11 Polarizability Enhancement of Ionic 1+
Conductivity for
A
in
1+
A MX 2
6
Series
A. W. SLEIGHT, J. E. GULLEY, and T. BERZINS
Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch011
Central Research and Development, Ε. I. du Pont de Nemours & Co. Experimental Station, Wilmington, Del. 19898
1+
The ionic conductivity of A is studied in the isostructural series of the general formula A M X where A is Rb, Cs, or Τl; M is Ta, Nb, or W; and X is Ο or F. High polariza bility of either A or the (M X ) framework enhances the ionic mobility of A . Thus although Tl is essentially the same size as Rb , the mobility of Tl is much higher in a given (M X ) framework. This is attributed to the higher polarizability of Tl . The polarizability of the (Μ Χ ) framework is related to the polarizability of M-X bonds. The influence of polarizability in β-alumina and anion con ductors is also discussed. 1+
1+
2
1+
6
1-
2
6
1+
1+
1+
1+
1-
2
6
1+
1-
2
Tn
6
a s e a r c h f o r n e w m a t e r i a l s e x h i b i t i n g fast i o n t r a n s p o r t , v a r i o u s t h o u g h t s h a v e b e e n expressed c o n c e r n i n g t h e s t r u c t u r a l a n d c h e m i c a l
r e q u i r e m e n t s f o r s u c h m a t e r i a l s . T h e s t r u c t u r a l features h a v e g e n e r a l l y b e e n stressed; h o w e v e r , there a r e other i m p o r t a n t c o n s i d e r a t i o n s . T h e difficulty i n assessing factors o t h e r t h a n s t r u c t u r a l is t h a t o n e m u s t s u b d u e t h e g e o m e t r i c effects w h i c h g e n e r a l l y a p p e a r to d o m i n a t e . T h i s is d o n e b y c o m p a r i n g t h e i o n i c c o n d u c t i v i t y o f ions w i t h n e a r l y i d e n t i c a l sizes (e.g., L i v s . C u o r R b v s . T l ) i n t h e same s t r u c t u r e . +
+
+
+
A l t h o u g h s o m e d a t a exist o n t h e r e l a t i v e m o b i l i t i e s o f T l a n d R b i n +
+
/ 3 - a l u m i n a , a c o m p a r i s o n i n this s t r u c t u r e is h i n d e r e d b y t h e c o m p l e x behavior
of R b - / ? - a l u m i n a ( I ) .
Another approach
is to c h a n g e t h e
c h e m i s t r y of a host s t r u c t u r e w i t h o u t s i g n i f i c a n t l y a l t e r i n g its g e o m e t r y . T h e effect of this n o n g e o m e t r i c c h a n g e c o u l d t h e n b e d i r e c t l y e s t a b l i s h e d . A family of compounds
w i t h the general formula A M X e h a v i n g a 2
p y r o c h l o r e - r e l a t e d s t r u c t u r e w a s d i s c o v e r e d b y B a b e l et a l . ( 2 ) i n 1967. T h i s is a n ( M X e ) " f r a m e w o r k s t r u c t u r e f o r m e d b y c o r n e r - s h a r i n g M X e 2
195
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
196
SOLID STATE
CHEMISTRY
o c t a h e d r a . T h e r e are i n f i n i t e t u n n e l s i n this f r a m e w o r k w h i c h f r e q u e n t l y intersect. I n this A M X e f a m i l y , X c a n b e Ο a n d / o r F . T h e M c a t i o n 2
can be practically any cation w h i c h w i l l readily accept octahedral ordination to Ο or F . T h e A
+
co
cation can be nearly any univalent cation,
b u t i f this c a t i o n is too s m a l l , w a t e r also enters t h e f r a m e w o r k as i n zeolites.
T a b l e I presents s o m e examples of c o m p o u n d s
i n this A M F 2
e
family.
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Table I.
Ionic Conductivity D a t a for A M X e
Compound
a
CsTaWOe CsNbWOe RbTaWOe RbNbWOe TITaWOe TINbWOe TlTa 0 F TlNb 0 F RbTa 0 F RbNb 0 F CsTa 0 F CsNb 0 F
10.39 10.40 10.35 10.37* 10.36 10.36* 10.48 10.49 10.49 10.49 10.51 10.52
2
5
2
5
2
5
2
2
5
5
2
β
Phases
2
5
(A)
Ε
(kcal)
σο
( ohm'
1
