Mixed alkali metal sulfate proton conductors: The structure of Cs0.9Rb0.1HSO4 at room temperature

SOLID STATE ELSEVIER

IONICS

Solid State Ionics 91 (1996) 323-330

Mixed alkali metal sulfate proton conductors: The structure of Cs,.,Rb,. I HSO, at room temperature ’ B.V. Merinova**, D.J. Jonesa, J. Rozib-e”, T. Mhirib “Laboratoire des Agr&ats

Molkulaires

et Mate’riaux Inorganiques URA CNRS 79, UniversitP Montpellier II, Sciences et Techniques Place Eugkne Bataillon. 34095 Montpellier, France de 1’Etat Solide, Ecole Nationale d ‘Inginieurs de Sfax, 3038 Sfar, Tunisia

du Lmguedoc,

‘Laboratoire

Received 26 February

1996; accepted

15 March

1996

Abstract has been performed at room A single crystal X-ray diffraction investigation of the superionic conductor Cs ,,Rb,,HSO, temperature with the aim of studying the influence of the introduction of a heteroalkali metal ion on the crystal structure with respect to that of the end members, and its influence on the temperature of the phase transition to the superionic phase. M, = 225.23, h(MoK,) = 0.70930 A, monoclinic, P2,, a = 7.752(2) A, b = 8.117(2) A, c = 14.577(4) A, p = 99.09(2)“, V= 905.7(4) A’, Z = 8, D, = 3.29 g cm-‘, p = 90.2 cm-‘, F(000) = 818, R(F) = 0.034, R(F), = 0.034 for 2009 unique reflections. Inclusion of a small amount of rubidium ions is sufficient to perturb the metastable room temperature phase III structure of the end member CsHSO,, and the structure can be considered instead as similar to that of phase II, usually stable only above 330 K. Cs,,Rb,,HSO, exhibits a strictly one-dimensional character, with sulfate groups being linked into zigzag chains through hydrogen bonds as short as 2.48(2) A. Keywords:

Alkali metal sulfate proton conductor;

X-ray diffraction;

1. Introduction One of the important

reasons

causing

the

great

of the MXA04 family (M = NH,, K, Rb, Cs; X = H, D and A = S, Se) is the observed phenomenon of temperature-dependent structural phase transitions [1,2], the most important in the context of research for protonic electrolyte materials being the transition to the superprotonic interest

in solid

acids

*Corresponding author. On leave from: Institute of Crystallography of the Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia. ‘This paper is dedicated to Dr. Hatem Mhiri. 0167-2738/96/$15.00 Copyright PII SO167-2738(96)00375-X

01996

Crystal

structure

phase, which occurs at temperatures as low as 300 K [(NH,),H(SeO,),] but can be as high as circa 445 K in RbHSeO, and Cs,H(SeO,),. Superprotonic conductivity of the order of 10m2 S cm-‘, e.g. in the hydro- and deuterosulfates and selenates of caesium [3], is then observed up to the melting temperature. This phase transition temperature is known to be affected by substitution of hydrogen by deuterium and by selenate ion for sulfate ion. Some of our recent studies have been devoted to characterising the influence of the partial substitution of caesium in CsHSO, by another alkali metal ion [4-81 on physical properties and spectroscopic characteristics, and on the crystal structure. In the CsXAO, family,

