Thermal conductivities of some Ni-S, Ag-S, and Ag-Te melts
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high l e v e l s of i m p u r i t i e s ( n o n s t o i c h i o m e t r y ) . T h e r e i s , however, s o m e evidence to i n d i c a t e that a band gap m a y not be p r e s e n t . Ioffe and Regel have p r e s e n t e d an e x c e l l e n t review of much of the e a r l i e r work in this field. 1 One of their p r i n c i p a l o b s e r v a t i o n s is that i n liquid s e m i c o n d u c t o r s , the c a r r i e r s probably have low m o b i l i t i e s and that u n d e r these conditions the t e m p e r a t u r e dependence of the m o bility may be quite a b n o r m a l . In t h e i r d i s c u s s i o n of the b e h a v i o r of the Cu2Te c l a s s of compounds (Cu2Te, Cu2Se, Ag2Se, Ag2Te), they point out that the c r y s t a l s t r u c t u r e i n v o l v e s a s t a t i s t i c a l d i s t r i b u t i o n of m e t a l a t o m s in i n t e r s t i t i a l s i t e s and the t r a n s p o r t p r o p e r t i e s of the solids a r e t h e r e f o r e much like those of the liquid. R o s e n q v i s t p r e s e n t e d a s i m i l a r view of the p r o p e r t i e s of Ag2S. 9 This a r g u m e n t s u g g e s t s that the band s t r u c ture of the MeX liquids is much like that of the c o r r e sponding s o l i d s , as i s also s u g g e s t e d by much of the c u r r e n t t h e o r e t i c a l work. 1~ Data for the high t e m p e r a ture M2X c r y s t a l s indicate weak m e t a l l i c conduction ~'~1'~ o r s e m i m e t a l b e h a v i o r , and thus no band gap. T h e r e f o r e , u n l e s s i m p u r i t y conduction f r o m n o n s t o i c h i o m e t r y i s invoked, the solid state data s u g g e s t a band model which is somewhat different f r o m the one which has been u s e d to r a t i o n a l i z e the liquid state ~r and S results. U n f o r t u n a t e l y , t h e r m a l c o n d u c t i v i t y data cannot help to e x p e r i m e n t a l l y r e s o l v e this difficulty. E n h a n c e d (ambipolar) e l e c t r o n i c heat t r a n s p o r t i s p o s s i b l e for an i n t r i n s i c s e m i c o n d u c t o r 13 and C u t l e r and Mallon x4 have shown that T1-Te solutions n e a r the composition T12Te exhibit enhanced conduction. U n f o r t u n a t e l y , a m b i p o l a r t r a n s p o r t i s not unique to a b a n d - g a p model. Gallo is has shown that band o v e r l a p can also lead to an a n o m a l o u s l y l a r g e e l e c t r o n i c heat conduction if c a r r i e r s f r o m two bands play a s i g n i f i c a n t p a r t in the conduction p r o c e s s . T h e r m a l conductivity data f o r a composition close to " i n t r i n s i c " may t h e r e f o r e i n d i c a t e that two types of c a r r i e r s a r e a c t i v e , but cannot be used to det e r m i n e whether a band gap i s p r e s e n t or a b s e n t . EXPERIMENTAL T h e r m a l conductivity data were obtained on three s i l v e r - t e l l u r i u m solutions(31.3, 37.7 and 42.3 at. pct Te), one s i l v e r - s u l f u r (32.0 at. pct S) melt and a n i c k e l s u l f u r s o l u t i o n n e a r the e u t
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