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Colour centres in alkali metal azides 1958

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COLOUR CENTRES IN ALKALI METAL AZIDES hy JOHN PETER SCOTT PRINGLE B.A. Cantab., 1956 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Chemistry We accept t h i s t h e s i s <as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA DECEMBER 1958 Abstract Previous work by Heal had shown that X-Ivradiated sodium azide c r y s t a l s dissolved i n water produced small amotmts of nitrogen gas, hydroxyl i o n and ammonia, thereby i n d i c a t i n g that sone decomposition had occurred. Heal a l s o observed colours i n the material, s i m i l a r to those of the X - i r r a d i a t e d a l k a l i halides f o r which a whole s e r i e s of colour centres responsible have been postulated. I t was therefore decided to investigate the colour centres of the a l k a l i azides, p a r t l y to extend the colour centre research, and p a r t l y to i l l u m i n a t e the X-ray decomposition processes. C r y s t a l l i n e plates of NaN^, KNj, RbN^ and CsN-j were i r r a d i a t e d at l i q u i d nitrogen and room temperature, using a Machlett AEG-50 tungsten target X-ray tube, operated at 50 KVP. The absorption spectra of the i r r a d i a t e d samples were measured at l i q u i d nitrogen temperature with a Cary model 14 recording spectrophotometer. The low temperature spectra consisted of three bands. The A band, peaking at 612, 568, 578 and 592 mu f o r NaN_, KN-, HbN^ and CsN. r e s p e c t i v e l y , i s ascribed to F centres. The anomalous sodium azide band i s r e l a t e d to i t s t r i g o n a l c r y s t a l structure, d i f f e r i n g from the body centred tetragonal of the other azides. The B band, peaking at 361, 374 and 390 mu f o r KN^, RbN^ and CsN^ r e s p e c t i v e l y , was strong and t r i p l e , there being shoulders about 30 mu on each side of the main peak. For NaN^ i t was weak, sing l e and peaked near 330 mu. Tentatively, i t i s ascribed to the centre. The C band, peaking about 740, 790, 820 and 850 mu f o r NaKj, KN^, HbŴ and CsN^ i s weak and s i n g l e . I t may be due to FI centres. The room temperature spectra were s t r i k i n g l y d i f f e r e n t from each other, except f o r HbN^ and CsNj. For NaN-j f i v e bands were observed at 342, 560, 630 , 730 and 860 mu; the l a t t e r four were weak and may be an e l e c t r o n i c v i b r a t i o n a l spectrum. The strong 342 mu band i s ascribed to the presence of sodium metal i n some no n - c o l l o i d a l form; a c o r r e l a t i o n between the band and the i o n i s a t i o n p o t e n t i a l of the metal i s noted. In KN^ three bands at 760 (strong), 590 (strong shoulder) and 340 mu (weak) were obtained. The f i r s t two are ascribed to small F centre aggregates of the M,R type though no d e f i n i t e assignations are made. RBN3 and CsN^ spectra both consist of a broad peak showing f i n e structure,the highest peaks occurring at 330 mu and 375 mu r e s p e c t i v e l y . I t i s considered uncertain that a l l the absorption i s due to the impurity held responsible f o r the f i n e structure. I n 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 a n 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 a n d 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 by t h e Head o f my D e p artment o r by h i s r e p r e s e n t a t i v e . I t 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 not 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 . Department o f CAjUnwifovitf 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 S, C a n ada. TABLE OF COKTEHTS Page Introduction. 1 Experimental methods. 3 1. Preparation of samples. 3 2. Analysis of samples. 5 3. I r r a d i a t i o n c e l l . 7 4. X-ray tube c i r c u i t . 8 5. Production of X-rays. 9 6. F i l t r a t i o n and absorption of X-rays. 9 7. Spectrophotometric technique. 11 Discussion. 14 1. E f f e c t s of X-rays on s o l i d s . 14 2. Imperfections i n i o n i c s o l i d s . 15 3- Colour centres i n a l k a l i h a l i d e s . 17 ( i ) F centres 17 ( i i ) F 1 centres. 18 ( i i i ) P^, R_, M and N bands. 19 ( i v ) C o l l o i d a l bands. 19 (v). V bands. 19 (vi) Miscellaneous bands. 20 4. C r y s t a l structures. 21 Results. 21 1. Processing of spectra. 21 2. Spectra of u n i r r a d i a t e d azides. 22 ( i ) Spectra below absorption edge. 22 ( i i ) Spectra at absorption edge. 23 -2- Results (cont'd) Page 3. Spectra of azides i r r a d i a t e d at l i q u i d nitrogen temperatures. 26 ( i ) A band. 26 ( i i ) B band. 30 ( i i i ) C band. 33 4. Spectra of azides i r r a d i a t e d at room temperature. 33 ( i ) Sodium azide. 34 ( i i ) Potassium azide. 35 ( i i i ) Rubidium and Caesium azides. 36 References. 38 Tables Table I. Analysis of azides by Keal's Method. 6 Table I I . Calculations on Exciton absorption bands. 24 Table I I I . Comparison of A and F bands. 27 Table IV. Comparison of B and bands. 31 TABLE OF FIGURES Figure Ifo. Page 1. I r r a d i a t i o n c e l l . 7A 2. X-ray c i r c u i t . SA 3. F i l t r a t i o n and absorption of 50 PKV X-rays. 9A A. Mass absorption c o e f f i c i e n t s pf azides and f i l t e r s . 10A 5. Energy bands i n i o n i c s o l i d s . * 15A 6. Possible vacancies and vacancy aggregates i n a l k a l i h a l i d e s . 15A 7. Edge d i s l o c a t i o n i n an a l k a l i 'halide. 15A 8. Production of vacancies from an edge d i s l o c a t i o n . 15A 9. Colour centres i n a l k a l i h a l i d e s : S e i t z (16). 17A 10. C r y s t a l structures, of (a} UaN^ and (b) My 17A 11. Four spectra of un i r r a d i a t e d NaN^. 17A 12. Unirradiated azide spectra below absorption edge. 23A 13. Absorption edge spectra, (a) present work and (b)-reference (9) 23A 1A. NaKj i r r a d i a t e d at l i q u i d nitrogen temperature. 26A 15. KNj i r r a d i a t e d at l i q u i d nitrogen temperature. 26B fc 16. RbN3 i r r a d i a t e d at l i q u i d nitrogen temperature. 26C 17. CsK^ i r r a d i a t e d at l i q u i d nitrogen temperature. 26D 18. "C" band spectra of KN^, RbN-j and CsNy 33A 19. I r r a d i a t e d NaN^ spectra at room temperature. 34A 20. I r r a d i a t e d KN^ spectra at room temperature. 35A 21. I r r a d i a t e d HbN and CsML spectra at room temperature. 36A -1- Introduction The study of the chemical e f f e c t s produced i n matter by r a d i a t i o n s can be conveniently divided i n t o tvo f i e l d s , photochemistry and r a d i a t i o n chemistry. In photochemistry, the (electromagnetic) r a d i a t i o n energy i s of the same order as that of the electrons involved i n normal chemical bonding i . e . l e s s than 10 evj the absorption of such energy r e s u l t s i n a si n g l e e l e c t r o n i c e x c i t a t i o n or, more r a r e l y , a sin g l e i o n i s a t i o n , as the primary event. The much higher energies of r a d i a t i o n chemistry are d i s s i p a t e d by a s e r i e s of such primary processes, each incident p a r t i c l e or quantum being capable of producing multiple i o n i s a t i o n s or e x c i t a t i o n s . High energy r a d i a t i o n s can be c l a s s i f i e d ( l ) i n t o two types. The l i g h t p a r t i c l e group includes a l l those r a d i a t i o n s t r a n s f e r r i n g energy to the medium by means of f a s t electrons; beta and cathode rays where the electrons are produced e x t e r n a l l y , and gamma and X-rays i n which the electrons are pro- duced i n the medium i t s e l f v i a the p h o t o e l e c t r i c or Compton e f f e c t s , or p a i r production. Alpha p a r t i c l e s , neutrons, f i s s i o n products and accelerated ions form the heavy p a r t i c l e group; both groups produce broadly s i m i l a r chemical e f f e c t s . The present work describes an i n v e s t i g a t i o n of X-radiation induced e f f e c t s i n a l k a l i metal azides. Previous work by Heal (2,3) had shown that X - i r r a d i a t e d sodium azide evolved nitrogen gas on d i s s o l u t i o n i n water; the r e s u l t i n g s o l u t i o n contained appreciable amounts of hjrdroxyl i o n and ammonia. Solution i n l i q u i d ammonia at -78°C r e s u l t e d i n effervescence and a blue colour, i n d i s t i n g u i s h a b l e from that of a s i m i l a r s o l u t i o n of sodium metal; none of these r e s u l t s were reproduced with u n i r r a d i a t e d samples,thereby showing that some decomposition had occurred. S i m i l a r i o n i c azide decomposi- -2- t i o n s produced thermally (4,5,6) or by the a c t i o n of u l t r a v i o l e t l i g h t (7,8,9) have been studied, and mechanisms proposed; r e c e n t l y Groocock and Tompkins (10) have studied the decomposition induced by e l e c t r o n bombardment. Heal also observed colours produced by X - i r r a d i a t i o n of the normally colourless sodium azide; an unstable pale green turning to brown i n c r y s t a l s i r r a d i a t e d at or below room temperature, a stable brown colour on i r r a d i a t i o n between 51°C and 102°C, and a greyish or b l u i s h tinge, together with l o s s of transparency and waxy appearance, i n c r y s t a l s i r r a d i a t e d above 130°C. The brown colour was ascribed by Heal to the t a i l of an absorption band peaking at 3400A0, a band confirmed by the work of Rosenwasser, Dreyfus and Levy ( U ) , who placed i t at 3600A 0. The l a t t e r group, using sodium azide i r r a d i a t e d i n the Brookhaven reactor followed by various heat treatments, obtained f i v e other absorption bands i n the range 2700A 0 to 12000A 0. A s i m i l a r s e r i e s of absorption bands was found by Tompkins and Young (12) i n potassium azide i r r a d i a t e d with u l t r a v i o l e t l i g h t . The colours produced by X - i r r a d i a t i o n of the a l k a l i halides have long been known (13), and the absorption bands responsible have been extensively i n v e s t i g a - ted; reviews of this f i e l d have been given by Pohl (14), and S e i t z (15,16). A whole s e r i e s of "colour centres" have been postulated to account f o r the absorp- t i o n bands; these w i l l be considered i n a l a t e r s e ction of the present work. In view of the s i m i l a r i t i e s between the a l k a l i h a l i d e s and the a l k a l i azides, i t was considered that an i n v e s t i g a t i o n of the absorption bands produced by X - i r r a d i a t i o n of the l a t t e r would be valuable; f i r s t l y , as an extension the "colour centre" research, and secondly, because i t might i n d i c a t e the decomposition mechanism i n X - i r r a d i a t e d azides. Experimental Method 1. Preparation of Samples ( i ) Hydrazoic a c i d required f o r the preparation of the potassium s a l t and to suppress hyd r o l y s i s i n r e c r y s t a l l i s i n g s o l u t i o n s of the azides, was prepared by two methods. In