Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors

Mat. Res. Bul. Vol. 13, pp. 117-124, 1978. Pergamon P r e s s , Inc. Printed in the United States.

CRYSTAL

STRUCTURE AND OTHER

A N D IONIC C O N D U C T M T Y O F Lil4Zn(GeO4) 4 N E W Li+ S U P E R I O N I C C O N D U C T O R S *

H. Y-P. Hong Lincoln Laboratory, M a s s a c h u s e t t s Institute of Technology Lexington, M a s s a c h u s e t t s 02173

(Received D e c e m b e r 5, 1977; Communicated by A. W. Sleight)

ABSTRACT This paper reports the synthesis and characterization of a number of new Li+ superlonic conductors with the type formula LilS_2~Dx(TO4)4, where D is a divalent cation (Mg 2+ or Zn2+), T is a tetravalent cation (Si~+ or Ge4+), and 0< x < 4. One of these materials, Li14Zn(GeO4)4, has a resistivity of 8 ~ - c m at 300 ° C, lower than that of a~y Li+-ion conductor so far reported. The structure of this compound, which we have named LISICON (for Li superionic conductor), has been determined by singlecrystal x-ray analysis. The space group is Pnma, with cell parameters a=I0.828 ~, b-~.251~, c=5.140 ~, and z=l. The structure has a rigid three-dimensional network of LillZn(GeO4) 4. The three remaining Li+ ions have occupancies of 55 and 16%, respectively, at the 4c and 4a interstitial positions. Each 4c position is connected to two 4a positions and vice versa. The bottlenecks betweenthese positions have an average diameter that is larger than twice the sum of the Li + and O z- ionic radii, thus satisfying thegeometrical condition for fast I~ + -ion transport. Moreover, all four sp 3 orbitals of the O z- ion are shared by strong tetrahedral covalent bonds with the network cations. Therefore, the anion charge is polarized away from the interstitial Li+ ions, weakening the Li-O bond and increasing the Li+-ion mobility.

* This work was sponsored by the Defense Advanced R e s e a r c h P r o j e c t s Agency. The views and conclusions contained in this document a r e those of the c o n t r a c t o r and should not be i n t e r p r e t e d a s n e c e s s a r i l y r e p r e s e n t i n g the official policies, e i t h e r e x p r e s s e d o r implied, of the United States Government. 117

