Solid State Chemistry of Energy Conversion and ... - ACS Publications

10 New Solid Electrolytes H . Y-P. HONG Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Mass. 02173

Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch010

This paper discusses several new solid electrolytes for fast alakli-ion transport in terms of the applicable crystallo graphic principles. These materials are oxides in which the mobile alkali ions partially occupy a two- or three-dimen sionally linked interstice within a rigid three-dimensional network. The networks, which are stabilized by electrons from the mobile ions, are formed from cation-oxygen poly hedra. They are classified into three groups depending on whether the polyhedra are tetrahedra, octahedra, or a com bination of the two. Each group is further divided into subgroups depending on the number of network cations to which the oxygen anions are bonded. The first group is represented by the system K Mg Si O where MgO and SiO tetrahedra share corners in such a way that each oxygen is bonded to two cations to form the three-dimen sional network. The second group is represented by the systems NaSbO · 1/6NaF and Na Ta O F · O , in which the networks are formed by sharing corners and/or edges of SbO and TaO F octahedra, respectively. The third group is represented by the system Na Zr Si P O , in which the network is formed by corner-sharing of SiO and PO tetrahedra with ZrO octahedra. For one composition in this system, Na Zr Si PO , the Na -ion conductivity is 0.3 Ω cm at 300°C, which is comparable with that of the best Νa β"-alumina. 2-2x

1-x

1+x

4

4

4

3

6

1+2x

2

5

x

5

1+x

2

x

3-x

12

4

4

6

+

3

-1

2

2

12

-1

Ç o l i d electrolytes f o r fast a l k a l i - i o n transport a r e u n d e r i n v e s t i g a t i o n at ^

m a n y laboratories f o r p o s s i b l e u s e i n h i g h - s p e c i f i c - e n e r g y

secondary

batteries, e l e c t r o l y t i c cells f o r e x t r a c t i n g metals f r o m m o l t e n salts, a n d t h e r m o e l e c t r i c g e n e r a t i o n . T h i s c h a p t e r s u m m a r i z e s t h e results of studies at L i n c o l n L a b o r a t o r y that h a v e l e d to t h e synthesis o f a n u m b e r o f n e w 179

Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

180

SOLID STATE

a l k a l i - i o n s o l i d electrolytes

( I , 2, 3 ) .

CHEMISTRY

O n e of these m a t e r i a l s is

com

p a r a b l e i n N a M o n c o n d u c t i v i t y at 3 0 0 ° C to ^ ' - a l u m i n a , w h i c h is p r e s e n t l y the l e a d i n g c a n d i d a t e f o r use i n N a - S h i g h - s p e c i f i c - e n e r g y batteries, a n d another has the h i g h e s t K - i o n c o n d u c t i v i t y so f a r r e p o r t e d . +

T h e c h a p t e r b e g i n s w i t h a g e n e r a l d i s c u s s i o n of t h e c r y s t a l l o g r a p h i c p r i n c i p l e s r e l a t i n g to a l k a l i - i o n t r a n s p o r t t h a t h a v e e v o l v e d d u r i n g these studies a n d h a v e b e e n e m p l o y e d i n the s e a r c h for n e w s o l i d electrolytes. I n t h e r e m a i n d e r of the c h a p t e r the n e w m a t e r i a l s d e v e l o p e d

are d i s

c u s s e d i n d i v i d u a l l y i n terms of these p r i n c i p l e s . N o a t t e m p t has b e e n m a d e to present details c o n c e r n i n g the p r o c e d u r e s

u s e d f o r synthesis,


c e r a m i c f a b r i c a t i o n , x - r a y d i f f r a c t i o n analysis, or e l e c t r i c a l c h a r a c t e r i z a t i o n . S u c h details h a v e b e e n g i v e n i n a p r e v i o u s p u b l i c a t i o n ( 3 ) . Crystallographic

Principles

T h e essential s t r u c t u r a l feature of the s o l i d electrolytes c o n s i d e r e d i n this p a p e r is a r i g i d , t h r e e - d i m e n s i o n a l n e t w o r k h a v i n g a n i n t e r s t i c e c o n n e c t e d i n at least one d i m e n s i o n a n d p a r t i a l l y o c c u p i e d b y a l k a l i ions (A ). +

