Thermodynamic properties and ordering in PdAl
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S O L I D a l l o y s b e t w e e n t r a n s i t i o n m e t a l s of the f i r s t long p e r i o d and a l u m i n u m show e i t h e r a v e r y e x t e n s i v e bcc solid solution of a l u m i n u m in the t r a n s i t i o n m e t a l which m i g h t (vanadium, iron) o r m i g h t not ( c h r o m i u m ) i n cl u d e the e q u i a t o m i c c o m p o s i t i o n o r an e q u i a t o m i c compound with a v e r y wide s o li d s o l u b i l i t y (Ti, Mn, Co, and Ni). The c r y s t a l s t r u c t u r e of TiA1 is o r d e r e d fct of the CuAu ty p e , 1 t h e o t h e r c o m p o u n d s c r y s t a l l i z e in the o r d e r e d cubic CsC1 s t r u c t u r e . The solid solution of a l u m i n u m in i r o n is a l s o o r d e r e d n e a r 50 at. pct (CsC1), t h o s e of a l u m i n u m in c h r o m i u m and v a n a d i u m show no e v i d e n c e of long r a n g e o r d e r n e a r the e q u i a t o m i c c o m p o s i t i o n . In a r e c e n t d i s c u s sion 2 it was shown that the d e g r e e of o r d e r and the s t a b i l i t y of the e q u i a t o m i c c o m p o s i t i o n a r e at a m i n i m u m f o r CrA1 and i n c r e a s e s t e a d i l y with i n c r e a s i n g a t o m i c n u m b e r of the t r a n s i t i o n m e t a l e l e m e n t . A c c o r d i n g to H u m e - R o t h e r y 3 the t r a n s i t i o n m e t a l s do not add e l e c t r o n s to the bonding in t h e s e b i n a r y c o m p o u n d s so that it would s e e m u n l i k e l y that e l e c t r o n i c d i f f e r e n c e s b e t w e e n the t r a n s i t i o n m e t a l s play a l a r g e r o l e in d e t e r m i n i n g t h e extent of o r d e r . Since d i f f e r e n c e s in e l e c t r o n e g a t i v i t i e s a l s o cannot e x p l a i n the o r d e r p a t t e r n , it w a s t e n t a t i v e l y a s s u m e d that the change in o r d e r could be a t t r i b u t e d to a s i z e e f f e c t . F o r a given coordination number the atomic size diff e r e n c e b e t w e e n a l u m i n u m and the t r a n s i t i o n e l e m e n t s of the f i r s t long p e r i o d i n c r e a s e s with i n c r e a s i n g a t o m i c n u m b e r wh i c h should lead to a c o r r e s p o n d i n g i n c r e a s e in o r d e r due to a r e d u c t i o n in s t r a i n e n e r g y on o r d e r i n g . Th e Pd-A1 s y s t e m w a s c h o s e n to t e s t t h i s h y p o t h e s i s , s i n c e at h i g h e r t e m p e r a t u r e s P d A l c r y s t a l l i z e s in the CsC1 s t r u c t u r e and t h e a t o m i c d i a m e t e r of p a l l a d i u m is c l o s e r to that of a l u m i n u m than M. ETTENBERG, formerly Department of Metallurgy and Materials Sciences, New York University, New York, N. Y., is now with RCA Research Laboratories, Princeton, N . J . K . L . KOMAREK, formerly Department of Metallurgy and Materials Sciences, New York University, is now Chairman, Department of Inorganic Chemistry, University of Vienna, Vienna, Austria. E. MILLER, formerly Department of Metallurgy and Materials Sciences, New York University, is
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