118

H. Y-P. H O N G

Vol. 13, No. 2

Introduction A number of crystallographic principles underlying fast alkali-ion transport in solid electrolytes have been established (I-3). The essential structural feature of such materials is a rigid, three-dimensional, cation-anion network having an interconnected interstitial space that is partially occupied by mobile alkali ions. Ion mobility is governed physically by the size of the bottlenecks between the interstitial alkali-ion positions and chemically by the bonding energy between the mobile ions and the network anions. The shortest diameter of the bottlenecks should be larger than twice the sum of the mobile-ion and anion radii. The covalent bonding between the mobile ion and the anion should be weak as possible, which m a y be achieved if the anion forms a strongly covalent complex with the network cations, if the anion is bonded to more than two of these cations, or both. Since the Li-O bond is normally more covalent than the Na-O bond, for the same network structure Li + ions should be less mobile than Na + ions. Consistent with r/~s expectation, the r e s i s t i v i t y 0 at 300°C is 30 t i m e s higher for L i - 8 - a l u m i n a than f o r Na-8-alumina (4), and I000 t i m e s higher for Li3Zr2PSi2OI2 than for the Na+-ion conductor Na3Zr2PSi2OI2 (I). The strength of the Li-O bond reduces the likelihood of discovering oxides with high Li+-ion conductivity. The best oxide conductors reported previously are L i - 8 - a l u m i n a (4), Li3.755i0.75P0. 2504 (5), and Li3. sSi0. 5P0. 504 (6), with P300ot~ = 110, 100, and 31 fi-cm, respectively. In Li31~, single c r y s t a l s of which have 0300Oc = 11 ~-cm perpendicular to the c - a x i s (7), the conductivity is increased by avoiding the Li-O bond. Alternatively, however, the Li-O bond can be made m o r e ionic, and therefore weakened, by using s t r u c t u r e s in which the 0 2 - ions a r e bonded to four network cations, so that all four of their sp3 orbitals a r e occupied and the oxygen charge density is polarized away f r o m the Li + ions. The application of this concept is d e m o n s t r a t e d in the p r e s e n t paper, which r e p o r t s the d i s c o v e r y of a number of new Li+ solid e l e c t r o l y t e s . One of these, Li14Zn(GeO4) 4 (which we have named LISICON, for Li superionic conductor), has the lowest r e s i s t i v i t y at 300°C so f a r reported for any L-['~-ion conductor. Experimental Procedure To p r e p a r e each of the m a t e r i a l s studied in this investigation, a mixture of Li2CO 3 with MgO or ZnO and SiO 2 or GeO 2 was placed in an alumina crucible and reacted by firing overnight in a i r at 1100°C. The f i r s t such m i x t u r e had the composition, after loss of CO 2 from the Li2CO 3, of LisMg2(SiO4) 3. The others had compositions given by Li16.2xDx(TO4)4 , with D = Mg or Zn, T = Si or Ge, and 0 < x < 4. F o r each composition, a pellet for r e s i s t i v i t y m e a s u r e m e n t s was f a b r i cated by c o l d - 2 r e s s i n g the reacted m a t e r i a l at 650 at-m and then Sintering for 2 hours in a i r at 1150~C. This procedure yielded specimens with densities between 85 and 90% of theoretical. The r e s i s t i v i t y m e a s u r e m e n t s were made with pseudoreversible colloidal graphite e l e c t r o d e s , in m o s t c a s e s at 300 and 400°C. F o r each composition, an x - r a y powder diffraction pattern for the r e a c t i o n product was obtained by d i f f r a c t o m e t e r m e a s u r e m e n t s . The patterns indicated that the basic c r y s t a l s t r u c t u r e was the same in all c a s e s . A detailed s t r u c t u r e d e t e r mination was p e r f o r m e d by s i n g l e - c r y s t a l x - r a y a n a l y s i s of Li14Zn(GeO4) 4, which was of p a r t i c u l a r i n t e r e s t because it had been found to have the lowest r e s i s t i v i t y . To obtain s m a l l single c r y s t a l s , r e a c t e d m a t e r i a l of this composition was heated overnight at 1250 oC, close to the melting point of 1300 o C determined by our DTA m e a s u r e m e n t s . A cube-shaped c r y s t a l about 0. 1 mm on an edge was selected for a n a l y s i s . After oscillation and Weissenberg photographs had been made, t h r e e dimensional intensity data to 28 = 50 o were collected with a d i f f r a c t o m e t e r using Mo radiation. In total, 333 reflections were m e a s u r e d . The h e a v y - a t o m method was used to solve the s t r u c t u r e .

Vol. 13, No. 2

SUPERIONIC

CONDUCTORS

Crystal Structure of L I S I C O N ,

119

Lil4Zn(GeO4)4

Before presenting the data obtained for r e s i s t i v i t y as a function of composition, we shall d e s c r i b e the c r y s t a l structure determined for LISICON. Oscillation and Weissenberg photographs showed diffraction s y m m e t r y m m m . Systematically absent were 0kl, k + £ = 2n + 1, and hk43, h = 2n + 1, consistent with space groups Pn2a and Pnma. In using the h e a v y - a t o m method, the strongest peak in the Patterson map was a s s u m e d to r e p r e s e n t an interaction between the Ge4+ ions. A s t r u c t u r e - f a c t o r calculation based on the C-e4+ positions gave a difference-function R value of 0.25. With this model and the assumption of Pnma s y m m e t r y , it was possible to identify all atoms from a difference F o u r i e r synthesis. The atom p a r a m e t e r s , scale factors, and isotropic t e m p e r a t u r e f a c t o r s were then refined with a f u l l - m a t r i x , l e a s t - s q u a r e s p r o g r a m to give R = 0. 055 and R~ = 0.045 for all reflections. The final values are listed in Table I, and a projection of the structure on the a-b plane is shown in Fig. 1. TABLE I Final Atomic P a r a m e t e r s for Lil4Zn(GeO4) 4 Space group Pnma; cell p a r a m e t e r s a = 10.828(2) ~, b = 6.251(1) ~., c = Site

o.f. a

x

5. 140(I)~, z = I. y

z

B O

Li(1)