T h e m o b i l i t y of the A

+

ions is g o v e r n e d b y the t r a n s i t i o n p r o b a

b i l i t y for i o n transfer f r o m a n o c c u p i e d i n t e r s t i t i a l p o s i t i o n to a n e i g h b o r i n g e m p t y one. atoms M =

I n a l l these m a t e r i a l s the rigid n e t w o r k s consist of m e t a l

αΜι -f- βΜ

+

2

formed from M 0

4

. . . a n d o x y g e n atoms Ο .

tetrahedra, M O

e

They may

be

o c t a h e d r a , o r f r o m b o t h types

of

p o l y h e d r a . I f o n l y a l k a l i ions o c c u p y the interstice, the g e n e r a l c h e m i c a l f o r m u l a is A/(M O ) ~, v

z

w h e r e t h e r i g i d n e t w o r k (M O ) ~

x

y

b y a c c e p t i n g χ electrons f r o m the m o b i l e A

+

e

x

is s t a b i l i z e d

ions. I t is n o t u n c o m m o n f o r

m o l e c u l e s s u c h as H 0 , A 0 , o r A F also to b e present i n the i n t e r s t i c e . 2

2

B e c a u s e these m o l e c u l e s t e n d to act as c o n t a m i n a n t s t h a t interfere w i t h a l k a l i - i o n t r a n s p o r t , t h e y w i l l b e i g n o r e d i n this g e n e r a l d i s c u s s i o n . F o r fast A M o n transport, the A - O p o l y h e d r a a r o u n d adjacent

A

+

positions m u s t share a c o m m o n face. T h e smallest d i a m e t e r of s u c h faces, w h i c h act as bottlenecks to i o n m o t i o n , s h o u l d b e greater t h a n t w i c e the s u m of the a l k a l i - i o n a n d o x y g e n - i o n r a d i i . T h u s for fast N a M o n t r a n s p o r t the smallest d i a m e t e r s h o u l d e x c e e d 4.8 À since the r a d i i of the N a O " ions are r e s p e c t i v e l y 1.0 a n d 1.4 A (4). 2

F o r fast transport of K

+

and

+

ions,

w i t h a r a d i u s of 1.33 A ( 4 ) , the smallest d i a m e t e r s h o u l d e x c e e d 5.46 A . I n a d d i t i o n to these g e o m e t r i c a l constraints, c h e m i c a l b o n d i n g also p l a y s a r o l e i n d e t e r m i n i n g i o n m o b i l i t y . I n o r d e r to increase t h e a l k a l i i o n m o b i l i t y , the c o v a l e n t c o n t r i b u t i o n to the A - O b o n d s s h o u l d

be

m i n m i z e d a n d t h e i o n i c c o n t r i b u t i o n m a x i m i z e d b y p o l a r i z i n g the elec tron cloud away from the A

+

i o n a n d t o w a r d the

rigid

network.

This

p o l a r i z a t i o n c a n o c c u r i n t w o w a y s : ( a ) the anions c a n b o n d w i t h m o r e t h a n t w o M cations of the n e t w o r k a n d ( b ) the anions c a n f o r m s t r o n g l y c o v a l e n t M - O b o n d s w i t h i n the n e t w o r k .

H o w e v e r , b o n d i n g to a d d i -


10.

HONG

New

Solid

181

Electrolytes

t i o n a l cations results i n a n u n d e s i r a b l e l i m i t a t i o n o n t h e d i m e n s i o n a l i t y of the A M o n t r a n s p o r t . I n the extreme case w h e r e the anions b o n d to f o u r or m o r e n e t w o r k cations, t h e a n i o n a r r a y is c l o s e - p a c k e d , a n d the v o l u m e of the i n t e r s t i c e for A M o n t r a n s p o r t is greatly r e d u c e d .

I f e a c h a n i o n is b o n d e d to three

n e t w o r k actions, t r a n s p o r t is m o r e l i k e l y to b e either o n e - or t w o - d i m e n sional rather than three-dimensional.