the f i r s t , a column of the c a t i o n exchange r e s i n , Dowex 50W - X-8 i n the H + form was set up, 125 cms. long by 2.A cms. diameter; the t o t a l exchange capacity was about 1000 m i l l i - equivalents. A f t e r washing with 500 ml. d i s t i l l e d water, 10 gms. of NaN^ p r a c t i c a l grade, disso l v e d i n 200 ml. d i s t i l l e d water, was passed through at the slow flow rate of 2.4 ml/sq.cm/minute, the e l u t i n g agent being more d i s t i l l e d water. I t was observed that a deep red brown band with sharp lower and d i f f u s e upper boundary formed near the top of the column on a d d i t i o n of the NaN^ s o l u t i o n ; t h i s band moved slowly down the column as e l u t i o n proceeded, vanishing s h o r t l y a f t e r the l a s t of the WaN^ had been added, i n a p o s i t i o n corresponding roughly to the number of m i l l i - equivalents of Na + used. In the l a t t e r stages, a f a i n t brownish colour appeared below the band and eluted with the HN^; while the r e s i n above the band, presumably i n the Na + form, became more orange i n colour and contracted i n volume, an e f f e c t i n d i c a t e d by the makers. The hydrazoic a c i d obtained i n t h i s way was c o l o u r l e s s but weak, barely 2% s o l u t i o n s being obtained; a small amount continued to elute long a f t e r the majority had come through. On d i s s o l v i n g sodium azide i n t h i s s o l u t i o n with i n t e n t to r e c r y s t a l l i s e , a yellow colour was observed; f o r t h i s reason, and a l s o because only weak HN^ solutions could be produced, the method was discontinued. The second method used the procedure described i n Inorganic Syntheses (17) 40$ HgSO, was added slowly to a b o i l i n g s o l u t i o n o f Nal^, the HN3 di&MLl- -4- ing o f f i n the steam, being condensed and c o l l e c t i n g i n a r e c e i v e r containing d i s t i l l e d water. A reddish colour was observed i n the b o i l i n g s o l u t i o n on the i n i t i a l a d d i t i o n of the a c i d ; t h i s i s ascribed to impurities i n the commercialsodium azide,probably F e ^ + , the e f f e c t of which was studied by El-Shami and Sh e r i f (18). By reducing the quanti- t i e s of water used, HN^ concentrations up to 5% were obtained, ( i i ) Sodium_azide_ was obtained commercially from Eastman Kodak i n p r a c t i c a l grade,and r e c r y s t a l l i s e d from 3% hydrogen azide s o l u t i o n using an oven switched o f f at 90° to provide slow c o o l i n g . Suitable transparent p l a t e s of thickness about 0.1 mm. and area up to 30 sq. mm. were obtained; such p l a t e s had small regular corrugations i n the surfaces g i v i n g a feathery appearance by r e f l e c t e d l i g h t , but having no v i s i b l e e f f e c t on tr a n s - mitted l i g h t . ( i i i ) Potassium azide was prepared by n e u t r a l i s i n g hydrogen azide with reagent grade potassium hydroxide. The s o l u t i o n was then concentrated and attempts made to r e c r y s t a l l i s e the mat e r i a l . Much d i f f i c u l t y was encountered i n obtaining s u i t a b l e plates; r e c r y s t a l l i s a t i o n by the sodium azide method gave e i t h e r i r r e g u l a r lumps or plates too small to be of use. More success was obtained by using some of the l a t t e r as seeds, l a y i n g them f l a t on the bottom of a beaker containing a hot saturated s o l u t i o n . Rather t h i c k pl a t e s were obtained i n t h i s way, of area up to 45 sq. mm. and average thickness about 0.34 mm; the plat e s were a l s o a good deal l e a s transparent than those of the other azides. (iv) Rubidium -azide_was obtained from A. D. Mackay Inc. who stated i t contained at l e a s t 1% potassium and a trace of caesium. I t was r e c r y s t a l l i s e d from d i s t i l l e d water under slow evaporation i n a vacuum desiccator; excellent transparent p l a t e s of area up to 20 sq. mm. and thickness 0.3 mm. were obtained at the f i r s t attempt. (v) Caesium Azide was a l s o acquired from A.D. Mackay Inc. who i n d i c a t e d i t was about 99.8$ pure. Large transparent p l a t e s were obtained by r e c r y s t a l l i s i n g from hot d i s t i l l e d water; the area ranged to 40 sq. mm. and the thickness to 0.15 mm. (vi) Lithium,_Strontium_and -Barium azides.. Attempts were made to prepare the azides of l i t h i u m , barium and strontium as c r y s t a l l i n e p l a t e s s u i t a b l e f o r the measurement of absorption spectra. A l l were prepared by n e u t r a l i s i n g the oxide as hydroxide with excess hydrazoic a c i d . On concentrating the solutions, however, barium azide gave t h i c k i r r e g u l a r needles, strontium of azide opaque white deposits, and l i t h i u m azide came out/a very concentrated solutionsonly when the whole mass turned i n t o a brownish s o l i d . The brown colour could be removed by successive washings i n e t h y l a l c o h o l , i n which the l i t h i u m azide was not soluble, contrary to the published f i g u r e of 20 gm/ 100 ml. EtOH quoted by Audrieth (19). A f t e r drying f o r several weeks i n an oven at 90° C , a n a l y s i s , by the method described below, showed the white powder to be very impure; i t gave a strongly a l k a l i n e r e a c t i o n i n water from which i t i s i n f e r r e d that the major impurity was hydroxide. Analysis of Samples The samples were analysed by the method of Heal (2). About 0.002 gram equivalent weight of azide (0.13 gm. f o r NaN^, 0.35 gm. f o r CsN^) was weighed to the nearest 0.01 mg. i n a 5 ml. Pyrex beaker. Two m i l l i l i t r e s of normal HCl were then added to the beaker and the whole evaporated under an i n f r a - r e d lamp almost to dryness, when two more m i l l i t r e s of HCl were added. This time the system was evaporated c a r e f u l l y to complete dryness, followed by two more m i l l i l i t r e s of HCl, making 0.006;gram equivalent weight o f HCl added altogether; again the system was evaporated to complete dryness. The beaker and c h l o r i d e were weighed to 0.01 mg. as before; the r a t i o chloride to azide was c a l c u l a t e d Table I, Analysis o f azides by Real's method (2) Chloride: Azide r a t i o s . Azide S t a r t i n g M a t e r i a l Samples Used T h e o r e t i c a l value NaN3 0.9091 0.9028 0.8990 0.9056 0.9053 0.9088 0.9005 KN- 0.9237 0.9196 0.9190 0.9319 0.9177 0.9336 RbN- 0.9506 0.9502 0.94-85 0.9472 CsN 3 0.9585 0.9630 0.9265 0.9594 L i N 3 1.1114 0.8658 1.1118 1.1125 1.1124 S r ( N 3 ) 2 0.9215 0.9235 0.9182 B a ( N 3 ) 2 0.9399 0.9406 0.9336 and compared with the t h e o r e t i c a l value. Table I shows the r e s u l t s obtained f o r the s t a r t i n g materials, the samples used and the t h e o r e t i c a l values. I t i s considered that the standard err o r of the balance i s + 0.00004 grams i n the range used, 4-5 grams, and that the e r r o r i n the r e s u l t a n t r a t i o s a t t r i b u t a b l e to t h i s cause i s about 0 . 0 0 1 . The t o t a l e r r o r s i n the r a t i o s may be higher since i t was rather d i f f i c u l t to avoid s p i t t i n g as dryness was approached, though t h i s could be reduced by moving the lamp f u r t h e r away. Attempts to p r e c i p i t a t e the chlorides i n f i n e l y d ivided form by adding two m i l l i l i t r e s of acetone d i d not reduce the s p i t t i n g . The high values of the sodium r a t i o s are ascribed to traces o f water l e f t i n the sodium ch l o r i d e on evaporation. According to Duval ( 2 0 ) , NaCl does not a t t a i n constant weight below 407°Cj experiments with a thermocouple showed that under the normal conditions of evaporation i n which one, two or three i n f r a - r e d lamps were used the temperatures attained were about 1 6 0°C, 240°C and 3 1 0°C r e s p e c t i v e l y . This e f f e c t does not occur i n the other chlorides since the constant weight temperatures are much lower; 2 1 9°C f o r K C 1 , 8 8°C f o r RbCl and 1 1 0°C f o r CsCl. I r r a d i a t i o n c e l l The i r r a d i a t i o n c e l l used i s depicted i n f i g u r e I. The sample, i n the form of a c r y s t a l l i n e p l a t e , was cemented v i t h l a b e l varnish across a hole, 3 or 4 mm. square depending on the s i z e of the c r y s t a l , i n the s l o t t e d copper block, which i n turn was screwed to the bottom of the metal dewar. The c e l l was evacuated by connecting the metal flange on the r i g h t of the brass vacuum valve to a s i m i l a r flange attached to an evacuation t r a i n , con- s i s t i n g of mechanical fore pump, mercury d i f f u s i o n pump, l i q u i d a i r trap and P i r a n i gauge. The temperature of the sample was c o n t r o l l e d by putt i n g an appropriate coolant i n the dewar, and measured with the a i d o f a thermocouple. - 7 A - german silver tube 0.1 mm. wall i 1 0 2 cm thermocouple connections brass reservoir 'for liquid nitrogen etc. brass slotted copper block quartz windows position of beryllium window brass vacuum valve with Teflon seat crystal cemented across this hole thermocouple attached here Fig. 1. Irr*di»ti«n call To i r r a d i a t e the sample was turned p a r a l l e l to the beryllium- window by r o t a t i n g the top of the c e l l about the B2A j o i n t u n t i l two marks, one on each h a l f of the joint., coincided. Absorption spectra were taken by r o t a t i n g the c e l l through 90°, as measured by another mark on the j o i n t so that the c r y s t a l became p a r a l l e l to and i n l i n e with, the quartz windows through which the spectrophotometer beam passed. The c e l l could be attached to the port of the X-ray tube or placed i n the c e l l compartment of the Cary Model 1A spectro- photometer, by arrangements not shown i n the diagram. X-ray tube c i r c u i t Figure 2 gives the c i r c u i t supplying power to the non-shockproof Machlett AEG-50 type T X-ray tube. The c i r c u i t i s e s s e n t i a l l y a General E l e c t r i c X-ray un i t g i v i n g f u l l wave r e c t i f i c a t i o n , modified by the removal of two of the r e c t i f i e r tubes, shown dotted i n the f i g u r e , t o give an output voltage of 50 KVP instead of 100 KVP; the spare high voltage terminal was grounded. The mo d i f i c a t i o n also had the property of doubling the current obtainable, which was l i m i t e d by the heat generated i n the transformer. The voltage across the X-ray tube was c o n t r o l l e d by means of the H.T. Variac, and measured by the meter i n the H.T.primary c i r c u i t . An auto transformer was placed i n the c i r c u i t of the X-ray filament supply to s t a b i l i s e the filament current, on which the tube current markedly depends. This dependence was used to c o n t r o l the tube current by means of the v a r i a b l e inductor i n the filament c i r c u i t j the tube current was measured by means of a meter i n the grounded anode c i r c d t , whose readings had to be doubled since i t operated only on one h a l f of the c y c l e . Certain r e l a y s , incorporated as safety devices, are not shown i n the diagram; a water switch was placed i n the cooling water supply to the anode, which cut o f f the H.T. when the flow rate was too low. 6A- -9- 5. Production of X-rays The X-irradiation used in this work was obtained from a non-shockproof Machlett AEG-50 Type T X-ray tube, equipped with a water cooled tungsten target. Such tubes have a maximum power rating for continuous use of 50 milliamperes at 50 KVP; in practice, the tube was run at 50 KVP and 28 m i l l i - amperes or less. A 44° cone of X-rays is emitted through a 1 mm. thick beryllium window; the energy distribution of the continuous X-ray spectrum was calculated with the aid of De Waard's formula (21) and is shown in figure 3« It can be seen that there i s a steep rise at the short wave lengths, starting from the critical wavelength at O .248A 0 , which corresponds to the total energy of a 50 KV electron. Peak energy occurs at about 0 . 