Elsevier Science B.V All rights reserved

324

B.V. Merinov et al. I Solid State Ionics 91 (1996) 323-330

investigations made by various methods on polycrystalline and single-crystal samples have shown the existence of three phases for CsHSO, (III-330 KII-414 K-I) and only two phases for CsDSO, (II412 K-I), CsHSeO, (II-398 K-I) and CsDSeO, (II-401 K-I). All four compounds are isomorphic in phases I and II [9]. Phase III, which is observed only for CsHSO,, is metastable and has no analogue either among the other MHAO,-type structures. Furthermore, it does not appear on cooling the crystal from the high-temperature phase [lo] and, consequently, the III + II phase transition is irreversible. Although the structure of mixed alkali metal acid sulphates are, for the vast part, unknown, it is already recognised that crystallisation from aqueous solutions of Cs,SO,/M,SO,/H,SO, can give rise to members of the Cs, _,M,HSO, or of the Cs,_,M,H(SO,), families [8]. For Cs, _,Rb,HSO, four structural domains near x50.33, 0.33 10m2 S cm-i. The partial substitution of Cs+ by Rb+ increases the conductivity of the low-temperature phase but decreases that of the hightemperature one. A glassy state can be reached for compositions with 0.4 IX 5: 0.6 by quenching from the melt. Drastic changes with smoothing of the transition occur at x > 0.5, and the last area 0.9 5 x 5 1 is characterised by a ferroelectric phase transition at about 250 K, as in pure RbHSO,.

2. Experimental Transparent single crystals of Cs,,,Rb,,,HSO, were grown by slow evaporation above 300 K of an aqueous solution of Cs,SO,/Rb,SO,/H,SO, in a ratio 0.9:O. 1: 1 according to [6,9]. The crystals grown have a plate-like shape. Chemical analysis of two crystals was performed by X-ray fluorescence using a Castaing microprobe. The quantitative result [Cs-91(3)% and Rb-9(3)% of the total metal contents] was subsequently confirmed by X-ray crystallography.

The diffraction data were collected on an EnrafNonius CAD-4 automatic 4-circle X-ray diffractometer. The experimental details are listed in Table 1. The lattice constants were refined by least-squares methods from 24 reflections with 14 < 0 < 23”. The intensities of reflections with 12 3a(Z) were converted to moduli of structure amplitudes taking into account corrections for the Lorentz and polarization factors. All crystallographic calculations were carried out using the AREN program system [ 1 I]. Atomic scattering factors were taken from [12]. The absorption correction was introduced by means of the DIFABS program [13]. Extinction was taken into account in the isotropic approximation according to Zachariasen’s formalism [ 141. All non-hydrogen atoms were refined with anisotropic thermal parameters. Final positional and isotropic thermal parameters are listed in Table 2, the principal interatomic

Table 1 Experimental

parameters

and details of refinement

Radiation Wavelength (A) Monochromator Temperature (K) Crystal habit Size (mm) Space group a b c P v 2

(A) (A) (A) 0 (A’)

Dx &mm’) hkl range Scan mode Scan width (“) No. of reflections measured No. of reflections observed (I 2 3U,) Agreement factor on IFI No. of unique reflections 213 limits Linear absorption coefficient (cm-‘) Isotropic secondary extinction Weight R

R

W

MoK, 0.71030 graphite 295 plate-like 0.40 x 0.20 x 0.05 p2, 7.752(2) 8.117(2) 14.577(4) 99.09(2) 905.7(4) 8 3.29 -14shslO,Osks14, 051526 w/B 1.0+0.35tg0 6196 2116 0.021 2051 2~20~80’ 90.2 0.24 X 10m4 1/(2F,,, + F + 2F*/F,,,) 0.034 0.034

B.V Merinov Table 2 Fractional

atomic coordinates

and isotropic

et al. I Solid State Ionics 91 (1996)

thermal parameters

(A’) with e.s.d.‘s in parentheses

Atom

x

Y

Wl) W2) W3) Cs(4),Rb

0.2385(3) 0.2622(3) 0.4934(2) 0.0216(2) - 0.0223(7) 0.5081(10) 0.2301(g) 0.2539(g) 0.8498( 19) 0.6723(22) 0.8254( 15) 0.6555( 18) 0.3986(21) 0.0647(28) 0.1534(19) 0.3330(20) 0.6404( 19) 0.8313(30) 0.3856( 18) 0.0659( 22) 0.1067( 19) 0.4406( 19) 0.8988( 17) 0.6051(25) 0.7094 0.728(35) 0.7529 0.748(28) 0.5326 0.552(22) - 0.0272