T h u s , H - or L i M o n

transport

+

o c c u r s i n one d i m e n s i o n i n the t e t r a g o n a l r u t i l e s t r u c t u r e , w h i l e

two-

d i m e n s i o n a l t r a n s p o r t occurs i n l a y e r e d c o m p o u n d s s u c h as β-alumina. O n e - d i m e n s i o n a l i o n t r a n s p o r t is disadvantageous

b e c a u s e i t is r e a d i l y


b l o c k e d b y s t a c k i n g faults a n d i m p u r i t i e s , w h i l e i n p o l y c r y s t a l l i n e cer a m i c s t w o - d i m e n s i o n a l transport causes a r e d u c t i o n i n m o b i l e - i o n d e n s i t y a n d g r a i n - b o u n d a r y c o n d u c t a n c e i f the t r a n s p o r t i n g layers are w i d e l y s e p a r a t e d f r o m e a c h other, as i n β-alumina. M o r e o v e r , a n i s o t r o p i c t h e r m a l e x p a n s i o n r e d u c e s the m e c h a n i c a l s t r e n g t h a n d o p e r a t i n g l i f e of thermally cycled membranes. preferable.

T h r e e - d i m e n s i o n a l t r a n s p o r t is

therefore

T h i s is most p r o b a b l e i f the n u m b e r of M - O b o n d s p e r a n i o n

is l i m i t e d to t w o . I n this case, p o l a r i z a t i o n of t h e O " c h a r g e d e n s i t y a w a y 2

f r o m t h e A i o n r e q u i r e s s t r o n g l y covalent σ a n d π M - O b o n d s w i t h i n t h e +

rigid

network.

T o o b t a i n s t r o n g σ b o n d s i t is a d v i s a b l e to select

s t r o n g l y covalent c o m p l e x e s as

3

BO3 -,

C0

3

2

", N0

3

_

,

4

3

S1O4 ', P O 4 * ,

or

such 2

SO4 ",

a n d t r a n s i t i o n - m e t a l cations h a v i n g a n e m p t y d s h e l l s h o u l d b e u s e d f o r strong π bonding.

New

Solid

Electrolytes

T a b l e I s u m m a r i z e s the properties

of t h e n e w

solid

electrolytes

d e v e l o p e d d u r i n g this s t u d y b y e m p l o y i n g the c r y s t a l l o g r a p h i c p r i n c i p l e s d i s c u s s e d a b o v e . T h e s e materials are d i v i d e d i n t o three groups i n g o n w h e t h e r t h e i r r i g i d n e t w o r k s are f o r m e d f r o m M 0 MO

e

o c t a h e d r a , or b o t h t e t r a h e d r a a n d o c t a h e d r a .

4

These

depend

tetrahedra, groups

are

f u r t h e r d i v i d e d i n t o s u b g r o u p s d e p e n d i n g o n the n u m b e r of M - O b o n d s per O ' ion. 2

T h e same classification s c h e m e is u s e d i n the

following

d i s c u s s i o n , w h e r e the c r y s t a l l o g r a p h i c p r i n c i p l e s are a p p l i e d to these n e w electrolytes ( a n d r e l a t e d m a t e r i a l s , as a p p r o p r i a t e ) . Network

Formed b y Linked Tetrahedra.

A

rigid

f o r m e d e n t i r e l y b y l i n k e d t e t r a h e d r a m u s t h a v e z/y = 2,

network

M„O

0

corresponding

to t w o M - O b o n d s p e r n e t w o r k a n i o n , since n o other r a t i o c a n a l l o w a three-dimensional network.

A l l zeolites, for e x a m p l e , h a v e this r a t i o .

H o w e v e r , most of these a l u m i n o s i l i c a t e s h a v e too large a n interstice to b e effective s o l i d electrolytes; i f t h e i r c e l l e d g e exceeds 10 Â , t h e y are g e n e r a l l y s t a b i l i z e d b y w a t e r — o r some other m o l e c u l e — t h a t

fills

space a n d b l o c k s A M o n transport.