4 4 0 A 0 and there i s a pronounced t a i l at the longer wave lengths. The voltage used was insufficient to excite the characteristic K emission lines of tungsten, which requires at least 69 KV; and the intensity of the L lines is considered by Heal (22) to be negligible. 6. Filtration and Absorption of X-rays The X-ray absorption due to any element increases with the atomic number, and also with the wavelength of the incident radiation, except at absorption edges, where there, is a discontinuous change. In a given system, i t depends only on the number of atoms of that element present, and not at a l l on their chemical and physical environment. These properties make i t possible to measure a mass absorption coefficient for each element, which can then be suitably added together to give the mass absorption coefficients of compounds or mixtures. The mass absorption coefficientshave not been measured for a l l elements, but their variation is sufficiently well known to enable Victoreen (23) to produce empirical formulas for calculating them, which agree,so he claims, to within 1% of the best average measured values; the escperimental values being somevrhat variable. -9A- 3-6r F i g . 3. f i l t r a t i o n and a b s o r p t i o n of 50PKV X - r a y s . Using Victoreen's formulas the s p e c i f i c absorption c o e f f i c i e n t s o f the azides have been ca l c u l a t e d and are p l o t t e d i n Figure 4, together with data f o r aluminium and copper. The higher the s p e c i f i c absorption the more the X-ray beam i s absorbed i n the surface l a y e r s of the material r e l a t i v e to the bulk. In order to obtain reasonably uniform i r r a d i a t i o n , t h e r e f o r e , i t was desi r a b l e to reduce the i n t e n s i t y of the longer wavelengths at which the s p e c i f i c absorption c o e f f i c i e n t s are high. This was done by interposing metal f o i l s i n the path of the X-ray beam; f i g u r e 3 shows the s p e c t r a l d i s t r i b u t i o n of the X-radiation a f t e r f i l t r a t i o n by a) 6 2 . 5 milligrams per square centimetre of aluminium, used f o r NaN^, b) 373 milligrams per square centimetre of aluminium used f o r some KN^ runs, and c) 8 3 . 6 milligrams per square centimetre of copper and 4*6 milligrams per square centimetre of aluminium, used f o r KN-, RbNj, and CsN^. The attenuation of the X-ray beam at any wavelength was c a l c u l a t e d from the Beer-Lambert law: I x = 1^ e where 1°̂  , I x are the incident and emergent i n t e n s i t i e s r e s p e c t i v e l y , u.̂  i s the l i n e a r absorption c o e f f i c i e n t at wavelength A and x i s the thickness of the sample. For the purposes of c a l c u l a - t i o n , t h i s formula was modified to: I N = I ° x where i s "k n e mass absorption c o e f f i c i e n t and >c»* i s expressed i n grams per square centimetre, being the density of the m a t e r i a l . S i m i l a r c a l c u l a t i o n s were performed to f i n d the f r a c t i o n of the incident X-ray energy absorbed by the azide samples, which i s proportional t o 1 - e v ° ' , I t v a r i e s with the wavelength, and, m u l t i p l i e d by 1^, has also been p l o t t e d i n f i g u r e 3« As an approximation to uniform i r r a d i a t i o n throughout the c r y s t a l s , i t was considered by Keal advisable to keep the f r a c t i o n of the i n c i d e n t X-ray energy absorbed below 10$. This was not possible f o r KN3, RbN^, or CsN- because  - l i - the f r a c t i o n of the incident energy absorbed depends on the q u a n t i t y x ) /( and /o axe f i x e d q u a n t i t i e s , while x i s f i x e d f o r a given c r y s t a l , and was f a r too large i n the cases quoted. The quantity /> x was determined f o r the azide samples from the area of the c r y s t a l s , estimated by l a y i n g them f l a t on graph paper, and t h e i r weight as measured on a Cahn electro-balance. 7. Spectrophotometry technique ( i ) C h a r a c t e r i s t i c s of_wayglength regions The Cary model 1A spectrophotometer used covers the wavelength region from 26000A 0 to 2000A°, i n three ranges, r e f e r r e d to as "IR", "VIS" and "UV". The "VIS" region covers the range of wavelengths from about 8200A 0 to 2900A 0, '.though the u s e f u l range extends only from about 74O0A0 to 3200A0; below 3200A0 the s l i t width v a r i e s rather r a p i d l y , while above 7400A 0 there appears to be a drop i n the response of the IP28 photo-tube detector. The spectrum i s obtained from an ordinary General E l e c t r i c p r o j e c t i o n lamp, and analysed i n t o i t s wavelength components by successive passage through a prism and d i f f r a c t i o n grating; the monochromatic beam produced passes through a s l i t , and the sample i n t o the detector. The "UV" region uses a hydrogen discharge lamp, but the o p t i c a l path i s otherwise s i m i l a r to that of the "VIS" region; i t was used from 3500A 0 down to about 2500A 0, at which point the f i r s t fundamental absorp- t i o n band of the azides i s reached. In the "IR" region, which also uses a General E l e c t r i c p r o j e c t i o n lamp, the o p t i c a l path i s reversed. The whole s p e c t r a l range whose us e f u l l i m i t s were 26000A 0 to 3500A 0, passes through a s l i t and onto the sample; only then i s i t broken up i n t o i t s components by the mono- chromator, and f e d i n t o the PbS detector. - 1 2 - This d i f f e r e n c e between the "IR" and "VIS" l'egions was of immense s i g n i - f icance f o r absorption bands unstable to l i g h t , of which several were en- countered i n t h i s work. Scanning over the "VIS" and "UV" ranges combined would take about ten minutes at the normal scanning speed of 10A°/sec; under these conditions each wavelength would be incident on the sample f o r rather l e s s than one second, since the r e s o l v i n g power of the monochromator i s sa i d to be 1A° i n these regions. The whole scanning operation therefore would have as much bleaching e f f e c t on the unstable absorption bands as under one second's work i n the "IR" region, where a l l the wavelengths are incident on the sample a l l the time. ( i i ) C h a r a c t e r i s t i c s of_sp.ectrophotpmeter_beam The Cary model 14 i s a double beam type of spectrophotometer, the sample and reference beams passing through two separate compartments each about 10 cms. long i n which apparatus can be placed. The beams are focused at the centre of these compartments, where they form a rectangular v e r t i c a l image, and where the samples are placed i n normal use. The s i z e of the beams i s co n t r o l l e d by a) the S l i t Width c o n t r o l , automatically operated by the machine, which v a r i e s the s l i t width, and hence also the image width beWeen 0 and 3 mm. as recorded on a meter; and b) the S l i t Height c o n t r o l , which v a r i e s the s l i t height between 7 mm. and 20 mm. and whose f u n c t i o n i s to correct f o r mismatches of wavelengths by the monochromator at extreme range. According to the makers (24) the siz e of the image i s 15.8 mm. by 3 .15 mm; approximate measurements with a r u l e r i n the "VIS" region with the s l i t wide open i n d i c a t e d that the act u a l s i z e was nearer 12 mm. x 3 mm. or, with the s l i t height reduced to i t s f u l l extent, 7 mm. x 3 mm. Since the maximum height of the beam emerging from the sample was only 4- mm.,the si z e of the c r y s t a l aperture, the s l i t height c o n t r o l could have l i t t l e e f f e c t , and was therefore i n v a r i a b l y l e f t open to i t s f u l l extent. -13- ( i i i ) Alignment of Cr y s t a l s . The c r y s t a l s were aligned with the spectrophotometer beam by means of a spe c i a l carriage. The i r r a d i a t i o n c e l l was screwed to a brass block, which was f r e e to move along two v e r t i c a l brass rods to which i t could be screw clampedj t h i s allowed the samples to be c o r r e c t l y aligned i n a v e r t i c a l d i r e c t i o n . The brass rods were fastened to a h o r i z o n t a l aluminium t r a y r e s t i n g on supports i n the sample compartment; by means of a handle p r o j e c t i n g outside the com- partment the t r a y could be moved so that the samples were c o r r e c t l y aligned i n a h o r i z o n t a l d i r e c t i o n with respect to the beam. Horizontal alignment was i n - v a r i a b l y done at a wavelength near 8000A° on the "VIS" range, where the image width was nearly as wide as the sample i t s e l f , and was checked by minimising the absorption recorded. (iv) Operational o b s e r v a t i o n ^ At the s t a r t of the experiments, the samples were set by eye i n the middle of the "VIS" beam. I t was then discovered that, on changing to the "UV" beam at 3500A 0, there was a discontinuous jump i n the absorption and also that the slopes of the absorption curve were d i f f e r e n t i n the two regions. This trouble was traced to the f a c t that the "UV" beam was not following an i d e n t i c a l path to the "VIS",but. :was passing at a lower l e v e l ; lowering the c r y s t a l reduced the e f f e c t s u b s t a n t i a l l y . S l i g h t variations i n recorded absorption were a l s o ob- served i n the region 7400A0 to 6OOOA0 where the "VIS" and "IR" regions over- lapped; these are ascribed to a s i m i l a r e f f e c t , and also the f a c t that the two beams vrere i n c i d e n t on opposite faces of the c r y s t a l s . I t was observed that there was a marked gradation i n i n t e n s i t y across the "UV" beam, and to a l e s s e r extent i n the "VIS" a l s o . An aluminium sheet with a hole bored i n i t of approximately the same s i z e as that of the c r y s t a l holder, was placed i n the reference