4c

0.55(9)

0.206(4)

1/4

0.978(9)

4(1)

Li(2)

4a

0.16(9)

0

0

0

8(2)

LZ(1) b

4c

0.981(6)

0.426(1)

3/4

0. 163(3)

1.6(5)

LZ(2) b

8d

0,863(3)

0. 1653(3)

0. 9977(8)

0.3297(9)

0.8(1)

Ge

4c

0.4131(1)

1/4

0.3396(2)

0.82(3)

O(1)

8d

0.3353(5)

0.0245(9)

0.2185(9)

1.5(1)

0(2)

4c

0.0880(6)

3/4

0.1778(9)

0.8(1)

0(3)

4c

0.0633(6)

i/4

0.2789(9)

0. 9(1)

ao. f. = occupancy factor of Li + ions. bLZ = occupied by both Li + and Z 2 + ions. T h e r e is one Lil4Zn(Ge04) 4 f o r m u l a unit per cell in the LISICON s t r u c t u r e . As shown in Table I, the Li + ions are distributed among four different sites. F o u r of these ions occupy the 4c sites designated as LZ(1), while seven occupy the 8d sites designated as LZ(2), which they share with the Zn 2+ ion. (The designation of the 4c sites as LZ(1), indicating that they are occupied by both Li + and Zn2+ ions, has been adopted because the best fit to the diffraction data gives 0. 981 as the occupancy factor for the Li + ions. However, it is likely that these sites are e n t i r e l y occupied by i,i + ions, with the occupancy factor actually equal to unity. ) The LZ(1) and LZ(2) sites have n o r m a l values for the t e m p e r a t u r e coefficient ~ , implying that t h e i r eleven Li + ions form part of the rigid t h r e e - d i m e n s i o n a l networK, which is thus r e p r e s e n t e d by [Li.lZn(GeO4)~] 3-. In this network, each cation is t e t r a h e d r a l l y coordinated to four 02 -~ ions and e~ch 02- ion is bonded to four cations.

120

H. Y-P. HONG

.

_ Go

~_

/

ZX

~

Vol. 13, No. 2

Lz

~

LX FIG. 1

/

,,zx

P r o j e c t i o n of the LISICON s t r u c t u r e on the a - b plane.

