SOLID STATE IONICS

ELSEVIER Solid State Ionics 79 (1995) 152-157

Chloride ion conductor CH3NH,GeCl, studied by Rietveld analysis of X-ray diffraction and 35C1 NMR

Koji Yamada ay * , Kazuaki Isobe a, Emi Tsuyama a, Tsutomu Okuda a, Yoshihiro Furukawa b

a Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 724, fapan b Faculty of School Education, Hiroshima Universi& Kagamiyama l-l, Higashi-Hiroshima 724, Japan

Abstract

The chloride ion conductor CH,NH,GeCl, was characterized by DTA, Rietveld analysis of the powder X-ray diffraction, and 35C1 NMR. The mobile ion at the high temperature phase was deduced to be a chloride ion, because a disordered perovskite structure at the Cl site was found by the Rietveld analysis. This disordered model was also confirmed by 35C1 NMR. The mechanism of the ionic conductivity and the process to attain a conducting phase were discussed on the basis of the temperature dependence of the powder X-ray diffraction and the spin-lattice relaxation times of 35C1 NMR.

Keywords: Ionic conductivity - chloride; Rietveld analysis; X-ray diffraction; DTA

1. Introduction

In our previous paper we have reported high electric conductivity of CH,NH,GeCl, above 398 K [l]. A disordered cubic structure was found above 398 K by the Rietveld analysis of the X-ray diffrac- tion. In this cubic perovskite phase, a disorder of the type Ge-Cl . . . Ge c, Ge . . * Cl-Ge appears in con- trast to a symmetric bridge Ge-Cl-Ge in an ideal perovskite. This bond-switching disorder appears along three orthogonal directions because of the cubic structure. Furthermore, due to the characteris- tic fauns effect of the hypervalent atom such as Sn(I1) halide [2-41, two Ge-Cl bonds at the trulzs position are not strong simultaneously. Therefore, at every instant discrete GeCl; anions remain in the

* Tel: 0824-24-7418; Fax: 0824-24-0727, E-mail: gO58998.@ ue.ipc.hiroshima-u.ac.jp.

cubic phase. Considering these findings, we have proposed the following mechanism for the high ionic conductivity. The mechanism consists of two pro-

cesses, a translational displacement of the Cl ion between two Ge atoms, Ge-Cl . . . Ge * Ge . . . Cl- Ge, and a reorientaion of the GeCl; anion about the pseudo C, axis. The latter reorientational mode of the GeCl; anion was found by means of the 35C1 NQR technique even in the lower temperature phase [ll. Consistent with this model, 35C1 NMR was de- tected above 364 K, suggesting that the time-aver- aged quadrupole coupling constant at the Cl site disappeared in spite of the large quadrupole coupling constant at the rigid lattice. A cubic structure also appeared in (CH,),NGeCl, as the result of the disordered structure and disordered Cl sites were clearly seen on the Fourier map because of the large lattice dimensions [ 11.

In this paper we observed the temperature depen- dence of the powder X-ray diffraction pattern, and

0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00055-O

K. Yamada et al./Solid State Ionics 79 (1995) 152-157 153

10'

100

10.'

1o-2

7 E v) 10"

G

1o-4

1o-5

1o‘6

lo“

I I I I I I I

A : CsGeCI3 0 : CH3NH3GeC13 o : (CH3)4NGeCl3

I I I I I I I

2.2 2.4 2.6 2.8 3.0 3.2 3.4

1000 K I Temperature

Fig. 1. Temperature dependence of the electric conductivity for

MGeCl, (M = Cs, CH,NH, and (CH,),N) determined by the ac

complex impedance method [l]. Arrows indicate phase transition

temperatures.

the crystal structure of Phase IV (room temperature phase) was determined by the Rietveld analysis. The translational correlation times were also estimated by the 35C1 NMR T1 experiment. From these observa- tions, we will discuss the structural relationship be- tween low and high temperature phases. High ionic conductivity will also be discussed on the basis of the characteristic bonding property of the hyperva- lent element such as Ge(I1) and St@>.

2. Experimental

Preparation of CH,NH,GeCl, was described in our previous paper [l]. Powder X-ray diffraction was

V rnw III

I I I I 250 300 350 4txl

Temperature / K

Fig. 2. DTA curve for CH,NH,GeCI,.

2.05p I I 1

0

IV

2.00

_\

1.95 -

>>

1.9oc 1

300 I t

350 400

TemDerature I K

Fig. 3. Temperature dependence of the density calculated from the

lattice constants determined by the Rietveld analysis.

