Study of filled Ti 2 Ni-type phases with hafnium, tantalum, and tungsten
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F i l l e d Ti2Ni type (77) p h a s e s have been r e p o r t e d in the past and p r e s u m e d to e x i s t in the s y s t e m s H f - N i - O , T a - N i - O , and W - N i - O . An i n v e s t i g a t i o n of 1000~ i s o t h e r m a l s e c t i o n s in these t h r e e s y s t e m s shows that only the Hf-Ni oxide r e p o r t e d by Nevitt, Downey, and M o r r i s is stable. It has an H f - t o - N i r a t i o of 2:1 and an oxygen content of 4 to 8 at. pct. Its lattice constant v a r i e s from 12.135 to 12.108/~ and d e c r e a s e s with i n c r e a s i n g n i c k e l content. In T a - N i - O and W - N i - O the 77 phase is s t a b i l i z e d when n i t r o g e n i s a c c i d e n t a l l y picked up d u r i n g s a m p l e p r e p a r a t i o n . The s y s t e m s F e - W - O and C o - W - O , which were r e p o r t e d to have 77 oxides, only show b i n a r y phases at 1000~ New p h a s e s found in this i n v e s t i g a t i o n a r e ~Hf4Ni2N with the 77 c a r b i d e s t r u c t u r e , and Hfv4Ni2402 and Hf74Nie4N2 of i d e n t i c a l but unknown s t r u c t u r e .
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is now e s t a b l i s h e d that the s t r u c t u r e s of the filled Ti2Ni types and the 7/ c a r b i d e s a r e i d e n t i c a l x'2 and t h e r e f o r e the s y m b o l 77 will be used to designate this type even when the s t a b i l i z i n g m e t a l l o i d atom is not c a r b o n . P e r h a p s the f i r s t s u g g e s t i o n that oxygen is an 77 s t a b i l i z e r is due to K a r l s s o n 3 who e x p l a i n e d the occ u r r e n c e of 77 in the T i - C u s y s t e m on this b a s i s . Subsequent work by R o s t o k e r , 4 Kuo, 5 Sch6nberg, ~,7 and Nevitt 8 c o n f i r m e d the role of oxygen in s t a b i l i z i n g 7/ in m a n y s y s t e m s . The p r e s e n t study was begun to d e t e r m i n e the c o m p o s i t i o n of the 77 oxides r e p o r t e d in H f - N i - O , 8 T a - N i - O , 9 and W - N i - O . 6 The a b s e n c e of stable oxides in the l a t t e r two s y s t e m s u n d e r our exp e r i m e n t a l conditions p r o m p t e d an i n v e s t i g a t i o n of the influence of n i t r o g e n c o n t a m i n a t i o n . It also s u g gested a r e v i e w of the s t a t u s of the 77 oxides r e p o r t e d for the r e l a t e d s y s t e m s W - F e - O and W - C o - O . 6
EXPERIMENTAL
PROCEDURE
The samples were prepared from powders of the following e l e m e n t s (purity in wt pct): Hf (97.3; 2.5 Zr), Wa (99.8), W (99.9), Fe (99.4), Co (99.9), and Ni (99.5). Oxygen additions were made a s p o w d e r s of HfO2 (99.9) in Hf-Ni-O, NiO (99.7) in T a - N i - O and W - N i - O , as Fe2Os (99.9) in W - F e - O , and a s CosO4 (99.8) in W - C o - O . In the s y s t e m s H f - N i - O , T a - N i - O , a n d W - N i - O 3-g s a m p l e s were weighed, thoroughly mixed, and p r e s s e d into p e l l e t s without the use of b i n d e r s or l u b r i c a n t s . P e l l e t s were then p r e s i n t e r e d at 1000~ in a c l o s e d tube of fused s i l i c a e v a c u a t e d to 10 -4 t o
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