beam to equalise the -14- s i z e of the beams reaching the detectors. .The hole was aligned with the "VIS" and "UV" beam by inspection. Discussion 1. E f f e c t s of X - i r r a d i a t i o n on s o l i d s The primary e f f e c t of the passage of X - i r r a d i a t i o n through matter i s the production of f a s t electrons by one of three processes. I f the energy o f the X-radiation i s greater than one MeV, p a i r production i s possible; t h i s c o n s i s t s of the simultaneous production of a po s i t r o n and an e l e c t r o n when the photon passes close to an atomic nucleus, and i s the most probable process at very high energies. For lower energies, such as the 50 KeV used i n the present work, f a s t electrons are produced e i t h e r by the Compton e f f e c t or by the p h o t o e l e c t r i c e f f e c t . In the former, an X-ray photon i n t e r a c t s with an ele c t r o n and i s scattered, giving up part of i t s energy to-the e l e c t r o n , and appearing as X-radiation of longer wavelength. The ph o t o e l e c t r i c e f f e c t occurs when a l l the energy of the incident photon i s absorbed by an e l e c t r o n i n an atomic inner o r b i t a l , r e s u l t i n g i n the e j e c t i o n of the e l e c t r o n from the atom. Calculations by Lea (25) show that f o r a 51 KeV X-ray quanta absorbed i n water the proportion of primary photoelectrons i s 0.151, the r e s t being Compton r e c o i l s ; but the photoelectrons, being more energetic, are responsible f o r O.684 of the t o t a l e l e c t r o n energy. The average primary e l e c t r o n energy i s c a l c u l a t e d to be about 15 KeV f o r t h i s system. The f a s t primary electrons produced lose energy i n traversing matter because t h e i r r a p i d l y moving charge perturbs the e l e c t r o n i c system of the ma t e r i a l , r e s u l t i n g i n e l e c t r o n i c e x c i t a t i o n and i o n i z a t i o n ; such i o n i s a t i o n i s frequently so vigorous that the secondary e l e c t r o n freed has s u f f i c i e n t energy (^lOOev) to cause i o n i s a t i o n on i t s own account. In i o n i c s o l i d s , such as the azides, i o n i s a t i o n corresponds to the r a i s i n g of an e l e c t r o n from the valence band in t o the conduction band, leaving a p o s i t i v e hole i n the band from whence i t came. -15- The production of excitons corresponds to the e l e c t r o n i c e x c i t a t i o n discussed above; an exciton, which i s mobile i n the l a t t i c e , can be considered as an e l e c t r o n bound to a p o s i t i v e hole i n the same manner as i t i s bound to a proton i n the hydrogen atom. Figure 5 i s an energy diagram showing the t r a n s i t i o n s ! involved. I t i s the i n t e r a c t i o n of these conduction band electrons, excitons, and holes with the l a t t i c e defects described i n the next section that produce the colour centres described i n section 3. 2. Imperfection i n i o n i c s o l i d s The necessary existence of l a t t i c e imperfections was f i r s t pointed out by Frenkel, (26) who showed that the e l e c t r o l y t i c conductivity observed i n the a l k a l i halides at high temperatures was not e x p l i c a b l e i n terms o f a perfect i o n i c l a t t i c e ; but required that a c e r t a i n f r a c t i o n of the ions be f r e e to wander round the l a t t i c e . An applied e l e c t r i c f i e l d o r i e n t s the motion, and r e s u l t s i n the passage of the current. In order to explain the f r e e ions, Frenkel proposed that an i o n could jump to an i n t e r s t i t i a l p o s i t i o n (Frenkel defect) leaving a vacancy at the normal l a t t i c e s i t e ; the current could be c a r r i e d e i t h e r by the i n t e r s t i t i a l i o n moving through the l a t t i c e , or by the vacancy t r a v e l l i n g i n the opposite d i r e c t i o n through successive jumps of ions i n t o i t . Schottky (27) pointed out that the displaced i o n could be added to the surface of the c r y s t a l instead of an i n t e r s t i t i a l p o s i t i o n ; thus le a v i n g i s o l a t e d vacancies (Schottky defects) which were p e r f e c t l y capable of explaining the observed i o n i c c o n d u c t i v i t y and d i f f u s i o n e f f e c t s . Calculations by Mott and L i t t l e t o n (28) i n d i c a t e that i n t e r s t i t i a l ions are u n l i k e l y to occur i n the a l k a l i h a l i d e s since the energy required f o r t h e i r formation i s high. Thus i n NaCl, an i n t e r s t i t i a l Na+ requires - 2-.9 e V f o r i t s formation, while a p a i r of separated vacancies requires only 1.9 eV; even at the melting point the number of i n t e r s t i t i a l -15A- Conduction Electron band [Exciton -J bands Excitons Figa S» Energy bands In ionic solidso v 1 "\Valence Positive^ band hole j + + + + + • + + + + + + + + - + + + + - 4 - 4 - + + + + + + + + + +.+ +•• Figo 7o Edge dislocation in an a l k a l i halideo + "IS + " + " *zA " + - + -.+ - + - + - + - - +EEf- + m * 4 - + - > - — 4- — +• + — + - — h - + - + — + Figo 6a Possible vacancies and vacancy aggregates in a l k a l i halides< Figo 8o Production of vacancies from an edge dislocation,, -16- ions would be l e s s than 1% of the number of vacancies on the b a s i s of these c a l c u l a t i o n s . Frenkel e f f e c t s predominate, however, i n the s i l v e r h a l i d e s where the i o n i c r a d i i are much more unequal. Possible combinations of vacant l a t t i c e s i t e s i n the a l k a l i halides are shown i n f i g u r e 6. A and B represent a c a t i o n and an anion vacancy, with a negative and p o s i t i v e charge r e s p e c t i v e l y . C represents a vacancy p a i r formed by combina- t i o n of A and B with n e u t r a l i s a t i o n of charge; D depicts a quartet. Impurity multivalent ions present i n the l a t t i c e w i l l have vacancies associated with them, E and F, f o r reasons of e l e c t r i c a l n e u t r a l i t y which w i l l tend to be i n nearest neighbour p o s i t i o n s on account of the e l e c t r o s t a t i c a t t r a c t i o n . I t was o r i g i n a l l y thought that l a t t i c e vacancies were formed on the surface of the c r y s t a l or on the surfaces of i n t e r n a l cracks, and then d i f f u s e d i n t o the bulk. Obviously, equal numbers of c a t i o n and anion vacancies must d i f f u s e at the same rate, other- wise portions of the c r y s t a l w i l l become charged; t h i s suggests that p a i r s of vacancies are responsible, as does the f a c t that the jump frequency of anion vacancies i s very low-Seitz (15)• Recently, mechanisms f o r the formation of vacancies from c r y s t a l d i s l o c a - t i o n s have been proposed. A simple such d i s l o c a t i o n i s the Taylor-Orowan or edge d i s l o c a t i o n , which can be formed,in the a l k a l i h a l i d e s , by the i n s e r t i o n o f two p a r t i a l extra planes of ions. Figure 7 shows such a d i s l o c a t i o n ; the plane of the paper represents a (001) face of the c r y s t a l . The d i s l o c a t i o n l i n e i s per- pendicular and passes through the black dot i n d i c a t e d ; the dashed l i n e represents the (110) plane at which the extra (110) planes terminate, and i s c a l l e d a s l i p plane. Motion of the d i s l o c a t i o n l i n e i s r e s t r i c t e d mainly t o the s l i p plane; i f two such d i s l o c a t i o n s of opposite o r i e n t a t i o n , whose s l i p planes are separated by a few i o n i c spacings should chance to coincide, then a s e r i e s of vacancies w i l l be formed between the s l i p planes. A more sophisticated mechanism proposed by S e i t z (29) requires an i r r e g u l a r edge to the extra planes. Schematically t h i s i s shown i n f i g u r e 8 where the extra -17- plane of atoms a l l numbered 3 extends across a l l the planes i n rows 1 and 2 i but only across planes C and D i n row 3» I f atom 3 row 2 plane B drops down to row 3 i n the same plane, the p a r t i a l plane of atoms i s extended by one u n i t , and a vacancy i s formed which can d i f f u s e away. Such a mechanism i s an e x c e l l e n t explanation f o r the formation and d e s t r u c t i o n of vacancies, and i n d i c a t e s that dislocations'?can act as a source and sink f o r them. 3». Colour centres i n a l k a l i h a l i d e s A l k a l i halide c r y s t a l s absorb electromagnetic r a d i a t i o n i n the f a r i n f r a ' red due to i o n i c v i b r a t i o n s and a l s o i n parts of the u l t r a v i o l e t , where excitons and conduction band electrons are generated o p t i c a l l y . Elsewhere, they are quite transparent i n the absence of coloured i m p u r i t i e s . On X - i r r a d i a t i o n , however, c h a r a c t e r i s t i c colours, varying with the temperature are produced; thus at room temperature NaCl becomes brown and KCl magenta. The colours are ascribed by S e i t z (16) to various "colour centres" considered below, and drawn i n f i g u r e 9. ( i ) The F centre;;. ("Farbenzentren" i . e . colour centres) i s characterised by a. prominent band ( c a l l e d the F band) peaking generally i n the v i s i b l e ; at 5630A0 f o r KCl, f o r example. I t i s considered to be a si n g l e e l e c t r o n trapped at an anion vacancy and can be produced e i t h e r by high energy r a d i a t i o n s , or by heating the a l k a l i halide i n a l k a l i metal vapour. In the former, a conduction band e l e c t r o n or exciton can i n t e r a c t with e i t h e r an i s o l a t e d anion vacancy or a p a i r , i n which case the c a t i o n vacancy d i f f u s e s awajr as does the hole associated with the ex- c i t o n . On heating i n a l k a l i metal vapour, some a l k a l i metal i s adsorbed on the surface of the c r y s t a l where i t i o n i s e s , the e l e c t r o n going i n t o the conduction band from which i t produces F centres as before (3)• Good evidence f o r the correctness of the F centre model i s obtained Erom the f a c t that the F band i s independent of the nature of added a l k a l i metal; a l s o Witt (31) has shown that the observed decrease i n density i s , within experimental error, compatible with the formation of a s i n g l e anion vacancy f o r each F centre formed. 