Isotope effect on the solvus of the V-H system
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DURrNGa
study ~ of the s t r u c t u r e s of v a n a d i u m d e u t e r i d e s , we b e c a m e a w a r e that d e u t e r i u m is c o n s i d e r ably m o r e soluble than hydrogen in v a n a d i u m . Recently, C a n n e l l i and Mazzolai z r e p o r t e d s i m i l a r findings f r o m i n t e r n a l f r i c t i o n m e a s u r e m e n t s at s u b a m b i e n t t e m p e r a t u r e s . H a r d c a s t l e and Gibb 3 have p u b l i s h e d a phase d i a g r a m for the V-D s y s t e m as d e t e r m i n e d by X - r a y diff r a c t i o n . The solvus above r o o m t e m p e r a t u r e is d e f i n i t e l y shifted to higher c o n c e n t r a t i o n s than the solvus in M a e l a n d ' s 4 V-H d i a g r a m . The s t r u c t u r e s of the h y d r i d e s and d e u t e r i d e s a r e not c o m p l e t e l y analogous. For e x a m p l e , at r o o m t e m p e r a t u r e V4D3 does not exhibit the l a r g e t e t r a g o n a l d i s t o r t i o n of the m e t a l l a t t i c e that has b e e n r e p o r t e d for V4Hs. ' Another study, by Wiswall and Reilly, 5 shows that VD2 is m o r e s t a b l e than VH2. The d i f f u s i v i t l e s of H and D a r e not r e l a t e d by the factor ~ in V, Nb, o r Ta; 8 thus the o b s e r v e d effects cannot be a s c r i b e d s i m p l y to the m a s s e s of the i s o topes. In fact, Sicking 7 r e p o r t s that d e u t e r i u m diffuses f a s t e r than h y d r o g e n in p a l l a d i u m at r o o m t e m p e r a t u r e . It can be concluded that the s u b s t i t u t i o n of d e u t e r i u m for hydrogen in m e t a l - h y d r o g e n a l l o y s has s e v e r a l s i g n i f i c a n t and unexplained effects. The p u r pose in the p r e s e n t study was to make a q u a n t i t a t i v e d e t e r m i n a t i o n of the V-D solvus over a wide r a n g e of c o n c e n t r a t i o n s with the r e s i s t o m e t r i c technique a l r e a d y used s u c c e s s f u l l y for the V-H s y s t e m . 3'9
of F r e o n 11. The t e m p e r a t u r e of the bath was m e a s u r e d with a c a l i b r a t e d p l a t i n u m r e s i s t a n c e t h e r m o m eter. RESULTS AND DISCUSSION Plots of e l e c t r i c a l r e s i s t a n c e vs t e m p e r a t u r e exhibited a d i s c o n t i n u i t y in slope when the f o r m a t i o n of d e u t e r i d e was i n i t i a t e d upon cooling. T h e r m a l h y s t e r e s i s was o b s e r v e d and its magnitude i n c r e a s e d with i n c r e a s ing d e u t e r i u m c o n c e n t r a t i o n . For e x a m p l e , a l m o s t no h y s t e r e s i s was o b s e r v e d for c o n c e n t r a t i o n s below 2 at. pct D, but at 4.36 pct t h e r e was a h y s t e r e s i s of about 4 K, and for 27.44 at. pct D, about 13 K. The l a t t e r is g r e a t e r than the h y s t e r e s i s o b s e r v e d in the V-H s y s t e m at about the s a m e c o n c e n t r a t i o n (Ref. 9, Fig. 1). Scholtus and Hall 10 suggested that the t h e r m a l h y s t e r e s i s in plots of h y d r o g e n p r e s s u r e v e r s u s c o m
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