this

182

SOLID S T A T E

Table I. 3D Network

P r o p e r t i e s of

A*-ion Transport

System

Linked Tetrahedra O ' bonded to 2 cations K . 2*Mgi. J 5 i i 0 , x =

CHEMISTRY

A* Ions per cc

2

3D

16.4 Χ

10

21

3D

17.4 Χ

10

21

3D

10.9 Χ

10

21

ID

5.4 Χ

10

21

3D

7.6 Χ

10

21

Linked Tetrahedra and Octahedra O " bonded to 2 cations N a i + eZr SieP3.*Oi2 x = 2

3D

11.1 X

10

21

O " b o n d e d to 2, 3, a n d 4 cations /3"-alumina

2D

5.6 Χ

10

21

2

+ Λ

Linked

0.05

4

Octahedra

O " bonded to 2 cations NaSb0 · l/6NaF 2


3

Nai

+ 2

. T a a O F · Ο., χ — 0.28 B

Ο " bonded to 3 cations K^Mg./aTie.^Oie, x = 2

0.8

O " b o n d e d to 2,3 cations K Sb 0u 2

2

4

2

2

y

2

C a r n e g i e i t e , the h i g h - t e m p e r a t u r e f o r m of N a A l S i 0 , has a 4

cubic

( A l S i 0 ) ~ n e t w o r k w i t h c e l l e d g e a — 7.38 A ( 5 ) , too s m a l l f o r w a t e r to 4

be a contaminant.

U n f o r t u n a t e l y this s t r u c t u r e is n o t stable at

t e m p e r a t u r e . A t this t e m p e r a t u r e , o n l y N a C a S i 0 2

4

and N a M g S i 0 2

room 4

are

r e p o r t e d to f o r m stable structures w i t h n e t w o r k s r e l a t e d to t h a t of c a r n e g i e i t e , a n d these ( M t h e N a ions ( 6 ) .

2 +

S i 0 ) ' n e t w o r k s are d i s t o r t e d b y b o n d i n g w i t h 2

4

These networks have distorted-hexagonal

+

bottlenecks

w i t h a smallest d i a m e t e r less t h a n 4.31 A , s i g n i f i c a n t l y less t h a n the v a l u e r e q u i r e d for fast N a - i o n transport. P r e s u m a b l y this e x p l a i n s o u r o b s e r v a +

t i o n that m a t e r i a l s i n the system N a . « C a S i i . « Ρ * 0 2

4

have poor ionic con

d u c t i v i t y . T h e carnegieite ( A l S i 0 ) " n e t w o r k has b e e n s t a b i l i z e d to r o o m 4

t e m p e r a t u r e b y the i n c o r p o r a t i o n of N a 0 m o l e c u l e s i n t h e i n t e r s t i c e t o 2

give: cubic N a A l S i O 2

2

2

s

· (Να 0) , 2

χ

w h e r e χ — 0.5 or 1. A p p a r e n t l y these

m o l e c u l e s g r e a t l y r e d u c e i o n m o b i l i t y since a c c o r d i n g to o u r m e a s u r e ments (3)

c o n d u c t i v i t i e s of these c o m p o u n d s are three orders of m a g n i

t u d e l o w e r at 3 0 0 ° C t h a n t h a t r e p o r t e d f o r / ? " - a l u m i n a . S i n c e the N a i o n is a p p a r e n t l y too s m a l l to s u p p o r t the ( A l S i 0 ) ' +

n e t w o r k of carnegieite, w e p r e p a r e d analogs w i t h the l a r g e r K

4

+

ion.


It

10.

HONG

New

Solid Electrolytes, Shortest A -A (A)


+

+

Solid

183

Electrolytes

A MO +

x

y

z

Bottleneck A+-0 (A)

psoo°c (a cm)

c (eV) a

3.35

hexagon 2.74

0.35

28

2.87

triangle 2 38 hexagon 2.67

0.35

13

0.40

150

2.94

square 2.34

—

3.50

rectangle 2.52

0.40

3.52

hexagon 2.49

0.24

3

3.23

rectangle 2.71

0.16

4

3.70

— 4 Χ 10

5

w a s f o u n d that at r o o m t e m p e r a t u r e the system K - 2 a j M g i . S i i , 0 4 has 2

a;

