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a n o m a l y in the e l e c t r i c a l r e s i s t i v i t y of v a n a d i u m hydride ( m e t a l atoms in a bct a r r a y with c/a ~- 1.1) has been r e p o r t e d ~ at t e m p e r a t u r e s r a n g i n g f r o m 224 to 184 K as the hydrogen c o n c e n t r a t i o n is r e d u c e d f r o m 41.2 to 35.7 at. pct. Hydrogen o r d e r i n g into c e r t a i n of the m a n y p o s s i b l e i n t e r s t i t i a l s i t e s was p o s t u lated as the c a u s e . Such o r d e r i n g has b e e n studied in Nb(Cb)-D a l l o y s 2 and in T a - D alloys a'4 by n e u t r o n d i f f r a c t i o n . The r e s i s t i v i t y m e a s u r e m e n t s , ~ h o w e v e r , could not tell us whether a phase t r a n s i t i o n had o c c u r r e d . If a phase t r a n s i t i o n is c o n c o m i t a n t with the a n o m a l y in r e s i s t i v i t y , and if such a t r a n s i t i o n i n v o l v e s the p o s i t i o n s of v a n a d i u m a t o m s , then these changes should be detected by X - r a y d i f f r a c t i o n , the technique u s e d in this study. The p o s i t i o n s of hydrogen might b e t t e r be a s c e r t a i n e d by s u b s t i t u t i n g d e u t e r i u m for hydrogen and n e u t r o n s for X - r a y s , a study which has b e e n i n i t i a t e d at this L a b o r a t o r y . EXPERIMENTAL A specimen of V-41.2 at. pct H had been prepared for the resistometric study described in Ref. I. Vanadium with a resistivity ratio P300 K/P6 K -~ 27 was allowed to react with pure hydrogen obtained by thermal decomposition of UH3. A piece of this specimen was powdered for use in a Debye-Scherrer camera built in such a way that the specimen could be cooled by nitrogen gas from a liquid nitrogen source. Another piece of the same polycrystalline specimen was used in bulk form in an X-ray diffractometer which was fitted with a c r y o s t a t . RESULTS AND DISCUSSION

tion of d i f f r a c t i o n angle 2 0 for 2 2 5 , 2 1 0 , and 180 K. The i n t e g r a t e d i n t e n s i t y above b a c k g r o u n d is n e a r l y the s a m e at all t h r e e t e m p e r a t u r e s , but the m a x i m u m i n t e n s i t y is much lower at 210 than at 225 K, and line b r o a d e n i n g is o b s e r v e d for the lower of the two t e m p e r a t u r e s . F r o m F i g . 1, one s e e s that 210 K is within the t r a n s i t i o n t e m p e r a t u r e r a n g e . One notes that at 180 K, where the t r a n s i t i o n s e e m s to have b e e n c o m pleted, F i g . 1, the m a x i m u m i n t e n s i t y is only slightly l e s s than at 225 K, and the peak width at half the m a x i m u m i n t e n s i t y is i d e n t i c a l to that at 225 K. A v e r y i n t e r e s t i n g a s p e c t of the d i f f r a c t o m e t r y is that the 002 peak does not b r o a d e n in this t r a n s i t i o n t e m p e r a t u r e r a n g e , Fig. 2(b). However, as the t e m p e r a t u r e is l o w e r e d to 120 K, b r o a d e n i n g of the 002 peak b e c o m e s obvious and it i n c r e a s e s with d e c r e a s i n g t e m p e r a t