LZ

LZ MIXED Li*

AND

Znl *

The t h r e e r e m a i n i n g Li + ions p e r unit cell a r e di st ri but ed among the 4c sites designated as Li(1) and the 4a s i t e s designated as Li(2), which a r e located in the i n t e r s t i t i a l space within the rigid network. The occupancies of the Li(1) and Li(2) s i t e s a r e 55 and 16~o, r e s p e c t i v e l y . T he s e sites have a b n o r m a l l y high t e m p e r a t u r e c o e f f i c i e n t s , indicating that t h e i r Li + ions a r e mobile. Each Li(1) site is connected to two Li(2) s ites , and vice v e r s a . As shown in Fig. 1, the bottlenecks to L i +-i on t r a n s p o r t between these sites a r e p a r a l l e l o g r a m s , with c r o s s d i a m e t e r s of 3.54 and 5.22 A, that a r e tilted with r e s p e c t to ~he a - b plane. The connected sites lie in the s a me a - b plane, so that ion t r a n s p o r t is t w o - d i m e n s i o n a l , as it is in B-alumina. In LISICON, h o w e v e r , the conducting planes a r e adjacent to each ot her, w h e r e a s in B-alumina they a r e s e p a r a t e d by thick spinel blocks that do not contain alkali ions. Ionic C onductivit~r The p r e s e n t investigation began with an e f f o r t to synt hesi ze a Li compound that would be s t r u c t u r a l l y s i m i l a r to the Na+-ion solid e l e c t r o l y t e Na3Zr2PSi2OI2, NASICON (1), in which the t h r e e - d i m e n s i o n a l network is com posed of both cationoxygen o c t a h e d r a (ZrO6) and t e t r a h e d r a (PO 4, SiO4). Since d i r e c t substitution of Li f o r Na in NASICON was known to r educe the ionic conductivity by t h r e e o r d e r s of magnitude, it was decided to at t em pt the p r e p a r a t i o n of a compound with the f o r m u l a LixM2M~O12,where M and M' r e p r e s e n t network cations that a r e , r e s p e c t i v e l y , o c t a h e d r a l l y and t e t r a h e d r a l l y c oor di nat ed to the 0 2 - ions. The p a r t i c u l a r c o m p o s i tion s e l e c t e d was Li8Mg2Si3012, with M = Mg and M' = Si, and a m i x t u r e of Li2CO 3, MgO, and SiO2 in p r o p o r t i o n s c o r r e s p o n d i n g to this c o m p o s i t i o n was fi red in a i r at I1 00 °C . An x - r a y powder p a t t e r n f o r the r e a c t i o n product showed that the NASICOE s t r u c t u r e had not been obtained, but the m a t e r i a l was found to have significant ionic conductivity (P300Oc = 1500 ~l-cm). To d e t e r m i n e w h e t h e r the conductivity could be i n c r e a s e d by a change in s t o i c h i o m e t r y , we p r e p a r e d a s e r i e s of Li16.2xMgx(SiO4) 4 s a m p l e s with values of x f r o m 0 to 3. F o r even the best of t h e s e , with x = 2 . 5 , the conductivity was still quite low (P300Oc = 1000 ~-cm ). Since r e p l a c e m e n t of the Si 4+ ion by the l a r g e r Ge 4+ ion could be expected to e n l a r g e the bottlenecks to Li+-ion t r a n s p o r t and might t h e r e f o r e i m p r o v e the conductivity, we next p r e p a r e d a s e r i e s of Lil6_2xMgx(C-eO4) 4 s a m p l e s , with 0. 5_< x_< 3. 5. In this c a s e t h e r e was a strong i n c r e a s e in conductivity, with p 2 0 n o c = 40 Cl-cm for x = 1, the optimum s t o i c h i o m e t r y Finally, in an a t t e m p t to enlarg~ ~ e ' L i + - i o n bottlenecks still f u r t h e r by r e p l a c i n g the Mg 2÷ ion with the l a r g e r Zn 2+ ion, we p r e p a r e d a s e r i e s of L i l 5 2yZnx(C-eO4) 4 s a m p l e s , with 0 . 5 _ x _< 3.5. This r e p l a c e m e n t r e s u l t e d in a n o t h e r significant inc r e a s e in conductivity, with ~300Oc = 8 ~ - c m for x = 1, which was again the optimum value. The compound with this s t o i c h i o m e t r y , Lil4Zn(GeO4)4, is the one that we have named LISICOE.

Vol. 13, No. 2

SUPERIONIC CONDUCTORS

121

The r e s i s t i v i t y of LISICON was m e a s u r e d as a function of t e m p e r a t u r e between 200 and 400°C. With i n c r e a s i n g t e m p e r a t u r e , D d e c r e a s e s monotonically, reaching 5 fl-cm at 400°C. Results derived from the r e s i s t i v i t y data a_~e shown in Fig. 2, where the product of conductivity ¢r (= 1/9) and absolute t e m p e r a t u r e T is plotted on a logarithmic scale v e r s u s 1/T. Between 250 and 400°C the points fit a straight line whose slope c o r r e s p o n d s to an activation e n e r g y e a = 0.24 eV in the usual e x p r e s s i o n for the t e m p e r a t u r e dependence of ionic conductivity, v = (¢ro/T) exp (-ea/kT). F i g u r e 2 also shows the data reported for L i - ~ - a l u m i n a (4) and Li3.5Si0.5P0.504 (6). "[he l a t t e r has the highest Li+-ion conductivities reported previously for the 200400°C range. It is seen that LISICON has significantly higher conductivity over the e n t i r e range, p a r t i c u l a r l y at the lower t e m p e r a t u r e s .

2.4

2.0

4O0 ~

~

°C 3OO I

I

2OO I

'

~"~

%%%%%% •

%X

o.

