HIGH PRESSURE STUDIES OF SmSe AT 77 K

EXAFS s t u d i e s w i t h h i g h s p a t i a l r e s o l u t i o n h a v e been per formed a t t h e Se K-edge e d g e o f SmSe a t 7 7 K a n d up t o 57kbar. An i n c r e a s e i n t h e f l u c t u a t i o n o f t h e Sm-Se bond l e n g t h o f -0.05 A is o b s e r v e d upon g o i n g t h r o u g h t h e i s o s t r u c t u r a l mixed v a l e n t t r a n s i t i o n which is c o n t i n u o u s a n d b e g i n s a t 2 5 kbar . For t h e s e measurements a p r e s s u r e ce l l was c o n s t r u c t e d f o r u s e a t low t e m p e r a t u r e . It is used i n t h e a x i a l geometry. I n t r o d u c t i o n I n t h i s p a p e r h i g h p r e s s u r e xray a b s o r p t i o n s p e c t r o s c o p i c s t u d i e s a r e e x t e n d e d t o low t e m p e r a t u r e s . T h i s p e r m i t s t h e s t u d y of e f f e c t s t h a t d o n o t o c c u r a t room t e m p e r a t u r e a n d , t h r o u g h t h e r e d u c t i o n o f t h e r m a l mot ion h a s t h e a d v a n t a g e o f p r o v i d i n g improved r e s o l u t i o n o f p h y s i c a l phenomena s t u d i e d a t h i g h e r t e m p e r a t u r e . F i r s t l y , we d e s c r i b e t h e h i g h p r e s s u r e c e l l d e s i g n e d f o r low t e m p e r a t u r e . Secondly , we p r e s e n t r e s u l t s on t h e p r e s s u r e i n d u c e d mixed v a l e n c e t r a n s i t i o n i n SmSe. A t room t e m p e r a t u r e SmSe u n d e r g o e s a c o n t i n u o u s v a l e n c e t r a n s i t i o n w i t h p r e s s u r e , showing a l a r g e d e c r e a s e i n volume w h i l e r e t a i n i n g t h e same NaCl s t r u c t u r e [1,2]. T h i s h a s been s t u d i e d a t room t e m p e r a t u r e w i t h xray a b s o r p t i o n s p e c t r o s c o p y [31. The L3-edge o f Sm i n v o l v e s d i p o l e c o u p l e d t r a n s i t i o n s from i n i t i a l 2p s t a t e s t o f l n a l d-symmetry. I n SmSe t h e h i g h d e n s i t y 3 f empty g s t a t e s n e a r t h e Fermi e n e r g y r e s u l t s i n s t r o n g w h i t e l i n e s f o r t h e Sm + and Sm + s t a t e s . The w h i t e l i n e o f Sm +, which c o r r e s p o n d s t o a h i g h e r b i n d i n g e n e r g y , is c h e m i c a l l y s h i f t e d r e l a t i v e t o Sm2+ by +7 eV. By f i t t i n g e a c h peak t o a L o r e n t z i a n convolved w i t h a Gauss ian , t h e e f f e c t i v e v a l e n c e shown i n F ig . 1 is o b t a i n e d a s a f u n c t i o n o f p r e s s u r e . T h i s s t u d y r e v e a l e d two f e a t u r e s . The c o l l a p s e i n volume o c c u r s a t a l o w e r p r e s s u r e t h a n t h e v a l e n c e change . Both t h e Sm L3and S e K-edge EXAFS show t h a t t h e Debye-Waller f a c t o r a12 f o r t h e n e a r e s t ne ighbour s h e l l a p p e a r s t o i n c r e a s e and t h e n d e c r e a s e on g o i n g t h r o u g h t h e t r a n s i t i o n , d e s p i t e t h e l a r g e d e c r e a s e i n n.n. d i s t a n c e RI. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868187 C8-976 JOURNAL DE PHYSIQUE F i g u r e 1. The p r e s s u r e d e p e n d e n c e o f t h e volume ()and v a l e n c e (---l i n SmSe a t room tempera ture . P R E S S U R E ( k b a r ) The s t u d i e s o f SmSe w e r e e x t e n d e d t o low t e m p e r a t u r e t o examine f u r t h e r t h e s e f e a t u r e s . The Se K-edge i s o f p a r t i c u l a r i n t e r e s t b e c a u s e o f t h e enhanceq s p a t i a l r e s o l u t i o n p o s s i b l e . At 77 K t h e S e EXAFS is s t i l l o b s e r v e d a t k = 2 4 8whereas t h e maximum v a l u e a t t h e Sm L3-edge is r e s t r i c t e d t o 12 .5 8 I due t o t h e o n s e t o f t h e LE a b s o r p t i o n edge. The Se K-edge a l s o e n a b l e s o n e t o answer t h e q u e s t i o n w h e t h e r o r n o t i ts Debye-Waller f a c t o r c o n t a i n s a n a d d i t i o n a l component r e f l e c t i n g c h a n g e s i n Sm b a c k s c a t t e i n a m p l i t u d e s d u e t o t h e d i f f e r e n t e l e c t r o n i c c o n f i g u r a t i o n s o f t h e sm5+ :nd sm3+ s t a t e s .