0.6841(O) 0.1899(4) O&84(3) 0.9408(3) 0.4468(6) 0.9450( 11) 0.6928(g) 0.1910(7) 0.8251(14) 0.3147(19) 0.3909( 15) 0.8900(21) 0.2623( 18) 0.7343(29) 0.0901(15) 0.5598( 16) 0.5832(24) 0. I 104(33) 0.0612( 19) 0.5746(20) 0.3189(19) 0.7981(19) 0.5203( 18) 0.0543(25) 0.5115 0.5 1 l(47) 0.0121 - 0.008(38) 0.1273 0.082(28) 0.6157

S(1) S(2) S(3) S(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(l0) (x11) (x12) O(l3) (x14) W15) Wl6) H(1) H(2) H(3) H(4)

a For hydrogen atoms the first and second lines correspond thermal parameters, respectively.

323-330

0.0151(l) 0.4705( 1) 0.2765( 1) 0.2333(2) 0.2424(4) 0.2703(4) 0.4765(5) 0.0100(5) 0.0161(8) 0.4581(10) 0.1783(g) 0.3391(10) 0.0806( 12) 0.4176(17) 0.0649( 10) 0.4530( 10) 0.0528( 13) 0.4277( 16) 0.2961(11) 0.1832(13) 0.2886( 12) 0.2213(12) 0.3185( 10) 0.2014(16) 0.0996 0.088(20) 0.3882 0.392(18) 0.1590 0.171(12) 0.3627 atomic coordinates

a B eq h

Z

to calculated

325

3.29(4) 3.35(4) 2.99(3) 3.08(4) 2.20(7) 2.86(9) 2.61(9) 2.51(9) 4.01(15) 3.00(20) 2.1 l(14) 3.97(22) 2.39(27) 7.35(56) 2.55(17) 2.39( 16) 3.21(24) 5.38(60) 2.89(21) 4.63(26) 3.23(22) 3.33(25) 2.90( 17) 3.68(38) 5(5) 4(4) 2(4)

and refined atomic coordinates

and isotropic

’ P,, = (4/3W,P,,v,.

distances in Table 3 and the thermal soids in Table 4.

motion

ellip-

3. Results and discussion Cso,,RbO,,HSO, crystallises in a structural arrangement which is different from either the phase III or phase II structures of CsHSO,, although it shows clear similarities to the latter. The projection of the crystal structure is shown in Fig. 1. In accordance with the symmetry of the structure (space group P2,) all atoms occupy general positions. The

crystal structure can be described as consisting of SO, tetrahedra linked by hydrogen bonds in infinite zigzag chains along the [ 1011 direction. These chains alternate in the [OlO] direction with rows of heavy ions along [ 1011. There are no hydrogen bonds (H-bonds) between the zigzag chains, and, therefore, Cs,,Rb,,, HSO, exhibits a strictly one-dimensional character in the room-temperature phase. The hydrogen bonds were identified from an analysis of the inter-tetrahedral 0. . *0 and intratetrahedral S-O distances. Two short (2.48 A) and two long (2.64 and 2.76 A) hydrogen bonds can be distinguished in the crystal structure of

326 Table 3 Principal

B.V. Merinov et al. I Solid State lonics 91 (1996) 323-330

interatomic

Cs polyhedra

Sulfate tetrahedra

distances

(A) Cs(l)-O(1) ’a Cs(l)-O(12) Cs( 1)-O(7) ” Cs( 1)-O(9) Cs(l)-O(1) ‘I’ Cs(l)-O(3) ’ Cs(l)-O(14) Cs(l)-O(5) ’ Cs(1)-O(7) IV Cs(l)- O(9) ’ Cs(3)-O(14) Cs(3)-O(2) Cs(3)-O( 15) Cs(3)-O(8) Cs(3)-O(3) Cs(3)-O( 13) Cs(3)-O(5) Cs(3)-O( 11) Cs(3)-0( 12) Cs(3)-0( 16) S( 1)-O(3) “‘I S(l)-O(15) “‘I S(l)-O(13) S(l)-O(12) [S( 1)-01,” S(2)-O( S(2)-O(

11) “‘I’ 14)

S(2)-o(4) S(2)-0( 16) ““’

w)-ol,, W-W) W-O(6) S(3)-O(2) ’ S(3)-O( 10) X

[S(3)-01,” S(4)-O( 1) X’ S(4)-O(7) S(4)-O(5) S(4)-O(9)

_I’

[S(4)-01,” Hydrogen

bonds ’

0(9)-H(l).