+a

the c u b i c n e t w o r k w i t h no d i s t o r t i o n . A s i n g l e - c r y s t a l structure d e t e r m i n a t i o n gave space g r o u p F d 3 m . A p r o j e c t i o n of the M O y

z

network on

t h e a-b p l a n e is s h o w n i n F i g u r e 1, a n d the properties of t h e m a t e r i a l w i t h χ = 0.05 are l i s t e d i n T a b l e I . E a c h K i o n is s u r r o u n d e d b y t w e l v e +

O " ions at a distance of 3.2 A . A s s h o w n i n F i g u r e 1 the b o t t l e n e c k is a 2

r e g u l a r h e x a g o n f o r m e d b y M 0 - t e t r a h e d r a edges; its d i a m e t e r of 5.48 A 4

is just l a r g e e n o u g h for fast K M o n transport. B e c a u s e n o m o l e c u l e s are i n t r o d u c e d i n t o the interstice to s t a b i l i z e t h e structure, g o o d K M o n t r a n s port was anticipated a n d found.

T h e r e s i s t i v i t y m e a s u r e d at 3 0 0 ° C f o r

χ = 0.05, 28 Ω c m , is t h e lowest r e p o r t e d f o r K M o n s o l i d electrolytes. Network Formed b y Linked Octahedra. I f the M cations in

AMO x

y

z

are a l l c o o r d i n a t e d o c t a h e d r a l l y b y the o x y g e n anions, f o r m a t i o n of a t h r e e - d i m e n s i o n a l n e t w o r k r e q u i r e s z/y < 3.

Such networks m a y

d i v i d e d i n t o three s u b g r o u p s d e p e n d i n g o n the v a l u e of z/y:

( a ) z/y =

be 3,

c o r r e s p o n d i n g to t w o M - O b o n d s p e r n e t w o r k a n i o n , ( b ) z/y = 2, c o r r e s p o n d i n g to three M - O b o n d s p e r n e t w o r k a n i o n , a n d ( c ) 2 < z/y < 3, c o r r e s p o n d i n g to some n e t w o r k anions w i t h t w o , others w i t h three, M - O


184

SOLID STATE

CHEMISTRY


α

Figure I . Projection on the a-b plane of the M Og network of cubic K . Mg _ Si ^ Pi . The 0 ~ ions are represented by open circles, the Mg * and Si * ions by the letter M. The ions that lie above the plane are identified by numbers giving their height as a percentage of the lattice constant. A simihr representa tion is used in several of the following figures. The bottleneck to K M o n transport, which has a diameter of 5.48Â, is the regular hexagon indicated by the heavylines. y

i 2x

1

li

1

1

t

2

2

bonds.

4

E x a m p l e s of e a c h s u b g r o u p are l i s t e d i n T a b l e I a n d d i s c u s s e d

below. Two

M - O

BONDS PER NETWORK

ANION.

NaSbO

s

This

• l/6NaF.

c o m p o u n d has a c u b i c s t r u c t u r e ( 3 ) t h a t is s t a b i l i z e d b y the N a F m o l e cules.

The (Sb0 )" 3

n e t w o r k of this s t r u c t u r e contains p a i r s of

edge-

s h a r e d o c t a h e d r a t h a t are c o r n e r - s h a r e d to f o r m t h e t h r e e - d i m e n s i o n a l a r r a y i l l u s t r a t e d i n F i g u r e 2.

E a c h O " i o n is b o n d e d to t w o S b 2

5 +

ions.

T h e ( S b 0 ) " n e t w o r k contains t u n n e l s a l o n g the (111) axes that intersect 3

at t h e o r i g i n a n d b o d y - c e n t e r positions. T h e t u n n e l intersections i n t h e i n t e r s t i c e are o c c u p i e d b y F " ions ( 3 ) .

T h e t u n n e l segments

between

these intersections consist of three f a c e - s h a r e d o c t a h e d r a , s q u a s h e d a l o n g t h e t u n n e l axis, w h i c h are p a r t i a l l y o c c u p i e d b y N a

+

ions. T h e c e n t r a l

o c t a h e d r o n , site 8c of the s t r u c t u r e , is b o u n d e d b y t w o t r i a n g u l a r b o t t l e n e c k s of 0

2

atoms t h a t separate i t f r o m the t w o o u t e r o c t a h e d r a , site

16f, e a c h of w h i c h has a t r i a n g u l a r face of O i atoms i n c o m m o n w i t h one of the F - p o l y h e d r a . T h e N a

+

ions are d i s t r i b u t e d b e t w e e n the 8c a n d

16f sites, w i t h electrostatic forces r e s u l t i n g i n v a c a n c i e s at t h e 16f sites t h a t are adjacent to a n o c c u p i e d 8c site. T h e D e b y e - W a l l e r f a c t o r d e t e r -


10.