I I I I I I (A) Phase II at 398 K

I

(B) Phase III at 370 K I

-+---~~~ - I I I I I ll

10 20 30 40 50 60 70

201 degree

Fig. 4. Final plot of the Rietveld analysis of CHsNHsGeCl,.

Solid lines and dots are calculated and observed patterns, respec-

tively. The differences between them are shown by the lower

portions.

154 K. Yamada et al./Solid State Ionics 79 (1995) 152-157

Table 1

Crystal data and experimental details of the Rietveld analysis for CH,NH,GeCl,

Phase II III IV

Temperature (K) 398 381 298

Crystal system cubic rhombohedral orthorhombic

Space group Pm3m R3m Pnma

Lattice constant a = 5.658(l) A a = 5.659(l) A a = 11.194(l) A

cy = 90.90(l)” b = 7.417(l) A

c = 8.292(l) A

* 1 1 4

deal (g cmm3) 1.935 1.934 2.036

Number of param. 16 23 27

28 range (“1 10-80 lo-80 12-72

Step width (“1 0.04 0.04 0.04

R,, R, a,b 0.110, 0.115 0.103, 0.091 0.099, 0.054

DS, RS, SS (“1 ’ 0.5, 0.15, 0.5 0.5, 0.15, 0.5 0.5, 0.15, 0.5

a R, = ZZ 1 y (ohs) - y,(cal) @y-fobs), where y,(obs) and y&al) are the observed and calculated intensity at the ith step.

b R, =X(&i - &&X4-, w h ere IK is the intensity assigned to the Kth Bragg reflection.

’ Divergence slit, receiving slit and scatter slit.

observed by a Rigaku Rad-B system with a home- made high temperature cell using Cu Kcu radiation.

The diffraction patterns were analyzed by the Ri- etveld method using a program developed by Izumi

[5]. DTA was observed by a home-made apparatus using a sealed glass tube. 35C1 NMR was observed by the Matec Gating amplifier and the signals were

detected by a home-made apparatus.

3. Results and discussion

3.1. Powder X-ray difiaction and successive phase

transitions

At least five phases were confirmed for CH,NH,GeCl, in the temperature range between 77 K and 450 K by means of several different tech- niques (Fig. 1). Although the structure of the highest

temperature phase has not been characterized yet, each phase was tentatively abbreviated as Phase I, II,

III, IV, and V from the high temperature side. Fig. 2 shows a DTA curve observed in the temperature

range of 250-420 K. A strong endothermic peak was assigned to the phase transition from phase IV (or- thorhombic phase) to III (rhombohedral phase). On the other hand, no apparent peak assigned to the phase transition from phase III (rhombohedral phase)

to II (cubic phase) was found with increasing tem- perature. Fig. 3 shows the temperature dependence of the density calculated from the lattice constants determined by Rietveld analysis. Consistent with the DTA measurement, a discontinuous decrease of about

3% was found at the phase transition from phase IV to III.

Fig. 4 shows the final plot of the Rietveld analysis for phase IV, together with phases III and II. The experimental details and crystallographic parameters

Table 2 Positional and isotropic thermal parameters of CH,NH,GeCl, at 294 K (phase II)

Atom

CH,NH, b

Ge

Cl(l)

Cl(21

Position a

4cfm)

4&l)

8d(l) 4&n)

x Y z BisO cK,

0.520(l) 0.25 0.232(3) 34.605)

0.7300) 0.25 0.7450) 2.1(3)

0.727(l) 0.021(l) 0.581(l) 4.0(4) 0.93701 0.25 0.793(2) 3.3(6)

a Wyckoff notation and point symmetry.

’ Isoelectronic dummy atom K+ was used for the refinement.

K. Yamada et al./Solid State Ionics 79 (1995) 152-157 155

156 K. Yamada et at. /Solid State Ionics 79 (1995) 152-157

are summarized in Tables 1 and 2. Fig. 5 shows the

perspective drawings of phases IV and II along crystal a-axes. By comparison of the structures of

phases IV and II, it becomes apparent that the disor- der at the Cl site leads to a cubic phase. It is particularly interesting that this type of order-dis-

order transition reflects the characteristic bonding property of the hypervalent atom. In our previous

paper, we have analyzed the phase III to be R3m having no disorder. However, the disordered struc- ture may have already appeared in phase III because of the following three reasons: (1) a large endother-

mic peak was observed at the phase transition from

phase IV to III; (2) a step wise decrease of the density at the phase transition; and (3) 35C1 NMR was also detected in this phase. Therefore the space group of phase III may be Rjrn rather than R3m

and a disordered structure similar to cubic phase is

supposed to exist.