250 300 350 l*oo 500 6oo 800 1000 Figo 11 o Four spectra of u n i r r a d i a t e d NaN-jo The F band i s a good b e l l shaped band, which becomes very narrow at low temperatures, showing that the sing l e t r a n s i t i o n involved i s between two d i s c r e t e energy states. In some c r y s t a l s (KCi)a small shoulder and t a i l h a s been observed on the high energy side of the band; Duerig and Markham (32) have shown that t h i s shoulder bears a constant r a t i o of 0.047 to the height of the F band. S e i t z (16) c a l l s i t the K band, and Mott and Gurney (33) consider i t i s due to a t r a n s i t i o n to a higher state, while the t a i l i s due to a t r a n s i t i o n to the i o n i s a t i o n continuum. They add that i t i s not c l e a r why the continuous absorp- t i o n should be so much weaker than the band absorption. F centres can be bleached by i r r a d i a t i n g v i t h l i g h t l y i n g i n the F band, when such c r y s t a l s become photoconducting; the inc i d e n t l i g h t i s only s u f f i c i e n t to r a i s e the F electron to an excited state, from which i t i s thermally i o n i s e d to the conduction band even at temperatures of the order of -200°G. ( i i ) F_. centrej. When an a l k a l i h a l i d e c r y s t a l containing F centres i s i r r a d i a t e d with F l i g h t at f a i r l y low temperatures, below 25°C for- NaCl and -75°C f o r KCl, the F band diminishes and a new band, c a l l e d the F 1 band by Pohl appears on the long wavelength side; f o r KCl i t peaks about 7300A0. Pick (34) measured the quantum e f f i c i e n c y with which F centres are destroyed i n the formation of F"' centres and found i t i s close to 2 i n KCl f o r temperature around -100°C; to explain t h i s , he proposed that the F centre e l e c t r o n i s r a i s e d to the conduction band with a l i t t l e thermal help, and i s then trapped by another F centre. Two F centres are then destroyed per incident quantum absorbed; the e n t i t y produced, with two electrons trapped at an; anion vacancy, i s the F"' centre and confers a l o c a l negative charge on the l a t t i c e . The F"1 band i s i n v a r i a b l y much broader than the F band and overlaps v i t h i $ ; i t s width i s r e l a t i v e l y i n s e n s i t i v e to temperature, suggesting that the t r a n s i - t i o n i s to a continuum. S e i t z (15) notes that t h i s i s i n q u a l i t a t i v e agreement v i t h the behavior of such s i n g l e charged negative ions as H "*. F' centres can be bleached to F centres by F1' l i g h t , and also by heat. -19- ( i i i ) % , R2, M and N bands A l l these bands are obtained on i r r a d i a t i n g c r y s t a l s containing F centres with F l i g h t at temperatures above those at which F 1 centres are stable. Not much i s known about any of themj models proposed by S e i t z are given i n f i g u r e 9 f o r the f i r s t three. M centres are bleached by M l i g h t , and the c r y s t a l becomes photoconducting; but both R centres are stable to R l i g h t . S e i t z (15) proposes that such centres are analogues of diatomic molecules, and that i n the excited state the vacancies separate somewhat so that the f i r s t e l e c t r o n i c excited state gets much c l o s e r to the ground state. The system then makes an almost adiabatic t r a n s i t i o n and returns to the ground s t a t e . A l l these bands occur on the long wavelength side of the F bands; f o r KC1 at room temperature the values are about 6700A 0 and 7300A 0 f o r the R bands, 8200A 0 f o r the M band, and 9700A 0 f o r the N band. (iv) C o l l o i d bands. When c r y s t a l s containing F centres are heated to high temperatures, sometimes with i r r a d i a t i o n by F l i g h t as well, f u r t h e r bands are obtained on the long wavelength side of the F band which are ascribed to the presence of c o l l o i d a l metal. By i r r a d i a t i n g F centred KC1 with F l i g h t at 100°C, Scott and coworkers (35) obtained a broad h i g h l y composite band c a l l e d the R'1 band, which i s probably a combination of the R, M, N bands together with other aggregates. By heating alone to 300°C, gives a r e l a t i v e l y narrow pure c o l l o i d band which peaks at 7750A 0 f o r KC1, and which grows at the expense of the F band. Savostianova ( 3 6 ) has applied the Mie(37) theory of the s c a t t e r i n g of l i g h t by m e t a l l i c spheres to p a r t i c l e s of a l k a l i metal embedded i n a l k a l i halides, and obtained bands of s i m i l a r shape; on the basis that the diameter of the c o l l o i d s i s small compared tothe wavelength of the incident l i g h t , i t can be c a l c u l a t e d f o r K i n KC1 that the c o l l o i d band should peak at 7 3 0 0 A 0 , i n f a i r agreement with experiment. (v) V bands are produced by i r r a d i a t i o n at low temperatures simultaneously with F centres or by a d d i t i o n of excess halogen on heating i n halogen vapour, -20- without F centres; they are ascribed to the i n t e r a c t i o n of holes with l a t t i c e vacancies. The V system i s much l e s s c e r t a i n than the F band system; current assignations are i n d i c a t e d i n f i g u r e 9. Best characterised i s the V i band, produced i n KCl (38) by X - i r r a d i a t i o n at low temperatures; i t vanishes above -100°C and peaks, at 36OOA0. The centre responsible i s regarded as the a n t i - morph of an F centre; that i s , as a hole trapped at a c a t i o n vacancy, and Kanzig (39) has shown by e l e c t r o n spin resonance techniques that the hole i s l o c a l i z e d on two of the halide ions surrounding the vacancy. Tne V 2, and V^ centres are regarded as the antiniorphs of the Rg, R]_, and M centres r e s p e c t i v e l y ; a l l are more stable than the Vj_ centre, the V 2 and V^ centres being stable at room temperature. In KCl the bands peak at 2300A0 f o r V 2, 2150 A 0 f o r V^ and 2540A0 f o r V^. The diatomic nature of the V 2 and V^ centres was recognised by t h e i r discoverer Mollwo, who, using a d d i t i v e l y coloured c r y s t a l s , noted that t h e i r concentration v a r i e d with the f i r s t power of the pressure of the diatomic halogen vapour; s i m i l a r l y , the concentration of F centres v a r i e d with the f i r s t power of the monatomic a l k a l i metal vapour. The has a net negative charge which should make i t r e s i s t a n t to bleaching by electrons; Casler, Pringsheim and Yuster (38) have found that photoelectrons from F centres at room tempera- tures bleach V 2 centres with high e f f i c i e n c y , but hardly a f f e c t V^ centres at a l l . Evidence has a l s o been obtained by Dutton and Maurer (4-0 ) that V j centres bleach by evaporation of the hole which i s e i t h e r a n n i h i l a t e d at an F type centre, or trapped by a V3 centre to give a V 2 » (vi) Miscellaneous bands._ Various other bands have been obtained i n the a l k a l i h a lides by s u i t a b l e treatment. Impurity bands due t o H~ a d d i t i o n , c a l l e d U bands, and d i v a l e n t a l k a l i n e earth ions, c a l l e d Z bands, have been described. The H band, formed by i r r a d i a t i o n at very low temperatures, i s very s i m i l a r to, and peaks a l i t t l e on the u l t r a v i o l e t side of the V j band; S e i t z (16) ascribes i t to a hole trapped at a vacancy p a i r . The cL and (3 bands peak a l i t t l e to the long wavelength side o f the f i r s t fundamental absorption band, the cL being the longer; S e i t z ascribes them to e l e c t r o n i c e x c i t a t i o n s of the halid e ions to l e v e l s not found i n the perfect c r y s t a l . The close c o r r e l a t i o n i n i n t e n s i t y between the F and (5 bands i n d i c a t e s that the l a t t e r i s associated with a hali d e i o n adjacent to an F centre, while the oL band i s that adjacent to an anion vacancy. A. C r y s t a l Structures The a l k a l i metal halides c r y s t a l l i s e i n the face centred cubic form with the exception of CsCl, CsBr and C s l , which are body centred cubic. The azides, however, containing the l i n e a r N-j" i o n , have considerably lower symmetry; NaEj c r y s t a l l i s i n g i n the t r i g o n a l form (41) with the azide ions a l l p a r a l l e l . Figure 10a, while KN^, RbN^ and CsN^ c r y s t a l l i s e i n a body centred tetragonal mode (42) with successive azide ions at r i g h t angles to each other, Figure 10b. The c r y s t a l l o g r a p h i c measurements of the unimolecular u n i t c e l l of t r i g o n a l NaN ? are a= 5.488A? et = 38°A3''3 f o r tetragonal KN3, RBN3, CsN^, a=6 .09A,6.36,6.72A° and c=7.057,7.41,8.04A° r e s p e c t i v e l y , with four molecules to the u n i t c e l l . The N-N distances are equal at 1.16§ 0.02A 0 f o r a l l the azides quoted. Results 1. Processing of spectra The spectra f i g u r e d i n t h i s work were ca l c u l a t e d as the d i f f e r e n c e of two experimental records, which were obtained on a l i n e a r wavelengthscale; the r e s u l t s have been converted to a scale l i n e a r i n energy. For un i r r a d i a t e d c r y s t a l s the spectrum of the quartz windows with the c r y s t a l holder i n position£was subtracted from that obtained with the c r y s t a l i n place; the l a t t e r i n turn, was subtracted from the i r r a d i a t e d c r y s t a l record to give the spectrum due to the i r r a d i a t i o n . Measurements had to be made under the same temperature conditions as there was a considerable contraction of the inner part of the metal dewar on cooling to l i q u i d nitrogen temperatures,and a l s o because the spectrum of the u n i r r a d i a t e d -22- c r y s t a l s v a r i e d with temperature. Figure 11 shows four spectra o f u n i r r a d i a t e d NaN^ taken at l i q u i d nitrogen temperature, i n which f o r E and .' the quartz window spectra were taken at room temperature. Unless otherwise stated, a l l measurements were taken at l i q u i d nitrogen temperatures to get the best r e s o l u t i o n . The r e p r o d u c i b i l i t y of the experimental records was, f o r the "UV" and "VIS" regions, about that stated by the makers, namely i 0.002 absorption u n i t s at o p t i c a l density 1.0 and 1 0.00? near 2.0. In the "IR", however, there were sometimes discontinuous jumps occurring between successive I.R. spectra; these d i s c o n t i n u i t i e s may be due to changes i n the "IR" lamp filament r e s u l t i n g when the lamp i s switched on and o f f . Discrepancies between the regions have been removed i n the spectra f i g u r e d by matching the absorptions with the "VIS" region, which was taken as the standard since the alignment of the c r y s t a l s was checked i n that region. The temperature of the c r y s t a l was checked with a thermocouple soldered to the bottom of the c r y s t a l holder; i t was found impracticable to have the thermo- couple d i r e c t l y i n contact with the c r y s t a l . At l i q u i d nitrogen temperatures, -196°C, the reading was -180 + 5°G. Room temperature r e f e r s to temperatures about 20°C s t a b i l i s e d by means of an acetone bath i n the dewar. 