HONG

New

Solid

185

Electrolytes

m i n e d b y x - r a y d i f f r a c t i o n analysis shows t h a t t h e N a

+

ions i n 16f sites

h a v e a n o m a l o u s l y l a r g e t h e r m a l m o t i o n s a l o n g (100) d i r e c t i o n s , i n d i c a t i n g excellent i o n transport b e t w e e n 16f sites of t w o n e i g h b o r i n g t u n n e l s e g ments a r o u n d t h e F " i o n o c c u p y i n g the i n t e r s e c t i o n ; i n this case t h e a d d e d N a F m o l e c u l e s a p p a r e n t l y d o not h a v e a strong effect o n a l k a l i - i o n m o b i l i t y . T h e x - r a y d a t a also r e v e a l the presence of l a r g e n u m b e r of N a * ions at 8c positions, i n d i c a t i n g that these ions d o not differ too m u c h i n p o t e n t i a l e n e r g y f r o m those at 16f sites. W e c o n c l u d e t h a t the p r i n c i p a l b o t t l e n e c k to N a M o n m o t i o n is the 0

t r i a n g l e s e p a r a t i n g the 8c a n d 16f

2

positions. T h e distance f r o m the center of this t r i a n g l e to a n 0 Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch010

is 2.38 A , just l a r g e e n o u g h for fast N a M o n t r a n s p o r t .

2

position

This relatively

t i g h t b o t t l e n e c k a n d the absence of ττ-bonding o r b i t a l s at the S b m a y a c c o u n t for the r e l a t i v e l y l a r g e a c t i v a t i o n e n e r g y (0.35 e V ) for this c o m p o u n d .

5 +

ions

observed

Nevertheless, a relatively large pre-exponential factor

i n the c o n d u c t i v i t y e x p r e s s i o n — c a u s e d b y a h i g h N a M o n c o n c e n t r a t i o n of 17.4 Χ 1 0

cm" —lowers

21

the r e s i s t i v i t y m e a s u r e d

3

(3)

at 3 0 0 ° C to

13 Ω c m ( t h e a c t u a l v a l u e m a y b e e v e n l o w e r since n o c o r r e c t i o n

was

m a d e f o r contact resistance ). Defect

Pyrochlore

NaTa O t

s

· xNa O. t

T h e cubic pyrochlore struc

t u r e ( t y p e f o r m u l a A B X X , w i t h A the l a r g e r of the t w o c a t i o n s ) has a 2

2

e

/

Figure 2. Model of cubic structure of NaSbO · 1/6 NaF. Dark spheres represent Sb * ions, light spheres Na* ions. The O " ions are at the corners of the octahedra around the Sb ions. s

5

2

5+


186 rigid

SOLID STATE

B X 2

6

n e t w o r k of c o r n e r - s h a r e d o c t a h e d r a (7).

s t r u c t u r e o n the ( 1 1 0 )

CHEMISTRY

A p r o j e c t i o n of t h e

p l a n e is s h o w n i n F i g u r e 3.

W i t h space

group

F d 3 m , assignment of the Β cations to the 16c p o s i t i o n s places t h e A cations at 1 6 d a n d the X ' anions at 8 b . T h e t h r e e - d i m e n s i o n a l i n t e r s t i t i a l A X ' a r r a y consists of c o r n e r - s h a r e d Χ Ά 4 t e t r a h e d r a .