3.2. ‘jC1 NMR spectra and spin-lattice relaxation

times

In our previous paper we have reported 35C1 NQR

for a series of MGeCl, (M = Cs, Rb, CH, NH,, and (CH,),N) [l]. According to the NQR study, the quadrupole coupling constant at the Cl site is about 25 MHz which arises mainly from the Ge-Cl chemi- cal bond in the GeCl; anion. If the structure of

CH,NH,GeCl, is the ideal perovskite, about 10% smaller e’Qq/h is expected for 35C1 from the con- sideration of the ‘lBr NQR data on SnB$ anions

[2-41. It is usually impossible to detect Cl NMR having large quadrupole coupling constant such as CH,NH ,GeCl,, because the line broadening due to

the quadrupole effect is too Iarge to detect. As shown in Fig. 6(A), however, the 35C1 NMR signal could be

detected above ca. 364 K, and the signal intensity increased with increasing temperature. As far as we know, it is an unique case that both 35C1 NQR and NMR were detected below and above the phase transition temperature, respectively. These findings suggested that the e2Qq/h at the Cl site vanishes due to the isotropic motion and the correlation fre- quency of the motion is much higher than e2Qq/h, l/7, > e2Qq/h. On the basis of the disordered structure shown in Fig. 5(B), a two-step model was proposed for the translational diffusion of the chlo-

4 I 1 I

2.0 2.1 2.2

IOOOKIT

Fig. 6. 35Cl NMR spectra of CH,NH,GeCI, at selected tempera-

tures (A) and the temperature dependence of the spin-lattice

relaxation times (B).

ride ion. That is the translational jump between 6f sites (Wyckoff notation of space group Pm3m) and

the reorientation of GeCl; anion about the pseudo C, axis. Fig. 6(B) shows the temperature depen- dence of the spin-lattice relaxation time Tr observed at 26.58 MHz (6.37 T). Because the quadrupole coupling constant at the 35C1 site can be large, NMR relaxation is governed by quadrupole interaction [6]. Furthermore, in this case, the fast-motion limit (7, w

< 1, w: NMR Larmor frequency) is satisfied be- cause l/r, > e2Qq/h = w. Then the relaxation rate, l/T,, is given like a liquid state as,

3 (21+3)

lP1 = 40 P(2Z- 1) (I + v2/3)( ezQq/h)2~c9

(1) where Z is the spin quantum number, 7, the asym- metry parameter of the electric field gradient, and rc the correlation time of the motion. For a “Cl nucleus (I = 3/2) Eq. (1) reduces to

l/T, = (l/10)(e2Qq/h)2rc. (4 As is shown in Fig. 6(B), from the log@,) plot against l/T the activation energy for the transla-

K. Yamada et al. /Solid State Ionics 79 (1995) 152-157 157

tional diffusion was calculated to be 28 + 1 kJ/mol, which was a little smaller than that estimated from the conductivity measurement [l]. On the other hand, ionic conductivity can be estimated from the follow-

References

[l] K. Yamada, K. Isobe, T. Okuda and Y. Furukawa, Z. Natur-

forsch. 49a (1994) 258.

ing equation.

u=Nel/(k T T,), (3)

[2] K. Yamada, T. Hayashi, T. Umehara, T. Okuda and S Ichiba

Bull. Chem. Sot. Japan 60 (1987) 4203.

[3] K. Yamada, S. Nose, T. Umehara, T. Okuda and S. Ichiba

where N is the concentration of the conduction ions,

e the charge on the Cl atom, and 1 the length of the elementary jump. Using TV calculated from Eq. (2),

v was estimated to be ca. 20 Sm-’ (at 450 K) which is about one order lamer than the conductivitv

Bull. Chem. Sot. Japan 61 (1988) 4265.

[4] K. Yamada, H. Kawaguchi, T. Matsui, T. Okuda and S. Ichiba

Bull. Chem. Sot. Japan 63 (1990) 2521.

[5] F. Izumi, in: The Rietveld Method, ed. R.A. Young (Oxford

University Press, Oxford, 1993) p. 236.

[6] A. Abragam, Principles of Nuclear Magnetism (Oxford Uni-

measurement. versity Press, Oxford, 1961).