2. Spectra of u n i r r a d i a t e d azides ( i ) Spectra below absorption_edge (a) V a r i a t i o n i n Spectra. Figure 11 gives the spectra of four c r y s t a l s of NaNj at l i q u i d nitrogen temperatures, showing some v a r i a t i o n i n slope; t h i s can be accounted f o r by noting that the c r y s t a l s d i d not have plane surfaces and were probably not set at r i g h t angles to the spectro- photometer beam. The f i n e structure noticeable i n B and F may be due to impurities since these c r y s t a l s came from a d i f f e r e n t r e c r y s t a l l i s a t i o n to G and L. (b) Comparison of spectra. The spectra of the four azides i s given i n f i g u r e 12. -23- The magnitudes of the absorption are not s t r i c t l y comparable since the c r y s t a l s v a r i e d i n thickness from about 0.1 mm. f o r NaN^ and CsN^ to 0.3 mm. f o r KNj and RbN^j also d i f f e r e n t hole apertures were used, 3 mm. square f o r NaN^ and RbNj, 4 mm. square f o r KN^ and CsN^. Nevertheless, i t can be seen that the b e t t e r c r y s t a l s of RbN^ and CsN^ attenuate the l i g h t beam- l e s s than the other two azides. The explanation of the slopes of the spectra and t h e i r decrease from NaN^ to CsN^ i s uncertain. S i m i l a r slopes have been observed i n Ag, TI and Cd h a l i d e s , where they have been ex- plained (43) on the basis of impurijr atoms, atoms i n unusual p o s i t i o n , or forbidden t r a n s i t i o n s ; the l a t t e r has been used to account f o r t h e i r absence i n a l k a l i h a lides. I s e t t i and Neubert (44) working with NaCN have explained t h e i r slopes on the basis of Debye and Bueche's (45) c a l c u l a t i o n s on the s c a t t e r i n g of l i g h t by inhomogeneous s o l i d s , ( i i ) Spectra at_absorption edge (a) Experimental. The spectra of the absorption edges, s u i t a b l y s h i f t e d f o r c l a r i t y , are shown i n f i g u r e 13a. The absorption edge energy generally decreases f o r l a r g e r cations; a r e s u l t at variance with that obtained by Jacobs and Tompkins (9) quoting reflectance spectra of NaN-j, KN^ and BaKj shown i n f i g u r e 13b. (b) Interpretation. By analogy with the a l k a l i halides the absorption can be a t t r i b u t e d to the low energy t a i l of an exciton band, whose peak can be very approximately c a l c u l a t e d by the method of von Hippel (43) (44). The formula f o r the energy of the exciton t r a n s i t i o n i s : hv t a U = V - V/P - £1+ - St, Here W i s the work done i n t r a n s f e r r i n g an e l e c t r o n from an anion to a neighbouring cation, and i s c a l c u l a t e d from a c y c l e , giving the following formula: W = (2d- 1) e 2 + E - I where Y 0 i s the smallest i n t e r i o n i c distance i n the c r y s t a l and 300 350 U00 500 600 800 1000 2000 2U0 250 260 270 2 80 290 300 Figo 12. Unirradiated azide spectra below absorption edge. Fig. 13(»)» Absorption edges0 -24- Table I I . Calculations on Exciton Absorption Bands. A l l energies i n e l e c t r o n v o l t s S a l t s W W P h VC«Ac. h vovr s. NaBr 10.4 1.7 1.7 0.3 6.7 6.50 KBr 10.1 1.4 1.6 0.2 6.9 6.58 RBBr 9.8 1.4 1.5 0.2 6.7 6.43 HaN3 7.0 1.2 1.7 0.3 3.8 KN 3 9.6 1.1 1.6 0.2 6.7 RBN 3 8.9 1.0 1.5 0.2 6.2 CsN 3 8.7 1.0 1.4 0.2 6.1 -25- was c a l c u l a t e d from the c r y s t a l l o g r a p h i c data. od i s the Madelung constant r e f e r r e d to f 0 5 f o r NaN^, a value due to Ryjendahl ( 4 8 ) was used, while f o r the others i t was ca l c u l a t e d from the Madelung energies quoted by Gray and Waddington ( 4 9 ) to whom i s a l s o due a value f o r E the ele c t r o n a f f i n i t y of the azide r a d i c a l . I represents the i o n i s a t i o n p o t e n t i a l of the cation; values are taken from Evans and Y'offe ( 4 . 2 ) Wp of the f i r s t equation, represents the p o l a r i s a t i o n energy change and i s given by a formula due to KLemm ( 5 0 ) Wp = 2.027 e 2 ( c t j +«< 2) v- 4 where ©C ̂  and are the p o l a r i s a b i l i t i e s of the anion and cations. For the cations, the data of Tessman, Kahnand Shockley (51) have been used, and a value, 4 « 4 , f o r the azide anion has been obtained from the formula quoted therein: o £ K - - 3 7 M N 2 - 1 _ 4TT N 2 + 2 where V m i s the c r y s t a l l o g r a p h i c volume (r\)pev molecule of Kw^ and N> i s the r e f r a c t i v e index, given as 1 . 6 6 by Dreyfus and Levy ( 5 2 ) . + and are the i n t e r a c t i o n energies of the neutral atoms, formed as a r e s u l t of the el e c t r o n t r a n s f e r with neighbouring ions and values quoted ( 4 7 ) f o r the corresponding teomides have been used. The r e s u l t s are shown i n Table I I , together with some data f o r the corresponding bromides f o r comparison. I t should be pointed out that while these c a l c u l a t i o n s can f a i r l y accurately be applied to the a l k a l i h a lides, where the error i s u s u a l l y l e s s than 0 . 4 e l e c t r o n v o l t s , Mott -26- and Gurney (53) express doubts whether they would hold f o r other polar c r y s t a l s . The value c a l c u l a t e d f o r NaN^ i s obviously i n er r o r , since from the experimental data i t must be greater than 5 eV; but the value _ r KN^ may well be f a i r l y accurate, since f o r the s i m i l a r KCNS the ex- perimental value i s 6;7 e l e c t r o n v o l t s (54). The c a l c u l a t i o n s do ind i c a t e however that the absorption edge energy should decrease f o r l a r g e r cations. 3. Spectra of azides i r r a d i a t e d at l i q u i d nitrogen temperatures When the a l k a l i metal azides were i r r a d i a t e d at l i q u i d nitrogen and room temperatures, two very d i f f e r e n t s e r i e s of spectra were obtained, as can be seen by comparison of Figures 14-17, 13, and Figures 19-21. The se r i e s were r e l a t e d since on warming samples i r r a d i a t e d at l i q u i d nitrogen temperature to room temperature, the room temperature spectra resulted. As the low temperature spectra are the more explicable i n terms of the a l k a l i halide models, they are considered f i r s t . Inspection of the f i g u r e s 14-17, 18 shows that these spectra have broadly s i m i l a r features; a prominent s i n g l e band near 600 mu, her e i n a f t e r r e f e r r e d to as the "A" band, a prominent (except i n NaN-j) mul t i p l e band peaking near 38O mu, the "B" band, and a vreak band beyond 700 mu which w i l l be c a l l e d the "C" band. ( i ) ÂJJ, band. The measurements of t h i s band are given i n table 3, along with data f o r c e r t a i n a l k a l i h a l i d e "F" bands measured at s i m i l a r temperatures; the correspondence between the tx-ro bands obviously suggests that the "A" band i s the F band of the a l k a l i metal azides. Note i n p a r t i c u l a r that at 612 mu f o r c max the six. coordinate NaN- with the int e r n u c l e a r distance-3.26A0-.-is almost i d e n t i c a l with the at 609 mu and 605 mu f o r s i x coordinate K B E and BSCl, max ' both with internuclear distance 3.29A0. In order to explain some other features of the A band the nature of the F centre, an electr6n trapped at a vacant anion s i t e , must be considered f u r t h e r , several t h e o r e t i c a l studies on such systems are quoted by S e i t z (16). Absorption     -27- Table I I I . Comparison of a l k a l i metal azide A band with a l k a l i halide F band (54fl at v ^ - l 8 0 ° C. S a l t I n t e r i o n i c distance r (A0) X (mu) E (eV) bandwidth (eV) © max max * A Band NaN- 3.26 612 2.03 0.32 KN 3 3.52 563 2.18 Q # 6 o RbN 3 3.68 578 2.I4 0 # 5 0 3.92 592 2.°9 0 . 4 3 CsN 3 F Band NaCl 2.81 455 2.72 0.34 KCl 3.14 548 2.26 0.23 KBr 3.29 609 2.04 0.22 RbCl 3.29 605 2.05 0.22 RbBr 3.43 665 1.86 0.19 -28- of l i g h t by an F centre r e s u l t s i n e x c i t a t i o n of the e l e c t r o n from an s to a p state, the energy of the t r a n s i t i o n being modified by the i n t e r a c t i o n of the el e c t r o n with the surrounding i o n s . This i n t e r a c t i o n r a i s e s the s and p energy l e v e l s and separates them furt h a ; as the p state with i t s l a r g e r radius i n t e r - acts more strongly than the s state; the i n t e r a c t i o n energy depends on the distance and number of the surrounding ions, but only s l i g h t l y on t h e i r nature ( i n the a l k a l i h a l i d e s ) . Now KN^ , RbN^ and CsN^ a l l have the same body-centred tetragonal c r y s t a l l a t t i c e i n which the i n t e r i o n i c distance ( r Q ) increases i n the order given. For an F centre i n such a l a t t i c e , the i n t e r a c t i o n energy would be greatest f o r KN^ and l e a s t f o r CsN^; i t i s therefore expected that the energy of the corresponding F band would decrease i n the same order, as found experimentally f o r the A band. Following Ivey (55) an empirical formula can be deduced r e - l a t i n g the i n t e r i o n i c distance Y « with \_«,„ of the form X = A t , where 9 max max ® A and n are constants; p l o t t i n g l o g r Q against logX^^ gives three points l y i n g almost p e r f e c t l y on a s t r a i g h t l i n e , and the formula: , 0.38 X m a x = 3520 r Q (A 0) with mean e r r o r l e s s than 10A°. For comparison, Ivey quotes: X 1.8A , n N max = 703 TQ U ) the a l k a l i h a l i d e F band at room temperature, with mean erro r of 130A°; the higher power r e f l s c t s the much greater d i s p e r s i o n of the F band. Applyingilrey 1s formula to the s i x coordinate t r i g o n a l NaN0 with r = 3 . 2 6 A 0 ,X. = 6 I 8 5 A 0 , which would move to shorter 3 o ' max 7 wavelengths at lower temperatures, the observed value of 612 mu at l i q u i d nitrogen temperatures i s therefore i n f a i r agreement. Noteworthy i s : the f a c t that .Iyey'sformula does not work well f o r KBr and RbCl with XQ - 3.29A°, which g i v e s = 629 mu; the observed values are 652 and 647 mu r e s p e c t i v e l y . So f a r only the i n t e r a c t i o n of the F e l e c t r o n with neighbouring ions i n t h e i r mean po s i t i o n s has been considered. Such ions, however, o s c i l l a t e s i n tune with the l a t t i c e v i b r a t i o n s or phonons, and therefore t h e i r i n t e r - actions with the F e l e c t r o n w i l l vary i n time. Since the time required f o r an e l e c t r o n i c t r a n s i t i o n i s much shorter than that required f o r one i o n i c v i b r a t i o n (the Franck-Condon p r i n c i p l e ) , the i n t e r a c t i o n e f f e c t on the ob- served t r a n s i t i o n w i l l depend on the instantaneous p o s i t i o n s o f the surround- ing ions and not on t h e i r mean p o s i t i o n . For t h i s reason, the t r a n s i t i o n i s observed not as a single l i n e , but as a band with the form of the Gaussian e r r o r curve; becoming narrower at low temperatures where the l a t t i c e v i b r a t i o n s are weaker. As a measure of the broadening the band width at h a l f height i s quoted i n table I I I . Comparisons of the bandwidths Cf KBr and RbCl (both 0.22eV) with NaN-j (0.32 eV), a l l three s a l t s having approximately the same i n t e r i o n i c d i s t - ance, and the same s i x f o l d coordination number, shows that the l a t t e r has a s i g n i f i c a n t l y greater bandwidth, which may be due to i n t e r a c t i o n s with the v i b r a t i o n s of the nearest azide ions. The much larger bandwidths o f KN^, RbN^ and CsN^ can be ascribed (12) to i n t e r a c t i o n s with a l a r g e r number of i o n i c o s c i l l a t o r s not only with the eight nearest cations, but a l s o with two l o n g i t u d i n a l i o n s . Another feature of these bands i s t h e i r markedly skew character as shown by the f l a t t e r slopes on the high energy .snde's.