2

(no) Figure 3. A (110) projection of cubic pyrochlore Α Β Χ Χ'· The linked octahedra represent the B X network, the shaded small circles represent the interstitial A cations in positions 16d, and the large open circles projected onto the shaded circles represent the two nearneighbor X ' ions in 8b sites on either side of an A cation. Ζ

2

2

6

6

+

+

T w o types of defect p y r o c h l o r e s h a v e b e e n i d e n t i f i e d p r e v i o u s l y . I n one the 8b positions n o r m a l l y o c c u p i e d b y X

anions are v a c a n t ; i n t h e

7

o t h e r t h e 16d positions are v a c a n t ; a n d a l a r g e A

c a t i o n replaces t h e X '

+

a n i o n at 8b. T h e first t y p e is i l l u s t r a t e d b y A g S b O 2

R b T a 0 F (3). 2

2

( 8 ) , the s e c o n d b y

e

[ I n the l a t t e r c o m p o u n d the F " ions are r a n d o m l y s u b s t i

5

t u t e d f o r O " ions of t h e ( B ^ e ) ' n e t w o r k . ] 2

I n b o t h cases one set of

i n t e r s t i t i a l positions is o c c u p i e d b y cations, a n d t h e other is v a c a n t ; h o w ever, since the 16d a n d 8 b positions are n o t c r y s t a l l o g r a p h i c a l l y e q u i v a lent, f a s t - i o n t r a n s p o r t r e q u i r e s either the presence of b o t h cations a n d v a c a n c i e s o n the same set of positions o r a n a d j u s t m e n t of t h e c e l l s i z e t h a t m a k e s t h e p o t e n t i a l energies n e a r l y e q u a l f o r A

+

p o s i t i o n s . T h e M a d e l u n g e n e r g y tends to s t a b i l i z e A w h i l e elastic forces t e n d to s t a b i l i z e l a r g e A

+

ions i n 16d a n d 8 b +

ions o n 16d sites,

ions o n 8 b sites, w h e r e t h e

A - O distances exceed 3.2 A . Stable A ( B X ) " defect pyrochlores can be p r e p a r e d w i t h the large +

Rb

+

2

6

a n d C s ions, a n d also w i t h K +

+

ions i f the size of the B X 2

6

a r r a y is


10.

HONG

New Solid

187

Electrolytes

s m a l l e n o u g h , as i n K M g A l F . e

I n t h e case of A ( T a a O F ) - , R b T a 0 F +

B

2

5

can be synthesized directly, b u t the K analog can be made only b y i o n +

exchange w i t h t h e R b c o m p o u n d . F u r t h e r m o r e , t h e i o n - e x c h a n g e d p r o d +

u c t is h y g r o s c o p i c , w i t h w a t e r o c c u p y i n g 8 b sites a n d d i s p l a c i n g t h e K

+

ions t o 1 6 d sites i n K T a 0 F · s H 0 , χ < 1. W h e n t h e K c o m p o u n d i s 2

5

+

2

i o n e x c h a n g e d w i t h N a , N a 0 ' m o l e c u l e s a r e i n t r o d u c e d to f o r m N a T a 2

2

0 F · x N a 0 ' , χ = 0.28, w i t h N a ions p a r t i a l l y o c c u p y i n g t h e 1 6 d p o s i 5

2

tions a n d t h e O ' ions o c c u p y i n g 8 b p o s i t i o n s . T h i s c o m p o u n d i s t h u s a t h i r d t y p e o f d e f e c t p y r o c h l o r e , w h i c h has t h e s a m e b a s i c s t r u c t u r e as a n i d e a l p y r o c h l o r e b u t has b o t h t h e 1 6 d a n d 8 b p o s i t i o n s o n l y p a r t i a l l y Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch010

occupied.

Its p r o p e r t i e s are l i s t e d i n T a b l e I . T r a n s p o r t of N a

+

ions

f r o m a n o c c u p i e d 1 6 d p o s i t i o n t o a v a c a n t o n e is p a r t i a l l y b l o c k e d b y t h e O ' ions. I n a d d i t i o n , t h e short 1 6 d - 8 b d i s t a n c e of 2.27 A i n d i c a t e s s t r o n g N a - O ' b o n d i n g , w h i c h is p r o b a b l y r e s p o n s i b l e f o r t h e h i g h a c t i v a t i o n e n e r g y , 0.40 e V , o b s e r v e d f o r i o n t r a n s p o r t i n t h i s m a t e r i a l . N e v e r t h e l e s s , at 3 0 0 ° C t h e r e s i s t i v i t y is o n l y 150 Ω c m .

