Standard free energy of formation of tin telluride
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Communications Standard Free Energy of Formation of Tin Telluride P . RAVINDAR, G. M, MEHROTRA, AND V. B. TARE T h e r m o d y n a m i c p r o p e r t i e s of t i n t e l l u r i d e , the only s t a b l e compound in the S n - T e s y s t e m , have b e e n i n v e s t i gated by a n u m b e r of workers.l-8 The s t a n d a r d f r e e e n e r g y of f o r m a t i o n of this compound has b e e n d e t e r m i n e d , in m o s t of the c a s e s , by u s i n g the data obtained f r o m c a l o r i m e t r i c m e a s u r e m e n t s . The d i r e c t d e t e r m i n a t i o n of this p r o p e r t y by emf method u s i n g m o l t e n s a l t e l e c t r o l y t e has been in the lower t e m p e r a t u r e r a n g e (533 to 668 K) ~ only. In the p r e s e n t work, u s i n g g a l v a n i c cells with c a l c i a s t a b i l i z e d z i r c o n i a (CSZ) a s the solid e l e c t r o l y t e , the s t a n d a r d f r e e e n e r g y of f o r m a t i o n of SnTe has been d e t e r m i n e d in the t e m p e r a t u r e r a n g e 930 to 1030 K. The cell used had the following s c h e m e : Pt,Air//ZrO~(+ CaO)//SnWe,Te,SnOJW. T i n and t e l l u r i u m were of 99.99 pet p u r i t y while SnO2 was 99.5 pet p u r e . The compound SnTe was p r e p a r e d by m e l t i n g t o g e t h e r the c o n s t i t u e n t s in the r e q u i r e d r a t i o in i n e r t a t m o s p h e r e . The e l e c t r o l y t e was in the f o r m of one end closed CSZ tubes, 400 m m long and 5 m m ID. The m i x t u r e of SnTe, T e and SnO2 was filled into the CSZ tube up to a height of about 50 m m . P u r i f i e d a r g o n gas flowed through the CSZ tube at about 20 m l / m i n . The cell a s s e m b l y was placed in a K a n t h a l - w o u n d v e r t i c a l f u r n a c e which could be cont r o l l e d to +2~ The m e a s u r e m e n t s were c a r r i e d out in the t e m p e r a t u r e r a n g e 930 to 1030 K. As the v a p o r p r e s s u r e of t e l l u r i u m is v e r y high (10 -2 a t m at 890 K), it was found n e c e s s a r y to add t e l l u r i u m into the z i r conia tube so a s to e n s u r e unit a c t i v i t y of t e l l u r i u m in the e l e c t r o d e m i x t u r e . When this was not done, the cell emf r e m a i n e d steady for s o m e t i m e , d r i f t e d a f t e r w a r d s and b e c a m e steady a g a i n but at a d i f f e r e n t v a l u e . Addition of t e l l u r i u m at this stage r e s t o r e d the e m f to o r i g i n a l steady value. T h e r m a l emf g e n e r a t e d by W - P t j u n c t i o n was m e a s u r e d in p u r i f i e d a r g o n a t m o s p h e r e in a s e p a r a t e e x p e r i m e n t . The e m f v a r i e d a p p r o x i m a t e l y l i n e a r l y f r o m 8.3 to 10.4 mV as the t e m p e r a t u r e v a r i e d f r o m 930 to 1030 K. The cell emf, a f t e r applying c o r r e c t i o n for t h e r m a l e m f of W - P t junction, is plotted in F i g . 1 as a function of t e m p e r a t u r e . In the t e m p e r a t u r e r a n g e of i n v e s t i gation, the emf (mV) v s
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