The crystal structures of Hf 3 N 2 and Hf 4 N 3
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e x i s t e n c e of p h a s e s i n t e r m e d i a t e in c o m p o s i t i o n b e t w e e n the m o n o n i t r i d e and the ~-Hf s o l i d solution in the Hf-N s y s t e m was f i r s t n o t i c e d in the c o u r s e of an i n v e s t i g a t i o n of the t e r n a r y alloy s y s t e m H f - B - N f Although not p u r s u e d in d e t a i l at that t i m e , the p r o nounced s i m i l a r i t y b e t w e e n the unknown d i f f r a c t i o n p a t t e r n s and those of the s t r u c t u r a l l y c h a r a c t e r i z e d n e i g h b o r i n g p h a s e s l e d us to b e l i e v e that the s a m e b u i l d i n g p r i n c i p l e , n a m e l y c l o s e s t packing of the m e t a l a t o m s , was a l s o i n v o l v e d in the f o r m a t i o n of the c r y s t a l l a t t i c e s in the newly d i s c o v e r e d p h a s e s . The two p r i n c i p a l t y p e s of c l o s e p a c k i n g , h e x a g o n a l c l o s e p a c k i n g a s in ~ - H f and cubic c l o s e packing of the m e t a l a t o m s a s in HfN, can be c o m b i n e d to y i e l d an infinite n u m b e r of d i f f e r e n t , but c l o s e l y r e l a t e d , s t r u c t u r e s with i d e n t i c a l packing d e n s i t i e s . The a d d i t i o n a l a l l o y i n g e l e m e n t s m a y then be r e g a r d e d a s r e s p o n s i b l e for s i n g l i n g out p a r t i c u l a r s t a c k i n g c o m b i n a t i o n s to p r o v i d e a f a v o r a b l e , and thus m o r e s t a b l e , c o n f i g u r a t i o n for d i s t r i b u t i n g the i n t e r s t i t i a l a t o m s a m o n g the o c t a h e d r a l i n t e r s t i c e s in the m e t a l l a t t i c e . E x a m p l e s of such s t r u c t u r a l i n t e r m e d i a t e s a r e known in MosC2 3 and Ta2VC2_x. 4 In the f o r m e r , the c l o s e p a c k e d m e t a l l a y e r s in the unit c e l l a r e s t a c k e d a c c o r d i n g to a sequence (cch)z,* o r ABCACB; in Ta2VCe_ x *h = stackingas in hexagonalclosepackmg;c = staclongas in cubm closepackhag. the s t a c k i n g s e q u e n c e i s 0ahc)3, o r ABABCBCAC. The expected close structural relationships between carb i d e s a n d n i t r i d e s and the a v a i l a b i l i t y of a d d i t i o n a l s a m p l e m a t e r i a l , p r e p a r e d in conjunction with a study of the H f - T a - N s y s t e m , s p r o m p t e d a n o t h e r a t t e m p t to d e t e r m i n e the s t r u c t u r e f r o m powder p h o t o g r a p h s . The p o s s i b i l i t y of obtaining single c r y s t a l s f o r the s t r u c t u r e d e t e r m i n a t i o n a p p e a r e d r e m o t e in v i e w of the l i m i t e d high t e m p e r a t u r e s t a b i l i t y of t h e s e n i t r i d e s and t h e i r slow r a t e of f o r m a t i o n . EXPERIMENTAL The r a w m a t e r i a l s u s e d in the p r e p a r a t i o n of the e x p e r i m e n t a l a l l o y s w e r e hafnium and h a f n i u m m o n o nitride. Hafnium m e t a l was p u r c h a s e d in p o w d e r f o r m f r o m ERWIN RUDY, formerly with Aerojet-Gener
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