LISICON and the other Li superionic conductors prepared in this investigation a r e all r e p r e s e n t e d by the formula Lil6.2xDx(TO4)4, where D is the divalent cation .~.g2+ or Zn2+ and T is the tetravalent cation Si4+ or C,e4+. The dependence of conductivity on stoichiometry and cation species is illustrated by Fig. 3, where log (¢T) is plotted v e r s u s 1/T for the samples with x= 1, 2, and 3 in each of the four s e r i e s with different p a i r s of D and T cations. The r e s u l t s for each of the 12 compositions are r e p r e s e n t e d by a straight line in Fig. 3. (Except for LISICON, these lines do not imply that cT v a r i e s exponentially with 1/T over the t e m p e r a ture range shown, since r e s i s t i v i t y m e a s u r e m e n t s for the other compositions were made only at 300 and 400 OC, corresponding to the end-points of the lines. )

The qualitative trends in the relationship between composition and conductivity illustrated in Fig. 3 can be 1 I I t.4 '1.6 t.8 2,0 2.2 seen m o r e c l e a r l y in Table II, where the t000 12 compositions a r e listed in o r d e r of d e c r e a s i n g cgnductivity at 300 °C. The compositions a r e listed in three FIG. 2 columns, for x = l , 2, and 3, and the four compositions in each column are Product of conductivity (~) and absolute t e m p e r a t u r e (T) as a function of 1/T for identified by specifying D and T. The o r d e r within each column is the same, LISICON, Li 3 5Si0 5P0,504 (Ref. 6 ) a n d with higher conductivity being observed Li -B -alumina "(Ref." 4 ) , for the two samples containing C,e than for those containing Si, and higher conductivity being observed for each Zn sample than for the Mg sample with the same tetravalent cation. In g e n e r a l , for a given D - T pair the conductivity d e c r e a s e s as x i n c r e a s e s , except tY,at the Zn-Si and Mg-Si c o m positions with x = 1 have lower conductivities than the correspondLug ones with x = 2. F o r 400°C, the o r d e r of d e c r e a s i n g conductivity within the x = 1 and x =2 columns is the same as at 300°C, but for x = 3 the conductivity d e c r e a s e s in the o r d e r Mg-Ge, Mg-Si, Zn-Ge, and Zn-Si. Li-BETA'ALUMINA/

122

H. Y-P. HONG

Vol. 13, No. 2

('C) 400

2""l

300

I

zo

I

~

1.3

L4

I.~

Li~4Zn(G*O,)4

1.6

1.7

1.8

1000 T(K}

FIG. 3 Product of conductivity (~) and absolute t e m p e r a ture (T) as a function of I / T for Lil6.2~Dx(TO4) 4 samples with D = Mg2+ or Zn2+, T = Si or Ge 4+, and x = 1, 2, or 3.

Discussion

TABLE II O r d e r of Decreasing Conductivity at 300°C in Lil6.2xDx(TO4)4 x = I

x =2

D

D

T

x

T

Zn Ge

Zn Ge Mg

Ge Mg Ge Zn Si

D

=3

T

The ionic conductivity of LISICON at 300°C is 0.13 fF i cm" I, only.a factor of three lower than the value for Na-B -alumina and NASICON, the best Na+-ion solid electrolytes known except for Na-B"-gallia. This high Li+ion conductivity is readily explained by applying the crystallographic principles developed for fast alkali-ion t r a n s p o r t (1-3) to the c r y s t a l s t r u c t u r e that we have determined for LISICON.

Like other superionic conductors, LISICON contains a high concentration of Zn Si mobile alkali ions that occupy sites in the interZn Ge stitial space of a rigid t h r e e - d i m e n s i o n a l netM g Ge work. (The Li + ions incorporated into the netZn Si work do not make an appreciable contribution to M g Si the conductivity. ) Although there are two M g Si inequivalent types of i n t e r s t i t i a l sites, both are Mg Si partially occupied, indicating that the difference in potential e n e r g y between the sites is not large enough to prevent e a s y t r a n s f e r of Li + ions from one to the other. The mean d i a m e t e r of the bottlenecks between adjacent sites (4.38 A) is considerably l a r g e r than twice the sum of the Li + and 0 2 - ionic radii (2.0k), r.he minimum required to satisfy the geometric condition for fast Li+-ion transport. Finally, the mobility of the interstitial Li + ions is i n c r e a s e d because the interaction between these ions and the 02° ions forming the bottlenecks is reduced by the chemical bonding between the 0 2 . ions