. .0(3)

O(9)-H( 1) H(1). .0(3) 0(16)-H(3). . .0(5) O( 16)-H(3) H( 3). O(5)

3.01(l) 3.10(2) 3.17(l) 3.18(l) 3.22(l) 3.25( 1) 3.29( 1) 3.39( 1) 3.46( 1) 3.55( 1) 2.96( 1) 2.99( 1) 3.15(l) 3.16(l) 3.17(l) 3.20(l) 3.21(l) 3.27( 1) 3.52( 1) 3.53(2) 1.45(l) 1.47(l) 1.52(l) 1.57(l) 1so 1.43(l) 1.44(l) 1.46(l) 1.61(2) 1.49 1.41(l) 1.46( 2) 1.49(l) 1.68(2) 1.51 1.37(l) 1.45(l) 1.51(l) 1.58(2) 1.48

2.64(2) 0.98 [0.9(3)] 1.66 [1.7(3)] 2.76(2) 0.96 [0.6(l)] 1.79 [2.1(l)]

Cs(2)-O( 13) Cs(2)-O( 11) Cs(2)-O(8) Cs(2)-O(4) ” Cs(2)-O(20) y Cs(2)-O(6) “’ Q(2)-O(8) ” Cs(2)-O(10) “‘I Cs(2)-O(2)

2.92( 1) 3.03( 1) 3.07( 1) 3.19(l) 3.23( 1) 3.24(2) 3.33( 1) 3.36(2) 3.36( 1)

Cs(4),Rb-0( Cs(4),Rb-O(7) Cs(4),Rb-0( Cs(4),Rb-O(6) Cs(4),Rb-0( Cs(4),Rb-0( Cs(4),Rb-0( Cs(4),Rb-O(4) Cs(4),Rb-0(

2.98( 1) 3.05( 1) 3.09( 1) 3.13(2) 3.21(l) 3.31(l) 3.36( 1) 3.46( 1) 3.47( 1)

11) ““’ “‘I’ 12) 13) ““’ 16) IX 1) “I “‘I 14)

O(3)“‘-O( 15) “‘I O(3)““-0( 12) O( 13)-O( 15) “‘I O(3)“‘-O( 13) O( 12)-O( 15) “‘I 0(12)-O(13) O(4)-O(14) 0( 1 l)“‘“-0( 16) ““’ O(4)-0( 16) ““’ O(ll)““-O(14) 0( 14)-0( 16) ““’ O(4)-O( 11) ““’ O(2)“-O( 10) x O(8)-O( 10) ’ O(2)“-O(8) O(6)-O( 10) ’

0(5)-O(7) 0(5)-O(9) X’ 0(7)-O(9) X’ O( I)“‘-O(9) I’

2.28( 1) 2.37(2) 2.38(2) 2.56( 1) 2.56(2) 2.57(2) 2.31(2) 2.35(2) 2.39(2) 2.46(2) 2.48(2) 2.50(2) 2.15(3) 2.34( 3) 2.44(2) 2.48(3) 2.50(2) 2.58(2) 2.24( 1) 2.25(2) 2.34(2) 2.40(2) 2.52(2) 2.65(2)

0(10)-H(2). . .0(4) O( 10)-H(2) H(2). . . O(4) 0(15)-H(4). . ‘O(6) 0(15)-H(4) H(4). . *O(6)