-.Theextent of t h i s skewness can be roughly measured by comparing the bandwidths on e i t h e r side of Xjnax, the r a t i o high energy: low energy bandwidth i s about 1.2 f o r a l l the azides. The skewness cannot be due to the sluggish response of the Cary recorder, since scanning i n e i t h e r d i r e c t i o n gave the same curve; nor can i t be due to overlap of the B band, since t h i s does not occur strongly i n NaN_. Tompkins and Young .(12) i r r a d i a t e d potassium azide at l i q u i d nitrogen temperatures with U.V. l i g h t and obtained a band s i m i l a r to the A band ; • peaking at 550 mu, which they ascribe- to F centres. They add: 'Limited F-centre -30- formation can also occur at vacant anion s i t e s within the s t r e s s f i e l d s of edge d i s l o c a t i o n s , and these F-centres may absorb at l i q u i d nitrogen temp- eratures at wavelengths s l i g h t l y d i f f e r e n t from 550 mu. The i n i t i a l forma- t i o n of a small concentration of centres which absorb at 535 mu i s ascribed to such an e f f e c t . The centres are formed f i r s t because excitons migrate p r e f e r e n t i a l l y towards d i s l o c a t i o n s , and they are formed i n l i m i t e d extent because vacant anion s i t e s cannot migrate towards d i s l o c a t i o n s under the conditions of i r r a d i a t i o n . 1 A continuous s e r i e s of such centres i n s l i g h t l y d i f f e r e n t environments could account f o r the f l a t t e r slope on the high energy side o f the A band. The absorption c o e f f i c i e n t f o r UV l i g h t i s much greater than that f o r X-rays, and hence r a d i a t i o n damage near the surface of the c r y s t a l w i l l be r e l a t i v e l y more important f o r the former. A l a r g e surface component might pos s i b l y account f o r the discrepancy between the 550 mu of Tompkins and the 568'j. 2 mu of the present work. ( i i ) B band_. The data f o r t h i s band i s summarised i n table TV, from which an Ivey (55) formula can be c a l c u l a t e d f o r the p r i n c i p a l peak i n the higher azides: T , , X = U 7 0 r - 7 1 6 (A°) max o with maximum erro r 10A°. The agreement between formula and experiment: not quite as good as that f o r the A band. The most noticeable feature of t h i s band i s i t s s i m i l a r i t y i n a l l three of the higher azides. There i s a steep low energy and f l a t t e r high energy slope; the shoulders on each side occur at about the same (equal) energy d i f f e r e n c e from the p r i n c i p a l peak, and the t o t a l bandwidth i s about 1 eV. The only di f f e r e n c e s are the s h i f t of the whole band to longer wavelengths from KN^ to CsN^ and the corresponding increasing d i s t i n c t n e s s of the shoulders. The close a s s o c i a t i o n of the shoulders and. peak i s -31- Table IV. Comparison of the a l k a l i metal azide B band with the a l k a l i h a l i d e V^ band (5£) at - 180°C. B Band S a l t High energy shoulder Peak Low energy shoulder KN3 X (mu) 34-0 3— 390 E (eV) 3.65 3.43 3.18 HbN3 X 350 374 400 E 3.54 3.31 3.10 CsN 3 X 365. 390 420 E 3.40 3.18 2.95 X 330 E 3.76 Vj^ Band mixed c r y s t a l s S a l t : KCl HbCl KBr RbBr 3KCl:RbCl 3KCl:2KBr X(mu) 356 359 410 420 359 385. E (eV) 3.48 3.45 3.02 2.95 3-45 3.22 suggested by the s i m i l a r behavior on bleaching with I.R. l i g h t , f i g u r e 17, and a l l three peaks may therefore be due to the same centre. The strong s i m i l a r - i t i e s between the higher azides i n d i c a t e that such a centre i s probably more c l o s e l y connected with the common azide anion than with the cations, and there- fore i s a V type centre i n v o l v i n g trapped p o s i t i v e holes. The most probably a l k a l i h a l i d e model responsible f o r the B band i s the VJL centre, since i t s band i s the only one occurring i n the same sp e c t r a l region; at 356 mu i s KC1 and 410 mu i n KBr, f o r example. Tjje-other V bands are much fur t h e r i n the u l t r a v i o l e t , j u s t on the long wavelength side of the f i r s t fundamental band; the longest, V^, occurs at 254 mu i n KC1 and 275 mu i n KBr. The centre i s a t t r i b u t e d to a hole trapped at a ca t i o n vacancy, and i t s band has a broad simple Gaussian form of bandwidth 0.7eV. There are i n d i c a t i o n s , i n the few a l k a l i halides i n which t h i s centre has been studied, that the p o s i t i o n of the V-̂  band i s strongly characterised by the anion type (see Table TV ( 5 4 ) ) ; not s u r p r i s i n g l y , i f Kanzig's (39) observations that the hold i s l o c a l i z e d on two of the anions bordering the ca t i o n vacancy are cor r e c t . Not explicable as yet, i s the t r i p l e nature of the band, i f indedd the three peaks r e a l l y are due to one centre. The e l e c t r o n i c energy l e v e l s of the tria t o m i c azide i o n or r a d i c a l w i l l be of very d i f f e r e n t character to those o f the monatomic halogen i o n or atom, and i t i s not unreasonable to suppose that such di f f e r e n c e s w i l l be r e f l e c t e d i n the V^ centre i n view of the strong dependence on anion type. Also c r y s t a l structure must have an e f f e c t since the B band of NaN^ i s so very d i f f e r e n t and may be due to another kind of centre. The increasing d i s t i n c t n e s s of.the shoulders from KN^ and CsNj can be a t t r i - buted to the decreasing bandwidths of the component bands, an e f f e c t a l s o noticeable i n the A band. The s h i f t to longer wavelength can be ascribed to an e f f e c t s i m i l a r to that discussed f o r the A band. -33- ( i i i ) C band.. These bands were more d i f f i c u l t to study experimentally since they occurred i n the "I.R." s p e c t r a l region, use of which r e s u l t e d i n consid- erable bleaching. For t h i s reason, they were p l o t t e d separately i n f i g u r e 13, together with the times since the s t a r t of s p e c t r a l recording; f o r NaN^ the band occurred more nearly i n the "VIS", and was therefore p l o t t e d with the r e s t of the spectrum i n f i g u r e I4. The p o s i t i o n and shape o f the C bands i s much l e s s c e r t a i n than those of the other bands, since they are much weaker, and occur as shoulders on the long wavelength side of the A band. Approx- imate p o s i t i o n s are: NaN^ 740 mu; 1.65 eV. Bandwidth 0.15eV KN^ 790 mu; 1.55 eV.. (The d i f f e r e n t shape of the upper curve of f i g u r e 18 i s a t t r i b u t e d to bleaching e f f e c t s ) . RbN^ 820 mu; 1.5 eV. CsN^ 850 mu; 1.45 eV. Very weak; rather doubtful. Of the a l k a l i halide models, the most l i k e l y to account f o r the C band i s the F"' centre; the R^, Rg or M bands, which a l s o occur i n t h i s region o f the spectrum, are not found under the given experimental conditions. Even l e s s has been published on the p o s i t i o n of the F' band than the V^ band; the only f i g u r e known to the w r i t e r being 730 mu f o r KCl, taken from a diagram i n S e i t z (16). That an F'1 band should be formed, and that i t should be weak, i s quite l i k e l y i f the designation of the A band as an F band i s c o r r e c t ; f o r an F"' centre i s formed by the capture of a conduction band e l e c t r o n by an F centre, and therefore, the number o f F1" centres i s always l e s s than, and dependent on the number of F centres. 4. Sfoectra of azides i r r a d i a t e d at room temperature. Since there was no common structure to the spectra at room temperature, except f o r RbN^ and CsN-j, each azide w i l l be considered separately.  -34- ( i ) Sjodium_azide A The room temperature spectrum i s given i n f i g u r e 191 and co n s i s t s of f i v e bands; a broad strong band peaking at 342 mu (3.62 eV) of bandwidth about 1.4 eV, and four weak bands of approximately equal energy separation at about 560 mu (2.22 eV), 630 mu (l.97 eV), 730 mu (1.70 eV) and 860 mu (1.44 eV). There i s some evidence to suggest that the l a t t e r four bands are associated since they were always observed together; f u r t h e r they disappeared together on standing at room temperature at the same time as the 342 mu band also decreased somewhat. Figure 14 TV shows the p a r t i a l l y bleached A band of KaN^ with the 860 mu and 730 mu peaks on i t s long wavelength side; the others, i f present, being masked by the A band. The 730 mu band may be present at lower temperatures i n view of the s l i g h t kink i n f i g u r e 14 H> but i t i s more reasonable to assume t h i s i s an unbleached p o r t i o n o f the 740 mu C band, since no attempt was made to bleach i t completely. At room temperature, these four bands broaden so much that only two of them could be. detected by Rosenwasser, Dreyfus and L e v y ( l l ) at 660 mu and 760 mu; the s h i f t of the f i r s t peak i s confirmed from f i g u r e 19 I I . In marked contrast, i s the behavior of the 342 mu band which i s hardly a f f e c t e d by temperature changes. A po s s i b l e explanation of the four small peaks, accounting f o r t h e i r associated character, i s that they are due to the e l e c t r o n i c v i b r a t i o n spectrum of an unknown constituent, which could be', an azide or impurity X-ray decomposition product (see f u r t h e r section 4(Hi))« Such an e n t i t y would be susceptible to l a t t i c e v i b r a t i o n s thereby accounting f o r the broad- ening observed at room temperatures. The 342 mu band i s considered analogous to the 440 mu (2.82 eV) band of u l t r a - v i o l e t i r r a d i a t e d KN3 observed by Tompkins and Young (12) above 60°C, and ascribed by them t o photoemission by potassium metal i n t o the conduction band of the s a l t ; the potassium metal being i n the form of filaments and l a y e r s so that no c o l l o i d bands are observed. Assuming that the p o s i t i o n of the conduction band i s the same i n both azides, the energy d i f f e r e n c e o f  -35- the observed bands should be approximately equal to the di f f e r e n c e i n the i o n i s a t i o n p o t e n t i a l s of the two metals, which i s ^O.SO - 0.85 eV. The observed energy diff e r e n c e i s 0.80 eV i n very good agreement. I t should also be noted that Kaiser (56) using t h i n f i l m s of KCl condensed at room tempera- ture, and