Vol. 13, No. 2

SUPERIONIC CONDUCTORS

12°3

and the cations of the t h r e e - d i m e n s i o n a l network. All t hree network cations, p a r t i c u l a r l y the Li + ion, f o r m strong covalent bonds to O2- ions that p o l a r i z e the oxygen c h a r g e d en s ity away f r o m the i n t e r s t i t i a l ions. Tl~is e f f e c t is e s p e c i a l l y i m port ant in the LISICON s t r u c t u r e because each 0 2 - ion is bonded to four network cations, leaving none of its sp 3 o r b i t a l s available f o r coval ent bonding to the i n t e r s t i t i a l Li + ions. In c o n t r a s t , the 0 2 . ions forming the bottlenecks in the ~-alumina and NASICON s t r u c t u r e s a r e each bonded to only two o r t h r e e network cations. This d i f f e r e n c e probably accounts to a l a r ge extent for the fact that LISICON has a much h i g h e r ionic conductivity than the Li compounds with the ot her two s t r u c t u r e s . In Li3PO 4, as in LISICON, each 0 2 . ion is bonded to four net w ork cations (8). The ionic conductivity of Li3PO 4 is v e r y low, how ever, because all the Li + ions f o r m p a r t of the network and cons e que nt l y have low mobility. In the pseudobinary solid solutions of Li3PO 4 with Li~SiO 4, which have the f o r m u l a Li3+xSixPl_xO4, the additional positive c h a r g e r~quired to c o m p e n s a t e f o r the r e p l a c e m e n t of p5+ ions by Si 4+ ions is provided by the i n c o r p o r a t i o n of i n t e r s t i t i a l Li + ions. Because of the high mobility of these ions, the conductivity is d r a s t i c a l l y i n c r e a s e d in the solid solutions (5, 6). In fact, as noted above, Li 3 5Si0..sP0. 504 has the highest Li+-ion conductivit i e s p r e v i o u s l y r e p o r t e d f o r the 20"0"--400~ C r a n g e (6). The mobility of the i n t e r s t i t i a l ions, which is i n c r e a s e d by the strong i n t e r a c t i o n between each 0 2 - ion and the four network catio n s to which it is bonded, would probabl y be e v e n h i g h e r if the bottlenecks between the i n t e r s t i t i a l s i t e s w e r e l a r g e r . Although detailed s t r u c t u r a l data a r e not a v ailab le f o r the solid solutions, the bottlenecks should be s i m i l a r in size to those in Li3PO4, which have c r o s s d i a m e t e r s of 3.24 and 5.00 A (8). The m e a n d i a m e t e r is thus 4.12 A, not much l a r g e r than the m i ni mum of 4 . 0 k r e q u i r e d f o r fast L i +-i on t r a n s p o r t . The small size of the bottlenecks probably accounts for the r e l a t i v e l y high a c t iv atio n e n e r g i e s r e p o r t e d (6) for the Li3+xSixPl_xO 4 solid solutions. Even the lowest value, e a = 0.51 eV (for L i 3 . 4 S i 0 . 4 P 0 . 604 ), is m o r e than twice the value for LISICON, 0.24 eV, which is c o m p a r a b l e to the activation e n e r g i e s of 0.2 - 0 . 3 eV o b s e r v e d f o r m o k e n salts (9). Although LISICON is the only L i l 6 . 2 x Z n x ( G e O 4 ) 4 com posi t i on f o r which we have p e r f o r m e d a detailed s t r u c t u r e d e t e r m i n a t i o n , the s i m i l a r i t y of t h e i r x - r a y powder p a t t e r n s indicates that all of these m a t e r i a l s have the sam e basic s t r u c t u r e as LISICON, with a t h r e e - d i m e n s i o n a l net~'ork in which each 0 2 - ion is bonded to four cations. T h e i r wide v a r i a t i o n in ionic conductivity at a given t e m p e r a t u r e , as shown in Fig. 3, can be attributed l a r g e l y to changes in the c o n c e n t r a t i o n of i n t e r s t i t i a l Li + ions and in bottleneck size r e s u l t i n g f r o m changes in s t o i c h i o m e t r y and network c a n o n s p e c i e s , r e s p e c t i v e l y , although d i f f e r e n c e s in s i n t e r i n g p r o p e r t i e s a r e probably also invo ived. As x in Lil6_2xDx(TO4) 4 is i n c r e a s e d f r o m I to 3, the num ber of i n t e r s t i t i a l Li + ions p e r f o r m u l a unit d e c r e a s e s f r om 3 to i. F o r this s t o i c h i o m e t r y range, in which l e s s than half the i n t e r s t i t i a l positions a r e occupied, for a given D - T p a i r such a d e c r e a s e in the n u m b e r of i n t e r s t i t i a l ions is expect ed to r e s u l t in a d e c r e a s e in conductivity. The e x p e r i m e n t a l r e s u l t s for both 300 and 4 0 0 ° C , as shown in Fig. 3 and Table II (300~C only), a r e g e n e r a l l y c o n s i s t e n t with this predi ct i on, except that f o r the Zn-Si p ai r the conductivity is lower f o r x = 1 than for x =2 and for the Mg-Si p a i r it is lower f or x= 1 than f or e i t h e r x = 2 or x = 3 . F o r Lil6_2xDx(TO4)4 s p e c i m e n s with the sam e s t o i c h i o m e t r y , t h e r e is an exc e l l e n t qualitative c o r r e l a t i o n between the ionic conductivity at a given t e m p e r a t u r e and the size of the D and T cations. In e v e r y c a s e the conductivity is h i g h e r f o r a sample containing C,e4+ (ionic r a di us 0.40A) than f o r the c o r r e s p o n d i n g sample containing Si 4+ (ionic r a di us 0.26 £). In addition, except at 400°C for s p e c i m e n s with x = i, the conductivity is hi ghe r f or each sample containing Zn 2+ (ionic r a d i u s 0 . 6 0 A) than f o r fl~e c o r r e s p o n d i n g one containing Mg2+ (ionic radi us 0.58 A). Ir~ genezal, h o w e v e r , the d i f f e r e n c e s in conductivity r e s u l t i n g f r o m ~he substitution of Zn 2+ for

