The solubility of hydrogen in transition metals
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T H E R E has long been a c o n t r o v e r s y r e g a r d i n g the nature of hydrogen d i s s o l v e d in t r a n s i t i o n m e t a l s in the c h e m i c a l l i t e r a t u r e s . F o r the l a s t two d e c a d e s a l a r g e amount of e x p e r i m e n t a l data have been a c c u m u lated, but s u r p r i s i n g l y l i t t l e in the way of a unifying p r i n c i p l e has e m e r g e d . The d i s c u s s i o n has c e n t e r e d around questions as to whether hydrogen is p r e s e n t as H ยง ions (protonic model) o r I-l- ions (anionic model) and whether c h e m ical bonding effects a r e important. In the anionic model the a s s u m p t i o n is made that e l e c t r o n s a r e t r a n s f e r r e d f r o m the conduction band of the metal to the hydrogen, and in the protonic model the oppos i t e a s s u m p t i o n is made. Recent developments in the t h e o r y of t r a n s i t i o n m e t a l s and the advent of new t e c h niques such as NMR have provided much new i n f o r m a tion about the nature of d i s s o l v e d hydrogen in m e t a l s . It a p p e a r s , however, that both m o d e l s a r e c o n s i s t e n t with NMR, magnetic s u s c e p t i b i l i t y , and other m e a surements. In this p a p e r , we would Iike to r e p o r t that t h e r e e x i s t s a c o r r e l a t i o n between the solubility of hydrogen and the difference in energy between the lowest e n e r g y l e v e l s of the dn-2sZ (or d n) configuration and the dn'ts configuration in the n e u t r a l gaseous atoms. This c o r r e l a t i o n p r o v i d e s a m e a n s of checking s o m e of the p r e v i o u s l y r e p o r t e d solubility data, and a b a s i s from which s o m e of t h e s e solubility d a t a can be e s t i m a t e d . Also, it provided new information on the nature of d i s solved hydrogen in t r a n s i t i o n m e t a l s . 1. METHOD OF CORRELATION We c o n s i d e r the s o l u b i l i t y of hydrogen in a m e t a l and w r i t e the equation of solution in the f o r m 89Ha -- HM
[1]
where a s u b s c r i p t M r e f e r s to hydrogen d i s s o l v e d in the metal. F r o m s t a n d a r d t h e r m o d y n a m i c c o n s i d e r a tions, we can e a s i l y deduce S i e v e r t ' s Iaw for the concentration of d i s s o l v e d hydrogen
HM = exp (- AG~
z
[2]
with AG~ = AH~ - T&S~ being the s t a n d a r d f r e e energT/ change for the r e a c t i o n [1]. To a good approximation, the solubilitv can be e x p r e s s e d as H, M. LEE, f o r m e r l y w i t h the D e p a r t m e n t o f Materials Science and E~agineering, N o r t h w e s t e r n University, E v a n s t o n , IL, is n o w w i t h A t o m i c s I n t e r n a t i o n a l Division, R o c k w e l l I n t e r n a t i o n a l , C a n o g a Park, CA 9 1 3 0 4 . Manuscript s u b m i t t e d S e p t e m b e r 4, 1974, METALLURGICAL TRANSACTIONS A
-
AH ~ + T~,S ~
[3]
Iog [HM] = - - ~ : ~ b 3 R T
when PH~ = 1 atm and the s o l u b i l i t y of hydrogen in m e t a l follows H e n r y ' s taw. The r e l e v a n t e x p e
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