containing excess sodium and potassium metal obtained s i m i l a r bands at 335 mu (3.70 eV) and 438 mu (2.83 eV) giv i n g an energy d i f f e r e n c e o f 0.87 eV. Since the i o n i s a t i o n p o t e n t i a l of the metal does not vary with temperature, the temperature invariance of the observed band can be explained on the assumption that the conduction band does not vary e i t h e r . (On t h i s b a s i s , i t i s predicted that s i m i l a r bands should be found i n (heated) RbK^ and CsW^ at about 466 mu (2.66 eV) and 523 mu (2.37 eV)). (ii) Potassium azide. The room temperature spectrum (fi g u r e 20) of KN^ consists of three peaks; a broad weak one at 34-0 mu not obvious i n spectrum I I , and two stronger ones at 590 mu and 760 mu of which the l a t t e r i s the higher. Tompkins and Young (12) using U.V. i r r a d i a t e d KN^, obtained a peak at 760 mu with structure at 550-570 mu, 725 mu, and 840 mu; of the 550-570 mu shoulder they write: "Unlike the main band, t h i s shoulder i s a f f e c t e d by temperature and i s doubtless due to the presence of r e s i d u a l F-centres. On i l l u m i n a t i o n at room temperature i n the spectrophotometer with F - l i g h t f o r 2 min. t h i s shoulder normally disappears with a r e s u l t i n g general increase i n the (rest of the R~') band, but i n a minority of c r y s t a l s s h i f t s t o 600 mu. This l a t t e r band i s probably due to an impurity". In the present work, no trace of the A>band was obtained at room temper- ature f o r any azide, so " r e s i d u a l F-centres" are u n l i k e l y t o be present under these conditions. The band at 600 mu may be the same as the experimental 590 mu; there i s no evidence to show that the l a t t e r i s due to an impurity. The two strong experimental bands may be due to R centres; the Ivey (5£) " • " I ' I _ J I I ' 1 1 -1 1 • 300 350 1*00 500 600 700 800 900 1000 1500 Figo 20o Irradiated KN̂  spectra at recm tenperature0 (Spectra measured at -196*C0) - 3 6 - formulas f o r the a l k a l i h alides are: ^ = aed 1 - 8 4 (A°) and f o r comparison, ,1 . 8* Ivey (55) attribute© the s i m i l a r powers to the f a c t that a l l these centres F X m a x = W S d ^ (AO) involve halogen vacancies only. Using the azide F centre formulas already obtained: F X m a x = 3520 r (A°) and m u l t i p l y i n g by the appropriate f a c t o r s 8 16/703 and 884/703: h X m a x = ^ ° 8 5 r 0 ° ' 3 8 < A°) \ X m a x = 4430 r O . 3 8 (A<>) For KKj, R^ X . ^ ^ 659 mu and = 714 mu; not i n agreement with the experi- mental bands. I t should be pointed out that Ivey !s formulae r e f e r to room temperature, while the azide F band formula was c a l c u l a t e d at l i q u i d nitrogen temperature. Tompkins and Young (12) a t t r i b u t e t h e i r 760 mu band, which v a r i e d mark- edly with the c r y s t a l , to that composite e n t i t y , the R1' band, by analogy with the work of Scott (35) on KC1; they also t e n t a t i v e l y ascribe, on k i n e t i c data, a 760 mu band obtained by UV i r r a d i a t i o n at l i q u i d nitrogen temperature, to the M centre. I t does not seem possible on.the basis of the present data toieach any d e f i n i t e conclusions as to the nature of the 590 mu and 760 mu bands, except that they are most probably due to ele c t r o n centres of the Mj R type, ( i i i ) P^bidium_and_CaesiuiTi_azides. The room temperature spectra ( f i g u r e 21) of these two azides are q u a l i t a t i v e l y the same, and very d i f f e r e n t from that of KN^. Both consist of a broad strong band i n the u l t r a v i o l e t , shox^ing f i n e structure; the highest peaks occur at 330 mu and 375 mu r e s p e c t i v e l y . 0-OL 30p 3^0 Flgo 21o Irradiated RbN^ & CaN^ spacer* at room temperatureo I RbNj X-Irradiated at room temperature II CsN^ X-Irradiated at room tonparattnroo III CsN 3 x " I r r a d i * t « d at -196*C &. warned to room temperatureo -37- Rubidium azide shows l e s s f i n e structure than CsN^j bands occur at about 6A0, 4.55, 410, 370, 330 and (?) 290 mu, while f o r CsN 3 bands about (?) 670, 560, 4B0, 4-30, 375, 350, 325 and 305 mu can be d i s t i n g u i s h e d . The f i n e structure can be assigned to a v i b r a t i o n a l e l e c t r o n i c t r a n s i - t i o n , but i t i s questionable whether t h i s i s s o l e l y responsible f o r the ob- served absorption. I t could be weak and superimposed on a broad strong band due to a V centre - presumably Vg or since these are the only ones stable at room temperature i n the a l k a l i h a l i d e s . However, both the and bands occur at short wavelengths very close to the f i r s t fundamental band; i n p a r t i c u l a r t h e i r wavelength i s much shorter than that o f the band. So i f the assignation of the B band to a centre i s correct, the peaks around 330 mu and 375 mu are u n l i k e l y to be due to e i t h e r of these centres. The absorption i s not n e c e s s a r i l y due to the same centre i n the two azides, but the f a c t that the peak occurs at longer wavelengths i n CsNj makes t h i s more probable. The v i b r a t i o n a l e l e c t r o n i c t r a n s i t i o n postulated to account f o r the f i n e structure i s d i f f i c u l t to explain except on impurity b a s i s . The responsible agent must contain at l e a s t two atomic n u c l e i ; some pos s i b l e azide decomposi- t i o n products f u l f i l l i n g t h i s condition are Ng11*, Ng, and N^. Ng + i s a very energetic molecule r e q u i r i n g at l e a s t 15.8 eV f o r i t s formation from (57); as such i t i s not l i k e l y to be formed on warming the CsN^ of f i g u r e 1<7 to room temperature. S i m i l a r considerations apply to Np and ; correspondends to a p o s i t i v e hole,- which would be trapped at a l a t t i c e defect to give a V centre, but the B band shows no such f i n e structure. i s equivalent to two p o s i t i v e holes, and would almost c e r t a i n l y dectmpos^ at once to give 3Ng; ordinary would not absorb i n the t h i s s p e c t r a l region. T r i p l e t N^, however, might absorb but would not be stable long enough to give .an absorption spectrum (58); the same applies to t r i p l e t N^"^ • -33- Therefore the e l e c t r o n i c v i b r a t i o n a l t r a n s i t i o n i s ascribed to imp u r i t i e s , of which the most probable are OH" and CO^" . Referencest 1. A l l e n , A.O., E f f e c t s of Radiation on M a t e r i a l s , U.3.A.E.C. l e a f l e t $5DD0-962j May 20, 194-7. 2. Heal, H.G., Can.J.Chem., 1953, 1153. 3. Heal, H.G., Trans. Faraday S o c , 1957, 5 J , 210. 4.. Wischin, A., Proc. Roy.Soc. A, 1939, 172, 314. 5. Thomas, J.G.N, and Tompkins, F.C., Proc. Roy. Soc. A, 1951, 210. 111. 6. Jacobs, P.W.M. and Tompkins, F . C , Froc.Roy.Soc. A, 1952, 215., 265.. 7. Garner, W.E., and Maggs, J . , Proc.Roy. Soc. A, 1939, 172 . 299. 8. Thomas, J.G.N, and Tompkins, F.G., Proc.Roy. Soc. A,1951, 209. 550. 9. Jacobs, P.W.M. and Tompkins, F . C , Proc. Roy. Soc. A, 1952, 215. 254. 10. Groocock, J.M., and Tompkins, F . C , Proc.Roy.Soc. A, 1954, 223.267. 11. Rosenwasser, H., Dreyfus,R.W., and Levy,F.W., J.Chem.Phys., 1956,24,, I84. 12. Tompkins, F . C , and Young, D.A., Proc. Roy. Soc. A, 1956, 236. 10. 1 3 . Goldstein, E., Z e i t s , F . Instfumentkunde., I 8 9 6 , 16,211.Quoted i n reference (15). 1A. Pohl, R.W., Proc. Fnys. Sod, 1937,^9. (extra p a r t ) , 4 . 15. S e i t z , F., Revs .Modern Fhys., 1946, 18, 384. 16. S e i t z , F., Revs. Modern Fhys., 1954, 26, 7. 17. Inorganic Syntheses, Volume I, 78. 18. El-Shami, H.K. and S h e r i f , F.G., Egypt. J.Chem., 1958, 1, 35. 19. Audrieth, L.F., Chem. Revs., 1934, 15., 169. 20. Duval, C , Inorganic Thermogravimetric Analysis, E l s e v i e r , 1953. 21. De Waard, R.H., Proc. Acad. S c i . Amsterdam, 1946, 49_,944« 22. Heal, H.G., Can.J.Chem., 1953, 3Jk, 91. 23. Victoreen, J.A., J . App. Fhys., 1949, 20, I I 4 I . -39- 24. Applied Physics Corporation, Instructions f o r Cary Recording Spectrophoto- meter, Sheet 4» 25. Lea, D.E., Actions of Radiations on L i v i n g C e l l s , C.U.P., 194-6, page 1 2 . •26. Frenkel, J . , Z e i t s . Fnys., 1926 , 3Jj, 652. 27. Schottky, W., and Wagner, C , Z e i t s . Phys. Chem. 1 9 3 0 , 1 1 B . 1 6 3 . 23. Mott, N.F., and L i t t l e t o n , M.J., Trans.Faraday S o c , 1938 , 24, 4 8 5 . 29. S e i t z , F., "Influence of P l a s t i c Flow on the E l e c t r i c a l and Photographic Properties of the A l k a l i Halide C r y s t a l s " , Symposium on the E l a s t i c Deformation of C r y s t a l l i n e S o l i d s , Mellon I n s t i t u t e , P i t t s b u r g , May 1950 , page 3 7 . 30. Mott, N.F., and Gurney, R.W., E l e c t r o n i c Processes i n Ionic C r y s t a l s , O.IT.P. 1948 , page 1 4 7 . 3 1 . Witt, H., Nachs. Akad. Wiss. Gottingen, 1952 , 17. Quoted i n Dekker, A.J., S o l i d State P n j s i c s , P r e n t i c e - H a l l , 1957, page 3 8 I . 32. Duerig, H.,and Markham,J., unpublished r e s u l t s , quoted i n reference (16) page 9. 3 3 . Mott, N.F., and Gurney, R.W., E l e c t r o n i c Processes i n Ionic C r y s t a l s , O.U.P., 1948,page I I 4 . 34. Pick, H., Ann. d. Physik, 1 9 3 3 , 32, 3 6 5 . 3 5 . Scott, A.B., Hrostowifeki, H.J., and Bupp,L.P., Piiys.Rev . , 1 9 5 0 , 72, 346 . 3 6 . Savostianova, M., Z e i t s . Physik, 1 9 3 0 , 64, 262 . 3 7 . Mie, G., Ann. Physik, 1908 , 25_, 3 7 7 . 3 8 . Casler, R., Pringsheim, P., and l u s t e r , F.H., J.Chem.Fnys.,1950, 18, 3 3 7 . 39. Kanzig, W., Fhys. Rev., 1955 , 22, 1 8 9 0 . 40. Dutton, D., and Maurer, R.J., Phys. Rev., 1953 , 22, 1 2 6 . 41. Hendricks, S.B., and Pauling, L., J . Am. Chem. Soc, 1 9 2 5 , £ 7 , 2904 . 42. Evans, B.L., and Yoffe, A.D., Proc. Roy. Soc. A., 1 9 5 7 , 238. 568. 43. Mott, N.F. and Gurney, R.W., E l e c t r o n i c processes i n Ionic C r y s t a l s , O.U.P. 1948,page 1 0 0 . 44. I s e t t i , G., and Neubert, T.J., J . Chem. Fhys., 1957, 26, 337. 45. Debye, P., and Bueche, A.M., J . App. Fhys., 1949, 20, 518. 46. von Hippel, A., Z e i t s . Pnys., 1936, 101, 680. 47. Garner, W.E., Chemistry of the S o l i d State, London, Butter-worths, 1955,page 61. 48. HjzJjendahl, K., K. Dansk. Videns. Selsk. Math-fys. Medd., 1938,16, 2,133. 49. Gray, P., and Waddington, T.C., Proc. Roy. Soc. A., 1956, 235. 481. 50. KLemm, W., Z e i t s . Fhys., 1933, 82, 529. 51. Tessman, J.R., Kahn, A.H. and Shockley, W., Phyg.Rev., 1953, 22, 890. 52. Dreyfus, R.W., and Levy, P.W., Proc. Roy. Soc. A., 1958, 246. 233. 53. Mott, N.F., and Gurney, R.W., E l e c t r o n i c Processes i n Ionic C r y s t a l s , O.U.F. 1948, page 98. 54. Landolt-Bornstein Tables, 6th E d i t i o n , 1954, I, 4> 981. 55. Ivey, H.F., Fhys. Rev., 1947, 72, 341. 56. Kaiser, R., Z e i t s . Phys. 1952, 132. 482. 57. Frost, D.C., and McDowell, C.A., Proc. Roy. Soc. A., 1955, 232.227. 58. Reid, C , Quart. Revs., 1958, 12, 205.

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