BRILLOUIN SCATTERING IN SILVER HALIDES

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Submitted on 1 Jan 1981

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BRILLOUIN SCATTERING IN SILVER HALIDES

T. Hattori, T. Imanishi, H. Kurokawa, A. Mitsuishi

To cite this version:

T. Hattori, T. Imanishi, H. Kurokawa, A. Mitsuishi. BRILLOUIN SCATTERING IN SILVER HALIDES. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-920-C6-922.

�10.1051/jphyscol:19816274�. �jpa-00221562�

JOURNAL DE PHYSIQUE

Colloque C6, suppZ6ment au n o 22, Tome 4 2 , dkcembre 1981

BRILLOUIN SCATTERING IN SILVER HALIDES

T. H a t t o r i , T . Irnanishi, H. Kurokawa and A. Mitsuishi

Department o f Applied Physics, Osaka University, S u i t a , Osaka 565, Japan

A b s t r a c t .- P o l a r i z e d B r i l l o u i n s c a t t e r i n g s p e c t r a i n AgBr and AgCl c r y s t a l s w e r e m e a s u r e d i n t h e t e m p e r a t u r e r e g i o n f r o m room t e m p e r a t u r e t o l i q u i d p h a s e t e m p e r a t u r e a b o v e t h e m e l t i n g p o i n t . The anomaly t h a t t h e a c o u s t i c phonon f r e q u e n c y shows a marked d e c r e a s e a t h i g h t e m p e r a t u r e c a n n o t b e i n t e r p r e t e d by a q u a s i - h a r m o n i c a p p r o x i m a t i o n . The d i s c u s s i o n i s g i v e n on t h e r e l a t i o n b e t w e e n t h e anomalous f e a t u r e s and t h e i n c r e a s i n g F r e n k e l d e - f e c t s .

1. I n t r o d u c t i o n . - I t i s w e l l known t h a t s i l v e r h a l i d e s show an anomaly i n i o n i c c o n d u c t i v i t y [ 1 ] , l a t t i c e c o n s t a n t [ 2 ] , s p e c i f i c h e a t [ 3 ] and o t h e r s a t h i g h t e m p e r a t u r e . The a n o m a l y i n t h e e l a s t i c c o n s t a n t C l l o f AgBr was o b s e r v e d by u s i n g an u l t r a s o n i c p u l s e t e c h n i q u e [ 4 ] . I t i s g e n e r a l l y assumed t h a t t h i s anomaly i s a s s o c i a t e d w i t h F r e n k e l d e f e c t , b u t t h e r e seems t o b e n o d e t a i l e d d i s c u s s i o n . I n o r d e r t o s t u d y t h e anomaly i n t h e c a s e o f AgBr a n d AgC1, p o l a r i z e d B r i l l o u i n s c a t t e r i n g s p e c t r a were m e a s u r e d i n t h e t e m p e r a t u r e r e g i o n from room t e m p e r a t u r e t o l i q u i d p h a s e t e m p e r a t u r e a b o v e t h e m e l t i n g p o i n t .

2 . E x p e r i m e n t . - S p h e r i c a l s i n g l e c r y s t a l s o f AgRr and AgCl were grown by t h e Bridgman method. The o r i e n t a t i o n o f c r y s t a l was d e t e r m i n e d b y t h e o b s e r v a t i o n o f t h e e t c h p i t s and was c h e c k e d by a Laue X - r a y t e c h - n i q u e . The c r y s t a l s were c u t and p o l i s h e d s o t h a t t h e w a v e v e c t o r c a n b e t r a n s f e r e d a l o n g < l o o > o r < 1 1 0 > d i r e c t i o n s . A f t e r p o l i s h i n g t h e c r y s t a l s were a n n e a l e d a t a b o u t 350°C and t h e n s l o w l y c o o l e d t o room t e m p e r a t u r e . The c r y s t a l f o r t h e o p t i c a l m e a s u r e m e n t s was l o c a t e d i n an e l e c t r i c f u r n a c e w i t h f u s e d q u a r t z - w i n d o w s . F o r t h e m e a s u r e m e n t s a t l i q u i d p h a s e t e m p e r a t u r e , t h e powder o f AgBr o r AgCl s e a l e d i n a s m a l l f u s e d q u a r t z - t u b e i n h i g h vaccum was m e l t e d i n t h e f u r n a c e . 3 . R e s u l t s . - T h r e e modes, w h i c h p a r t i c i p a t e i n t h e e l a s t i c c o n s t a n t s C l l , C 4 4 and 7(Cll+C12+2C44), 1 r e s p e c t i v e l y , w e r e o b s e r v e d i n t h e p o l a - r i s e d B r i l l o u i n s c a t t e r i n g s p e c t r a , w h i c h w e r e m e a s u r e d u s i n g a t r i p l e p a s s p r e s s u r e s c a n n i n g F a b r y - P e r o t i n t e r f e r o m e t e r . The f o l l o w i n g t h r e e v a l u e s f o r t h o s e t h r e e modes w e r e o b t a i n e d a s a f u n c t i o n on tem- p e r a t u r e : t h e v a l u e o f t h e B r i l l o u i n s h i f t , t h e h a l f w i d t h o f t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816274

B r i l l o u i n band a n d t h e i n t e n s i t y r a t i o ( I L / I T ) o f t h e l o n g i t u d i n a l a c o u s t i c (LA)

phonon t o t h e t r a n s v e r s e a c o u s t i c (TA) phonon. F i g u r e 1 shows t h e t e m p e r a t u r e d e p e n d e n c e o f t h e t h r e e v a l u e s f o r b o t h Cll and C44 modes o f AgBr. Both v a l u e s o f t h e B r i l l o u i n s h i f t f o r C l l a n d C 4 4 modes d e c r e a s e l i n e a r l y w i t h i n c r e a s i n g t e m p e r a t u r e up t o a b o u t 300°C a n d t h e r e - a f t e r show a marked d e c r e a s e t o m e l t i n g p o i n t . T o g e t h e r w i t h t h e phenomea m e n t i o n e d a b o v e , t h e v a l u e o f I L / I T i n -

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[ l ] J.K.Aboagye a n d R . J , F r i a u f , Phys . R e v . , B s ( 1 9 7 5 ) 1 6 5 4 .

[ Z ] A.W.Lawson, P h y s . R e v . , 2 ( 1 9 5 0 ) 1 8 5 .

[ 3 ] W.Jost a n d P . K u b a s c h e w s k i , Z.Phys.Chem.Neue F o l g e 60 ^(1968)69.

[ 4 ] D. S . T a n n h a u s e r , L . J . B r u n e r a n d A. W.Lawson, Phys .Rev. , 102 ( 1 9 5 6 ) 1276.

[ 5 ] K. F . L o j e a n d D . E . S c h v e l e , J . P h y s . C h e m . S o l i d , 2 ( 1 9 7 0 ) 2 0 5 1 . [ 6 ] F . S e i t z , P h y s . R e v . , 56 ( 1 9 3 9 ) 1 0 6 3 .