Synthesis and ionic conductivity of CuxLi3−xN

Mat. Res. Bull., Vol. 19, p p . 1377-1381, 1984. Printed in the USA. 0025-5408/84 $3.00 + .00 C o p y r i g h t (c) 1984 Pergamon P r e s s Ltd.

SYNTHESIS AND IONIC CONDUCTIVITY OF CUxLi3_xN Takeshi ASAI, Kunio NISHIDA, and Shichio KAWAI The Institute of Scientific and Industrial Research, Osaka University, 1-8 Mihogaoka, Ibaraki 567, Japan

(Received July 27, 1984; Communicated by M. Koizumi)

ABSTRACT

The effects of cation substitution in Li3N were studied on CuxLi3_xN. The results of Juza and Sachsez on the structural features were confirmed. The activation energy was reduced to 0.13 eV, which was attributed to partial covalency of the Cu-N bond. In spite of this reduction, the ionic conductivity also decreased because of decrease of a number of Li vacancy with substitution.

Lithium nitride, Li3N , is a good Li+-ion conductor. It has a layer structure belonging to the hexagonal space group P6/mmm and contains two kinds of Li atom [ i ] . One, Li(2), is located in the Li2N layer extending perpendicular to the c axis, and the other, Li(1), occupies a position between the layers and is coordinated with two N 3- anions. High ionic conductivity was attributed to easy migration of these Li(2) ions in the layer [25]. The conductivity is largely affected by the presence of anion impurities such as NH 2- and 02- at the N 3- site [5,6].

The present study aims to know effects of cation substitution to the ionic conductivity. Lapp et al. [7] doped 1 at% of Mg, Cu or AI, expecting to increase a number of Li vacancy for charge compensation. They reported negative results. According to Juza and Sachsez [8], Li3N takes in 0.4-0.85 Co, Cu or Ni between the layers. Thus these compounds may be a suitable system to study effects of cation substitution on the ionic conductivity in the layer. Among them, CuxLi3_xN was chosen in the present study because the 3d levels of the transition metal ion are filled in the case of the Cu + ion and the effects of an unpaired d-electron would be negligible. Because Juza and Sachsez unpublished their results, the present experimental results on the structure will also be reported.

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Experimental Samples were p r e p a r e d from Cu metal (99.85%) and Li3N. The Cu powder was t r e a t e d w i t h HNO 3 and HCI before use to e l i m i n a t e surface oxides. Li3N p o w d e r was o b t a i n e d by direct r e a c t i o n of Li (99%) and N 2 gas (99.999%) in a c l o s e d system. M i x t u r e s were p r e s s e d into a rod and fired in N 2 gas (=500 Torr at r o o m temperature) at 700°C for 6h. The p r o d u c t s w e r e i d e n t i f i e d by p o w d e r X-ray d i f f r a c t o m e t r y .

Ionic c o n d u c t i v i t y was m e a s u r e d by the ac c o m p l e x i m p e d a n c e method. S p e c i m e n s w e r e a p e l l e t s i n t e r e d at 500°C for 6h and w e r e e v a p o r a t e d w i t h Au as a b l o c k i n g electrode. Results

and D i s c u s s i o n

X-ray d i f f r a c t i o n p a t t e r n s of the p r o d u c t s are shown in Fig. i. They could be indexed using the same space group as and lattice c o n s t a n t s similar to Li3N. The lattice c o n s t a n t s are p l o t t e d in Fig. 2. C u x L i 3 _ x N w i t h x~ 0.4 always c o n t a i n e d some Cu. The m a x i m u m x was e s t i m a t e d to be 0.38.

In Fig. i, change of the r e l a t i v e i n t e n s i t y is significant, e s p e c i a l l y for the 100 and 001 reflections. Intensities relative to that of the 100 r e f l e c t i o n w e r e c a l c u l a t e d for two cases: one is a s t a t i s t i c a l d i s t r i b u t i o n of Cu e q u a l l y among all the Li sites, and the other is a s e l e c t i v e s u b s t i t u t i o n for the interlayer Li(1) atom. T h e y are c o m p a r e d w i t h the o b s e r v e d ones in Fig. 3. F r o m this c o m p a r i s o n it is c o n c l u d e d that the Cu a t o m is s u b s t i t u t e d s e l e c t i v e l y for the i n t e r l a y e r Li. This is reasonable b e c a u s e the linear 2 - c o o r d i n a t e d Cu + ion is known w h i l e the t r i a n g u l a r 3 - c o o r d i n a t e d Cu + ion is scarce.

that Cu. layer with

In spite of the larger ionic radius of Cu + (0.96 A) than of Li + (0.68 A), the c axis b e c o m e s shorter w i t h increase of This suggests a c o n s i d e r a b l y c o v a l e n t n a t u r e of the interC u - N bond. These structural features are in good a g r e e m e n t the results of Juza and Sachsez q u o t e d in ref. 8.