Figure 4. Projection on the a-b plane of the M O network of priderite, ^ x ^ g x / i T f . O j . (Ions lying in the plane are identified by zeroes). The bottleneck to K -ion transport in the one-dimensional tunnels of the structure is a square indicated by the dashed lines. 8

8

x / 2

ie

e

+


188

SOLID STATE

THREE M-O

Oie, χ =

BONDS PER NETWORK ANION.

Priderite,

CHEMISTRY

K^Mg^Tis-*^-

0.8, has the h o l l a n d i t e s t r u c t u r e . I n t h i s s t r u c t u r e a n Μ Ο ι 8

n e t w o r k is f o r m e d b y M O

β

o c t a h e d r a s h a r i n g corners a n d edges, w i t h

e

e a c h O " i o n b o n d e d to t h r e e M ions. A p r o j e c t i o n of t h i s n e t w o r k o n t h e 2

a-b p l a n e is s h o w n i n F i g u r e 4. T h e i n t e r s t i c e p a r t i a l l y o c c u p i e d b y K * ions

consists

another.

of

one-dimensional

t u n n e l s t h a t are i s o l a t e d f r o m

I n t h e i r e q u i U b r i u m positions, t h e K

+

one

ions o c c u p y l a r g e sites

s u r r o u n d e d b y t w e l v e O " ions at a d i s t a n c e of 3.3 A . A s s h o w n i n F i g u r e 2

4, t h e b o t t l e n e c k b e t w e e n adjacent positions i n a t u n n e l is a s q u a r e w i t h a d i a m e t e r of o n l y 3.6 A , m u c h too s m a l l f o r fast K M o n t r a n s p o r t . A c c o r d Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch010

i n g l y , n o d c c o n d u c t i v i t y is d e t e c t e d

(9).

H i g h frequency ac

conduc

t i v i t y w i t h a n a c t i v a t i o n e n e r g y of 0.2 e V has b e e n o b s e r v e d ( 9 ) , a p p a r e n t l y f o r K M o n d i s p l a c e m e n t s w i t h i n the l a r g e t u n n e l sites. Two

AND THREE M - O


K S b O n has a rigid n e t w o r k c o m p o s e d of S b O 2

4

e

The

compound

o c t a h e d r a t h a t share

corners a n d edges i n s u c h a w a y t h a t some O " ions are b o n d e d to t w o 2

Sb

5 +

ions, others to three ( 1 0 ) .

A p r o j e c t i o n of t h e s t r u c t u r e o n the

a-c

p l a n e is s h o w n i n F i g u r e 5. T h e K * ions o c c u p y i n t e r s t i t i a l p o s i t i o n s i n o n e - d i m e n s i o n a l t u n n e l s r u n n i n g p a r a l l e l to t h e b a n d c axes. A s s h o w n i n F i g u r e 5, the shortest r a d i i of the fc-tunnel, c - t u n n e l , a n d c r o s s - t u n n e l b o t t l e n e c k s are 2.58, 2.52, a n d 2.69 A , r e s p e c t i v e l y , a l l of w h i c h are less t h a n t h e m i n i m u m v a l u e of 2.73 A r e q u i r e d f o r fast K M o n transport. T h i s e x p l a i n s the h i g h r e s i s t i v i t y v a l u e , 4 Χ ΙΟ Ω c m , t h a t w e h a v e m e a s u r e d 5

at 3 0 0 ° C . α

Figure 5. Projection on the a-c plane of the structure of K Sb O . Diameters of the three types of bottlenecks to K -ion transport are indicated by the hatched bars. 2

k

n

+



10.

HONG

New Solid

189

Electrolytes

Figure 6. Projection of the hexagonal structure of NaZr P 0 . A Zr (PO ) unit of the three-dimensional network is indicated by shading in the lower left corner. g

Network M - O

Formed by Linked Octahedra


s

lz

2

and Tetrahedra.

T h e system N a i

+ a ?

k s

Two

Z r S i . P - , O i 2 (0 2

a

3

â

Solid State Chemistry of Energy Conversion and ... - ACS Publications

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