x i m a t e l y 825°K f o r 4 h o u r s r e m o v e s a l l F e and t h e 6 h y p e r f i n e l i n e s o f F e ^ r e m a i n a s shown i n F i g , 8 e , If t h i s s y s t e m i s now o u t g a s s e d a t 573°K no F e 2 ° 3 ' s p r o d u c e d . T h e m e t a l l i c i r o n now a p p e a r s t o be q u i t e s t a b l e , b o t h t o o u t g a s s i n g a t 573°K, and t o e x p o s u r e t o a i r a t room t e m p e r a t u r e . T h e s e r e s u l t s may be p l a u s i b l y e x p l a i n e d a s f o l l o w s . S i n c e t h e r e i s v e r y g o o d e v i d e n c e f o r t h e e x i s t e n c e o f two i r o n s p e c i e s , t h e i n i t i a l r e d u c t i o n t o f e r r o u s must be a t w o - p a r t p r o c e s s . T h e e x c h a n g e d F e ^ + i s 2+ r e d u c e d t o F e , t h e c h a r g e n e u t r a l i t y o f t h e s y s t e m p r e s u m a b l y b e i n g made up by p r o t o n s . T h e c o n d i t i o n s a r e s u f f i c i e n t l y m i l d (523°K) s o t h a t f u r t h e r r e d u c t i o n i s a p p a r e n t l y n o t f a v o u r e d . A t t h e same t i m e t h e r e i s r e d u c t i o n o f FeCI-j t o F e C l 2 v i a t h e e q u i l i b r i u m 2 F e C I 3 * 2 F e C I 2 + C l 2 t h e s t r e a m o f h y d r o g e n s e r v i n g t o sweep o u t t h e c h l o r i n e and f o r c e t h e e q u i l i b r i u m t o t h e r i g h t . S i m i l a r r e d u c t i o n s h a v e b e e n o b s e r v e d f o r F e C l y g r a p h i t e s y s t e m s ' ^ 0 ' . When t h e t e m p e r a t u r e i s r a i s e d t o 573°K, t h e e x c h a n g e d s p e c i e s i s f u r t h e r r e d u c e d t o F e ^ . T h e i r o n a t o m s must t h e n d i f f u s e t o t h e s u r f a c e ( t h e y s h o u l d be q u i t e m o b i l e a t t h i s t e m p e r a t u r e ) , where t h e y f o r m a g g r e g a t e s o f s u f f i c i e n t s i z e t o e x h i b i t Zeeman s p l i t t i n g . A g a i n , t h e c h a r g e n e u t r a l i t y w i t h i n t h e z e o l i t e i s p r e s u m a b l y m a i n t a i n e d by p r o t o n s . U n d e r t h e s e c o n d i t i o n s t h e F e C I 2 seems t o be a f f e c t e d o n l y s l i g h t l y , and i t a p p e a r s u n l i k e l y t h a t f u l l r e d u c t i o n t o m e t a l l i c i r o n c a n be a c c o m p l i s h -e d a t t h i s t e m p e r a t u r e . H e a t i n g t h e s a m p l e a t much h i g h e r t e m p e r a t u r e s - 53 (825°K) in hydrogen f i n a l l y causes the FeCI^ t o be reduced to Fe^, which must a l s o d i f f u s e t o the su r face and aggregate , s i n ce at the end of t h i s t reatment a l l the i ron in the system i s f e r romagne t i c . The mechanism f o r the o x i d a t i o n of Fe^ t o ^e^3 by s imple ou tgass ing i s not comple te l y c l e a r . S ince there are no oxygen atoms a v a i l a b l e except those a s s o c i a t e d w i th the Si and AI atoms of the z e o l i t i c framework, i t seems reasonable to suppose t ha t these oxygen atoms are employed. The su r f a ce Fe^ probably e x i s t s as very f i n e p a r t i c l e s in a h i g h l y a c t i v a t e d s t a t e and thus a t e l eva ted temperatures i t cou ld conce i vab l y a b s t r a c t 0 atoms from the z e o l i t e su r face t o form r~e,£>^. The indexed X-ray powder photograph of the z e o l i t e w i th sur face F^O-j was not very v a l u a b l e as the l i n e s were in almost the same p o s i t i o n s as in the neat z e o l i t e . However, there were some small s h i f t s as shown in Table I p o s s i b l y i n d i c a t i n g s l i g h t s t r u c t u r a l a l t e r a t i o n s . The small amount of su r f a ce f ^ O ^ (approximate ly 3% of the t o t a l sample) p rec ludes the obse r va t i on of l i n e s due t o t h i s s p e c i e s . The p l u ck i ng of oxygen atoms from the