124

H. Y-P. HONG

Vol. 13. No. 2

Mg2+ are considerably g r e a t e r than might be expected from the small difference in the radii of these ions. It is quite possible that these differences in conductivity are due in part to differences in the ceramic properties of the samples, whose densities did not exceed 90~0 of theoretical; it would not be surprising if the sintering conditions used in this study yielded better ceramic quality for samples in which Zn2+ rather than Mg2+ was the divalent cation, since materials containing Zn tend to be less r e f r a c t o r y than those containing Mg. To obtain an adequate understanding of the relationship between ionic conductivity and composition in the L 16.2xDx(TO4)4 solid electrolytes, it will be necessary to prepare and measure ceramic specimens with densities closely approaching theoretical, as well as to perform detailed crystal structure determinations for a number of additional compositions. Conclusion A new class of Li+-ion solid electrolytes has been discovered by applying the crystallographic principles underlying fast alkali-ion transport. One of these materials, LISICON, is the best Li+-ion conductor so far reported for the 300400°C range. The same principles should be useful in guiding ~ e search for still better superionic conductors. References

1. H. Y-P. Hong, Mater. Res. Bull. 11, 173 (1976). 2. J. B. Goodenough, H. Y-P. Hong, and J. A. Kafalas, Mater. Res. Bull 11, 203 (1976). 3. H. Y-P. Hong, "New Solid Electrolytes", American Chemical Society Meeting, New York, NY, April 1976; to be published in Advances in Chemistry. 4. M. S. Whittingham and K. A. Huggins, Solid State Chem., R. S. Koth and S. ]. Schneider, Eds., Nat. Bur. Standards Spec. Pub. 364 (1972), p. 139. 5. K. D. Shannon, B. E. Taylor, A. D. English, and T. Berzins, Electrochim. Acta 22, 783 (1977). 6. Y-W. Hu, I. D. l~listrick, and R. A. Huggins, J. Electrochem. Soc. 124, 1240 (1977). 7. U . v .

Alpen, A. Kabenau, and G. H. Talat, Appl. Phys. Lett. 30, 621 (1977).

8. J. Zemann, Acta Cryst. B 13, 863 (1960). 9. S. Pizzini, J. ~.ppl. Electrochem. 1, 153 (1971).