Partial excess entropies of hydrogen in palladium alloys
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The obvious d i f f e r e n c e b e t w e e n the two s i t u a t i o n s is that in the c a s e of the alloys the c o n f i g u r a t i o n a l e n t r o p y c o n t r i b u t i o n to AS~s has not b e e n p r o p e r l y evaluated; i . e . , /3 r 1 and the o c t a h e d r a l i n t e r s t i c e s cannot be c o n s i d e r e d to be e q u i v a l e n t for the alloys. S t a t i s t i c a l m e c h a n i c a l m o d e l s for h y d r o g e n a b s o r p t i o n by t h e s e alloys have g e n e r a l l y i g n o r e d t h i s p o s s i b i l i t y . 6-8 Models for these alloys have b e e n f o r m u l a t e d f r o m the model for p a l l a d i u m 9-n which has b e e n extended to account for the d - b a n d f i l l i n g of p a l l a d i u m if the s u b stituted m e t a l has a filled d - b a n d and e x t r a s - e l e c t r o n s , s'7,lz Allard, et al az have r e c o g n i z e d the p r o b l e m that/3 m a y not be 1 and have e x p l o r e d the p o s s i b i l i t i e s that/3 = f r a c t i o n of p a l l a d i u m atoms in the a l loy, but definite r e s u l t s were not obtained to indicate the c o r r e c t n e s s of this choice. Burch ls has argued for A g / P d alloys that s u b s t i t u t i o n a l s i l v e r r e d u c e s the a b i l i t y of p a l l a d i u m to a b s o r b h y d r o g e n and the following r e l a t i o n s h i p should be s a t i s f i e d : H/Pal + x A g / P d = Const. [2] He found that a c o n s t a n t value of x could be obtained b y e x a m i n i n g the h y d r o g e n s o l u b i l i t i e s in t h e s e alloys in the r e g i o n of l a r g e s o l u b i l i t i e s . T y p i c a l v a l u e s for/3 obtained by B u r c h a r e 0.54 (Ag (30 p c t ) / P d ) and 0.41 (Ag (40 p c t ) / P d ) . F a s t 14 has noted p r e v i o u s l y f r o m the t r e n d s with s i l v e r c o n t e n t within the v a l u e s of AS~IS for the A g / P d a l loys that not all of the i n t e r s t i c e s m a y be i d e n t i c a l . He points out however, that two facts appear to c o n t r a d i c t t h i s : i) hydrogen d i s s o l v e s e n d o t h e r m i c a l l y in p u r e s i l v e r and only to a v e r y s m a l l extent, and 2) the d e c r e a s e in e n t r o p y with s i l v e r content is too l a r g e to b e a c counted for by the p r e f e r e n t i a l occupation of c e r t a i n i n t e r s t i c e s . He s u g g e s t s t h e r e f o r e that in addition to the fact that/3 r I, v i b r a t i o n a l changes of the i n t e r s t i tial hydrogen with s i l v e r content occur. Since the p u b l i c a t i o n of his book 14 it has b e e n shown by G a l l a g h e r and Oates 15 and Chowdhury and Ross 16 that the v i b r a t i o n a l f r e q u e n c i e s of the hydrogen atoms a r e unaffected in t h e s e alloys up to Ag (30 p c t ) / P d . His two o b j e c t i o n s to c o n f i g u r a t i o n a l effects c a u s i n g all of the changes of AS~IS with s i l v e r content do not a p p e a r to be too s e r i o u s except they do s u g g e s t that the configu r a t i o n a l
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