2.48(2) 1.10 [1.2(2)] 1.37 [1.2(2)] 2.48(2) 1.10 1.37

0(6W@) O(2)“-O(6) O( l)“‘-O(7) O(l)“‘-O(5)

a Symmetry codes: (i) 1 -x, l/2 + y, -z; (ii) -x, l/2 + y, -z; (iii) x - 1, y, 2; (iv) X, 1 + y, z; (v) 1 -x, y - l/2, 1 - z; (vi) y - l/2, 1 - 2: (vii) x - 1, y. 2; (viii) X, 1 + y, z; (ix) x - 1, 1 + y, z; (x) 1 -x, l/2 + y. 1 - z; (xi) 1 -x, y - 112, - z. b Oxygen-hydrogen distances were calculated using both calculated and refined (in square brackets) hydrogen atom positions.

X,

B.K Merinov

et al. I Solid State Ionics 91 (1996) 323-330

327

Table 4. Continued Table 4 Thermal motion ellipsoids Atom

(xl)

C@)

Cs(3)

Cs(4), Rb

S(1)

S(2)

S(3)

S(4)

O(1)

O(2)

O(3)

O(4)

O(5)

O(6)

O(7)

O(8)

O(9)

r.m.s. amplitude 0.039 0.041 0.045 0.040 0.042 0.045 0.036 0.038 0.040 0.035 0.039 0.043 0.021 0.027 0.036 0.02 1 0.037 0.05 1 0.020 0.03 1 0.048 0.025 0.034 0.037 0.028 0.048 0.077 0.023 0.030 0.06 1 0.015 0.030 0.036 0.027 0.052 0.072 0.021 0.027 0.042 0.045 0.078 0.156 0.022 0.030 0.045 0.025 0.030 0.036 0.026 0.033 0.063

Atom

O(lO)

(A) 111.8 36.0 117.1 102.5 118.1 31.2 28.5 79.1 116.0 105.1 32.8 118.3 66.8 31.4 70.1 51.1 51.7 62.0 87.8 127.6 37.7 53.8 37.3 82.1 67.4 135.2 53.8 49.0 127.2 117.0 34.4 56.7 82.2 99.2 70.1 22.2 23.5 67.2 84.4 34.2 92.7 124.1 67.3 140.9 119.9 132.1 114.6 52.2 83.1 50.1 40.7

74.8 55.8 38.28 12.6 94.3 78.1 62.7 94.9 27.9 40.5 61.3 64.1 91.6 111.0 21.1 40.1 126.9 103.2 90.7 37.6 52.3 38.2 121.9 108.8 31.8 88.6 121.7 46.2 43.9 87.7 115.6 62.7 39.0 30.3 114.1 72.7 108.2 57.1 38.8 89.9 4.4 94.6 61.5 107.1 34.1 42.3 115.2 58.6 32.0 70.7 114.5

20.3 87.5 110.1 89.9 19.6 70.5 105.6 20.6 76.8 123.2 83.7 33.9 32.3 120.5 99.8 103.8 126.5 39.9 11.2 82.6 98.5 84.9 114.8 25.4 72.9 36.2 59.2 78.7 103.9 18.0 119.3 52.9 129.0 60.1 38.6 111.9 113.0 47.6 128.8 64.9 85.9 25.6 44.1 47.7 100.1 81.4 30.8 60.7 121.9 54.3 128.1

O(ll)

Wl2)

O(l3)

O(14)

(X15)

O(16)

r.m.s. amplitude 0.042 0.067 0.096 0.029 0.038 0.043 0.032 0.055 0.089 0.03 1 0.039 0.053 0.026 0.042 0.058 0.029 0.036 0.045 0.028 0.047 0.065

P”

9,

‘9,

107.1 39.2 124.0 86.2 66.2 24.1 63.4 148.8 74.8 15.6 99.3 102.4 63.4 70.7 33.8 32.7 57.3 88.6 32.3 57.7 88.1