z e o l i t e should not a l t e r the a I um inos i I i c a t e s t r u c t u r e a p p r e c i a b l y but i t w i l l leave a net p o s i t i v e charge on the donor atoms s i n ce the charge produced in the o x i d a t i o n of Fe^ to F e 3 + w i l l be d i s s i p a t e d by the negat ive charge of the oxygens. Th i s net p o s i t i v e charge cou ld be removed i f the p ro tons , present in the channels when the Fe^ i s depos i ted on the s u r f a c e , are reduced and pumped o f f as H 2 . Th i s would leave the a I um inos i I i c a t e framework e l e c t r i c a l l y n e u t r a l , Th i s e x p l a n a t i o n , however, i s on ly t e n t a t i v e and r equ i r e s f u r t h e r i n v e s t i g a t i o n . A f t e r a l l the i ron had been converted to Fe^ at 825°K, ou tgass ing d id not produce Fe . ,0 , . Th i s i s probably due t o the f a c t t h a t the i ron has 54 formed l a rge r p a r t i c l e s and become less a c t i v e by anneaI ing and, in i t s i n a c t i v e s t a t e , i s unable t o form F e 2 < - ) 3 by a b s t r a c t i n g su r face oxygens o r by any o the r p rocess . The z e o l i t i c su r f a ce Is by no means r egu l a r and thus i t seemed reasonable t o suppose t ha t there might be an EFG set up by the su r face t o g i v e a quadrupole i n t e r a c t i o n on the i r o n , present e i t h e r as Fe^ o r ^ e 2 ^ 3 * ' n P a r + I c u l a r , i f the re i s a quadrupole i n t e r a c t i o n , i t would be I n t e r e s t i n g t o determine whether a Morin t r a n s i t i o n e x i s t s between 80°K and 295°K. Bu lk Fe2<D3 shows a Mor in t r a n s i t i o n at 263 °K 3 2 , but as yet on l y a spectrum a t 295°K on the Fe^O^-zeol i te system has been o b t a i n e d . As shown in Tab le 6, the t~&2®3 d o e s show a quadrupole i n t e r a c t i o n . The Fe^ both in the presence and absence of F e 2 + appears t o show a quadrupole I n t e r a c t i o n a l s o , w i th a Mor in t r a n s i t i o n somewhere between 80°K and 295 °K. As the d i f f e r e n c e in the s p l i t t i n g s i s very s m a l l , and may depend a g rea t deal on the accuracy of the f i t , these r e s u l t s are by no means c o n c l u s i v e regard ing the presence o r absence of a Morin t r a n s i t i o n and quadrupole i n t e r a c t i o n in Fe^. However, there i s c l e a r l y a quadrupole i n t e r a c t i o n in Fe^O-^. The systems s tud i ed in t h i s i n v e s t i g a t i o n have been dep ic ted as f a i r l y c o m p l i c a t e d . Fu r the r i n v e s t i g a t i o n s are d e f i n i t e l y necessary before i t i s compl i ca ted more by adso rp t i on work. An e l e c t r o n microscopy study may y i e l d in fo rmat ion about the su r face spec ies and a Mor in t r a n s i t i o n temperature de te rmina t ion on the Fe^j-^ p e l l e t w i l l determine whether the observed quadrupole coup l i ng i s due to the EFG set up in the ^ 2 ^ 3 i t s e l f o r by the z e o l i t e su r f a ce s t r u c t u r e , Sweeping the i n i t i a l system wi th a more i n e r t gas such as l\L w i l l t e s t the 55 TABLE 6 Sample Temp ( °K ) A 1 0 - A a ' 12 56 F e 2 0 3 on z e o l i t e 295 0.151 Fe ° on z e o l i t e .295 -0,088 80 0,021 Fe°2<j>n z e o l i t e 295 -0.043 (Fe component present ) ' 80 0.048 In mm/sec S p l i t t i n g of the l a s t two peaks of from the f i r s t two. the hype r f i ne pa t t e rn subs t r ac ted 56 proposed mechanism f o r the p roduc t ion of Fe^ and the subsequent o x i d a t i o n t o Fe^O^.- More a n a l y t i c a l data w iI 1 be needed to desc r i be the system f u l l y and, f i n a l l y , low temperature s t u d i e s , down t o 4°K should g i ve v a l u a b l e i n fo rmat ion about the magnetic p r o p e r t i e s of the system. 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