The c o n d u c t i v i t y is shown in Fig. 4. In the l o w - t e m p e r a t u r e region, it is not of the A r r h e n i u s type. At p r e s e n t it is not clear w h a t type of c o n d u c t i o n was observed. At higher t e m p e r a tures, the A r r h e n i u s r e l a t i o n holds. An a c t i v a t i o n energy is 0.17 eV for x = 0.28 a n d 0.13 eV for x = 0.36. These v a l u e s are smaller than the s m a l l e s t value r e p o r t e d for the e x t r i n s i c ionic c o n d u c t i o n of Li3N [5], and the o b s e r v e d c o n d u c t i o n of C u x L i 3 _ x N was c o n c l u d e d to be extrinsic.

AS seen in Fig. 4, the s u b s t i t u t i o n of Cu for Li has two e f f e c t s to the ionic conduction. One is r e d u c t i o n of an activation e n e r g y and the other r e d u c t i o n of c o n d u c t i v i t y w i t h increase of Cu. The first effect seems due to the c o n s i d e r a b l y c o v a l e n t

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CuLi3_N

001

1379

3.9

I

I

I

I

X=O.O0

100 I

002

I

110

101

i

i

i

3.8

X=0.16 o

i

, I

i I

F

X=0.28

<

I

(J

I I

6 3.7

I

I

i

X=0.36

1

I

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3.(~

5O

~ o

I

I

0,1

0.2

0.3

0.4

XCu

FIG. i. X-ray diffraction pattern of CuxLi3_x N.

~,6

I 0

FIG. 2. Lattice constants us. composition of CUxLi3_xN.

,~ /

1.0

0

_-J

:

'

101

1.0

FIG. 3(left) Comparison of the relative diffraction intensities. Calculated for selective substit u t i o n ( m ) and for distribution among all sites( ). A circle is for the observed ones.

1.0

0.0

0.1

0.2

0.3

X in t i 3 _ x C u x N

0.4

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T. ASAI, et al.

10

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I

5

~

U

(n

'

I

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nature of the Cu-N bond. Covalency of this bond would decrease an effective ionic charge of the N 3- anion to the Li + cation in the Li2 N layer, and lower the activation energy of its migration.

o X= 0.28eV

Since o ~ n v e x p (-E a / R T ) , would increase with a b decrease of E a (i.e., the first effect mentioned above) so long as a number of Li vacancy, nv, remained the same o or increased. In effect the o conductivity decreased by the substitution, and the second 1 effect should be attributed to a decrease of nv in the Li2N , I 0.7 I layer. Colour change among 2.0 2.5 3.0 3.5 the products seems to support this conclusion. Li3N as a 10 3 K I T starting material was black because of the 02- impurity[5, 9], which brought colour cenFIG. 4. ters into the Li3N lattice. Ionic conductivity By substitution of Cu for Li, of CUxLi3_x N. its colour changed to brown, and became paler with an increase of Cu. This change seems due to elimination of the 02- impurity by the substitution. This results in a decrease of the number of Li vacancies, which were introduced for charge compensation, and also a decrease of the number of colour centers. The colour chanqe also shows lack of colouring by the charge transfer between Cu ~ and Cu 2+ ions, and that the Cu atom has a valency of +I. This valency state does not contribute to introduce vacancies at all. The net results of these two factors are a decrease of Li vacancy in the Li2N layer, and a subsequent decrease of the conductivity. B

A~parently, the substitution of Cu gave a negative effect on the Li ion conduction. Those metal cations that may occupy the interlayer Li(1) position and form a covalent Me-N bond, however, still have possibility to improve the ionic conductivity of Li3N through reduction of the activation energy of migration. References i. A. Rabenau and H. Schulz,

J. Less-Common

2. U. von Alpen, A. Rabenau and G. H. Talat, 3-0, 621 (1977). 3. R. Messer,

H. Birli and K. Differt,

Met.

50, 155

Appl.

J. Phys.

Phys.

C14,

2731

(1976). Lett.

(1981).

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CuLi3_xN

4. D. Brinkman, M. Mali, J. Roos, Rev. B26, 4810 (1982). 5. K. Nishida, (1983).

R. M e s s e r

T. Asai and S. Kawai,

6. J. Wahl,

Solid State Commun.

7. T. Lapp, (1983).

S. Skaarup and A. Hooper,

8. S t r u c t u r e Report, vol. t a l l o g r a p h y (1951). 9. K. Nishida, (1983).

K. K i t a h a m a

1381 and H. Birli,

Solid State Commun.

2_99, 485

Phys.

48, 701

(1979).

Solid State Ionics l_!l, 97

ii, p. 97, I n t e r n a t i o n a l

and S. Kawai,

J. Cryst.

Union of Crys-

Growth

6_~2, 475