116.4 66.0 37.1 22.2 71.6 102.0 137.8 101.0 49.9 83.6 119.1 30.0 106.6 20.9 102.5 111.2 53.5 135.9 110.7 59.3 38.4

28.1 69.2 71.9 112.1 38.3 119.5 65.0 52.3 48.1 84.9 29.2 61.4 40.0 85.0 129.6 122.0 59.8 46.9 122.1 54.5 128.1

(A)

A, vb and 9< are the angles between the principal motion ellipsoids and the coordinate axes (“).

axes of thermal

Cs,,,Rb,,,HSO,. This arrangement would seem to mark a transition between hydrogen bond geometries typical of phase III and phase II structures of CsHSO,, where space group symmetry imposes, in each case, a single H-bond length of 2.55 A (phase III) and 2.62 A (phase II), respectively. In Cs,,,Rb,,,HSO,, the decrease in symmetry leads to a distribution of hydrogen bond lengths (Table 5). We note, furthermore, that the average of the four hydrogen bond lengths in Cs,,9Rb,,,HS0, (2.59 A) is identical to the average of those in phases III and II of CsHSO,. The two short H-bonds (2.48 A) are close in length to those in phase III of CsHSO, (2.55 A), whilst one of the longer H-bonds (2.64 A) is almost identical to those in phase II of CsHSO, (2.62 A). Comparison of the S-O bond lengths containing oxygen atoms which participate in hydrogen bond formation shows all the hydrogen bonds to be asymmetric. This conclusion is confirmed by symmetry analysis as well, the oxygen atoms involved in hydrogen bond formation being crystallographically distinct. Positions of the hydrogen atoms were first calculated according to [15] and were then refined using the least-squares method.

328

B.V. Merinov

et al. I Solid State Ionics 91 (1996)

323-330

Fig. 1. Projection of the crystal structure of the room-temperature phase of Cs, ,Rb, ,HSO, on the (010) plane. The disposition of the unit cell of the monoclinic phase II of CsHSO, relative to that of the room temperature phase of Cs, ,Rb,,HSO, is shown by the dot-dashed line. The elements of the pseudosymmetry corresponding to the symmetry of phase II of CsHSO, are drawn using broken lines.

Table 5 Comparison of hydrogen bond lengths (A) in Cs,,,Rb, phases III and II of CsHSO, Phase III

Cs,

4 X 2.55

2 X 2.48 1 X 2.64 1 x 2.76

,Rb, ,HSO,

,HSO, and

Phase II 4 x 2.62

Acceptable thermal and coordinate parameters were obtained for three of the four independent hydrogen atoms (see Table 2). The (SO,)*complex anions have the usual

tetrahedral configuration, and although the average S-O distances are close to the standard average value of 1.49 A, the SO, tetrahedra are considerably distorted. Most probably the main reason for such distortions is hydrogen bonding. Indeed, the longer S-O distances generally correspond to those 0 atoms which participate in hydrogen bonding. The exception to this is the S(l)O, tetrahedron, where perhaps such an unusual geometry can be explained by its localisation between two symmetry-related sites of the partially Rb-occupied heavy atom position. The heavy atoms are 9- and lo-coordinated by

