Phase transformation in the oxides
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the s t u d i e s of p h a s e t r a n s f o r m a t i o n , o x i d e s p r e s e n t themselves as excellent prototype compounds. Various t y p e s of p h a s e t r a n s f o r m a t i o n s e x i s t in o x i d e s and a n u m b e r of t h e m m a y be found in the s a m e p r o t o t y p e s t r u c t u r e . This p a p e r p r i m a r i l y d e a l s with s o m e a s p e c t s of the p h a s e t r a n s i t i o n s in o x i d e s . As a c o u n t e r point to the s t u d i e s of p h a s e t r a n s f o r m a t i o n in m e t a l l i c s y s t e m s , the study of oxide s y s t e m s p r o v i d e s the f o l lowing d i s t i n c t i v e f e a t u r e s : 1) The p r e s e n c e of oxygen a t o m s in the o x i d e s d i s t i n g u i s h e s the two t y p e s of l a t t i c e s i t e s , n a m e l y , c a t i o n and anion. As a r u l e t h e s e two t y p e s of s i t e s do not i n terexchange. 2) Oxygen a t o m s can be c o n v e n i e n t l y u s e d a s b a s e s of r e f e r e n c e f o r the oxide s t r u c t u r e s . F o r e x a m p l e , the p l a n e s c o n t a i n i n g oxygen a t o m s can be a p p r o x i m a t e d a s l a y e r s of h e x a g o n a l c l o s e - p a c k i n g . The v o i d s in the c l o s e s t - p a c k i n g c o n s t i t u t e the p o s s i b l e c a t i o n s i t e s . A n o t h e r f o r m of r e p r e s e n t a t i o n f o r the oxide s t r u c t u r e is the p a c k i n g of c o o r d i n a t i o n p o l y h e d r a , with the oxygen a t o m s o c c u p y i n g the a p e x e s of the p o l y h e d r a and the c a t i o n s o c c u p y i n g the c e n t e r s of the p o l y h e d r a . 3) With only r a r e e x c e p t i o n s , the n e a r e s t n e i g h b o r s to a c a t i o n a r e anions, such a s oxygen a t o m s a r o u n d the m e t a l a t o m in an o x i d e . S t r u c t u r a l l y , anion p o s i tions a r e , t h e r e f o r e , e a s i l y a f f e c t e d b y d i f f e r e n t c a t ions. D i f f e r e n t c a t i o n s can c a u s e s i g n i f i c a n t l o c a l s t r u c t u r a l d i s t o r t i o n s although a v a r i e t y of c a t i o n s can e a s i l y be a c c o m m o d a t e d in any one of the cation sites. 4) C a t i o n s in the o x i d e s p o s s e s s definite v a l e n c e s t a t e s . V a l e n c e s t a t e s , in turn, g o v e r n the s e l e c t i o n of c a t i o n s i t e s f o r o c c u p a n c y . In the following p a g e s , a l i m i t e d n u m b e r of t o p i c s
FRANKLIN F. Y. WANG and KEDAR P. GUPTA are Professor and Graduate Student, respectively, Materials Science Department, State Univeristy of New York, Stony Brook, N.Y. 11790. This paper is based on a presentation made at a symposium on "Phase Transformations in Less Common Metals: A Dialogue," held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals. METALLURGICAL TRANSACTIONS
c o n c e r n i n g p h a s e t r a n s f o r m a t i o n in oxide
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