Point defects and thermodynamic properties in CsCl-type intermetallic compounds
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h a s b e e n s h o w n ~ '2 t h a t f o r a n i n t e r m e t a l l i c c o m p o u n d , A B I . 6 , the r e l a t i o n s h i p s b e t w e e n the a c t i v i t y of the B c o m p o n e n t , a B , a n d t h e d e v i a t i o n f r o m s t o i c h i o m e t r y , 5 , d e p e n d u p o n the t y p e s o f p o i n t d e f e c t s e x i s t i n g i n the l a t t i c e . A s s u m i n g t h a t a p r e p o n d e r a n c e of o n e type of d e f e c t i s p r e s e n t a t l a r g e d e v i a t i o n s f r o m s t o i c h i o m e t r y , the f o l l o w i n g r e l a t i o n s h a v e b e e n d e rived: ~ F o r A v a c a n c i e s : a B = CILIA (1+ - - ~ ) For B interstitials:
aB=CIB(ah--~_5)
[1] [2]
F o r B s u b s t i t u t i o n a l s ( B a t o m s on A s i t e s ) : a B = C B (A)
[ 3]
For B vacancies:
a B = C [ ~ B (1_~+66 )
[4]
F o r A i n t e r s t i t i a l s : a B = - C IA [ a ( l + 5)
+ ~]~'/~[~(1+ ~)] ~
[5]
F o r A s u b s t i t u t i o n a l s (A a t o m s on B s i t e s ) : a B = 2CA(B) (1+6)/[(2 + 6)(--6)] ~-
700°K. The r e s u l t s of c o m p a r i n g E q s . [ l ] t o [3] w i t h O l a n d e r ' s activity d a t a at positive deviations f r o m s t o i c h i o m e t r y ( e x c e s s c a d m i u m ) a r e s h o w n in T a b l e I. E a c h e n e r g y p a r a m e t e r was c a l c u l a t e d f r o m e a c h e x p e r i m e n t a l p o i n t f r o m Eqs. [l], [2], and[3]. If the p r o p e r f u n c t i o n of 5 i s u s e d , the c o r r e s p o n d i n g p a r a m e t e r s h o u l d not c h a n g e w i t h 5 . It c a n be s e e n f r o m T a b l e I t h a t a l t h o u g h C [ ] A u a n d C ICd v a r y w i t h 5 , C Cd(Au) d o e s n o t . T h u s , it a p p e a r s that the p r e d o m i n a n t d e f e c t p r e s e n t in A u C d c o n t a i n i n g e x c e s s c a d m i u m i s the c a d m i u m s u b s t i t u t i o n a l d e f e c t ( c a d m i u m o n gold sites). A s i m i l a r analysis for negative deviations from s t o i c h i o m e t r y u s i n g E q s . [4] t o [6] y i e l d s t h e r e s u l t s shown in Table II. Although the experinaental scatter appears to be greater in this case, C Au(Cd) is constant with 6. Consequently, it appears that gold substitutionals are the predominant defects. At the stoichiometric composition (5 =0), gold substitutionals and cadmium substitutionals are present i n e q u a l c o n c e n t r a t i o n s . T h u s , the intrinsic de.fecl in s t o i c h i o m e t r i c A u C d i s a g o l d a t o m on a c a d m i u m s i t e p l u s a c a d m i u m a t o m on a g o l d s i t e . T h i s i s u s u a l l y r e f e r r e d to as a n a n t i s t r u c t u r e d e f e c t , s T h e i n t r i n s i c d e f e c t c o n c e n t r a t i o n , ni, o r t h e c o n c e n t r a t i o n of a n t i s t r u c t u r e d e f e c t s a t the s t o i c h i o m e t r i c c o m p o s i t i o n
[6]
The terms C[~A, CIB, and so forth, are temperature dependent parameters containing the free energy of formation of the defect; and a represents the number of interst
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