B.V. Merinov et al. I Solid State Ionics 91 (1996) 323-330

oxygen atoms. Many different variants both of the distribution of Rb on the possible crystallographic positions and of the Rb content from x = 0.05 up to x = 0.2 were tested. The most satisfactory result, which was summarised from the values of the Rfactor, thermal parameters, interatomic distances and occupancy coefficients, was obtained for the variant in which x = 0.1 and Rb partially occupies only one of the positions of the heavy atoms (see Table 2). It is of particular interest to relate the observed symmetry and structure to those of known phases of the end members CsHSO, and RbHSO,, and their selenate congeners. From a study of the domain structure in CsHSeO,, it was concluded that the crystal symmetry increases from monoclinic to tetragonal at the II +I phase transition and that the transition is ferroelastic [16]. It was assumed that this resulted from a reconstruction of the monoclinic phase II, which can also be described as a pseudo-orthorhombic phase with cell parameters: a, = a, = 7.972(4), b, = b, = 8.427(4), c, = la, + 2c,l = 14.747(4) A and p,, = 99.1”, to the tetragonal phase I with a, = 4.18, c, = 7.20 A as a result of decrease in & by 9.1” and in the lattice constants a,, b,, c, by about half. Although, as subsequent investigations showed [ 17,181, the unit cell parameters of the tetragonal phase I were determined incorrectly, on the whole the approach turned out to be correct. Since the [102] direction in phase II is a pseudo-tetragonal axis, for the sake of analysis of the changes occurring at the II -+ I phase transition, it is very convenient to represent phase II as pseudo-orthorhombic. The inter-relationship between the cell parameters of phases II and I is then the following: Q, -a, + b,, b, - 6, - a, and c, -c,. Now, as clearly seen from Table 1, the unit cell parameters of Cs,,,Rb,,,HSO, are very similar to those of the hypothetical pseudo-orthorhombic CsHSO, phase having a, = 7.789, b, = 8.146, c, = 14.618 A, & = 98.96” and V, =916.2 A’ and Cs,,,Rb,,,HSO, can, therefore, be considered as an analogue of the phase II structure of MXAO, [19]. This result revises the conclusions of [7], according to which the unit cell parameters of Cs, _,Rb,HSO,, at least up to x = 0.5, remain close to those of phase III of CsHSO,: a = 7.3039(9), b = 5.8099(10), c = 5.4908(g) A, /3 = 101.51( 1)” [20,21]. In other words, the partial substitution (x = 0.1) of

329

Cs’ by Rbf in CsHSO, can be regarded as leading to destruction of the metastable phase III and transformation of the hypothetical pseudo-orthorhombic phase into reality. Since the ionic radius of Rb+ is smaller than that of Cs+, the unit cell volume of the room temperature phase of Cs,,,Rb,, , HSO, should be a little less than that of the pseudo-orthorhombic phase of CsHSO,, exactly what is observed. The cell volume is reduced from 916.2 A’ for CsHSO, to 905.7 A” for Cs,,,Rb,,,HSO,. structure of the crystal As expected, Cs,,,Rb,.,HSO, carries an imprint of the crystal structure of phase II of CsHSO,. Determining the structure was complicated by strong pseudosymmetry, elements of which correspond to the symmetry of phase II of CsHSO, (space group P2, / c) (see Fig. 2). In order to avoid the influence of the pseudosymmetry during the least squares refinement process, as the first step only the atomic coordinates were refined while the thermal parameters, to which approximate values were ascribed, were held constant. They were refined during later subsequent steps. The distortion of the sulfate tetrahedra referred to above may also arise in part from correlation between parameters owing to the strong pseudosymmetry. Finally, it may be expected from the unit cell ferroelastic phase of this parameters of Cs,,,Rb,, , HSO, that the superionic paraelastic phase of this compound will be isostructural with the corresponding phase of CsHSO, (spa$e group 14,/ amd, a, =5.729(9), b, = 14.21(l) A) [IS]. The superprotonic conductivity, which is observed in Cs,,,Rb,,,HSO, at high temperature [7], is, therefore, due to a loss of rigidity of the MSO, framework and transformation of the ordered one-dimensional hydrogen bond network into a dynamically disordered three-dimensional one at the superionic phase transition [ 18,221.

Acknowledgments Financial support from the International Association for the Promotion of Cooperation with Scientists from Independent States of the Former Soviet Union, INTAS, grant No. 93-913 is gratefully acknowledged.

B.V Merinov et al. I Solid State Ionics 91 (1996) 323-330

330

aa 5

/

s

s

/

/

s I

ccs Fig. 2. The relationship Cs, ,Rb, , HSO,.

between

t--s---_ the unit cell of the monoclinic

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phase

II of CsHSO,

and that of the room-temperature

phase

of

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