The concentration-dependent diffusion of chromium in nickel oxide

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N I C oxide, K NiO, E isLknownOto beUa m eSt a l d e f i cit, p - t y p e s e m i c o n d u c t o r 1-2 with v a c a n c i e s in the n i c k e l s u b l a t t i c e and e l e c t r i c a l c o m p e n s a t i o n by p o s i tive h o l e s o r Ni 3§ ions. The d e g r e e of i o n i z a t i o n of the c a t i o n v a c a n c i e s h a s b e e n a s u b j e c t of d e b a t e in the l i t e r a t u r e . B a u m b a c h and C. W a g n e r , 3 E r o r and J. B. 1/4 d e W a g n e r , 4 and B r a n s k y and T a l l a n 5 have found a P~2 p e n d e n c e of the e l e c t r i c a l c o n d u c t i v i t y which i n d i c a t e s that e i t h e r s i n g l y i o n i z e d c a t i o n v a c a n c i e s o r e x c e s s i v e h i g h e r v a l e n t i m p u r i t i e s a r e the p r e d o m i n a n t d e f e c t s in NiO. In a g r e e m e n t with the m o d e l f o r doubly i o n i z e d ,91/8 d e p e n d e n c e of v a c a n c i e s , Mitoff 8 h a s r e p o r t e d a --02 the c o n d u c t i v i t y of NiO. M e i e r and Rapp ~ have o b s e r v e d the c o n d u c t i v i t y to be p r o p o r t i o n a l to p~)/n w h e r e n a p p r o a c h e d 6 at the h i g h e s t t e m p e r a t u r e s for s p e c i m e n s of the h i g h e s t p u r i t y and n a p p r o a c h e d 4 for l o w e r t e m p e r a t u r e s and l e s s p u r e s p e c i m e n s . T h e s e r e s u l t s w e r e i n t e r p r e t e d to i n d i c a t e that doubly i o n i z e d c a t i o n v a c a n c i e s a r e the p r e d o m i n a n t d e f e c t s in s u f f i c i e n t l y p u r e NiO at l e a s t at t e m p e r a t u r e s above 850~ T r i p p and T a l l a n 8 have r e p o r t e d a p ~ 52 d e p e n d e n c e for the c o n d u c t i v i t y . They i n t e r p r e t e d t h i s r e s u l t a s i n d i c a t i n g that the c a t i o n v a c a n c i e s a r e doubly i o n i z e d but that the i m p u r i t y c o n c e n t r a t i o n of t h e i r s p e c i m e n s was too l a r g e to o b s e r v e the p~!8 d e p e n d e n c e which would be expected. The d e f e c t equation d e s c r i b i n g the f o r m a t i o n of d o u b l y i o n i z e d v a c a n c i e s and c o m p e n s a t i n g p o s i t i v e h o l e s (or Ni 3§ ions) in p u r e NiO is w r i t t e n a s I02(g)

~ O o + Y~i + 2h'

[1]

with the e q u i l i b r i u m c o n s t a n t gI

-

[Y~i]P

2

[21

RICHARD A. PERKINS is currently a Presidential Intern in the Metals and Ceramics Division at Oak Ridge National Laboratory, Oak Ridge, Tenn. ROBERT A. RAPP is with the Department of Metallurgical Engineering, Ohio State University, Columbus, Ohio 43210. Manuscript submitted August 23, 1971. METALLURGICAL TRANSACTIONS

w h e r e the d e f e c t notation of K r S g e r and Vink 9 h a s b e e n u s e d . F o r s u f f i c i e n t l y p u r e NiO, Eq. [2] with the s i m p l i f i e d e l e c t r i c a l n e u t r a l i t y condition of 2 [V~i ] = p a l lows the I n t e r p r e t a t i o n of g r a v i m e t r i c , c o n d u c t i v i t y , o r c o u l o m e t r i c t i t r a t i o n m e a s

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