Fast ion conductors with rotating sulphate ions

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Fast ion conductors with rotating sulphate ions

R. Aronsson, B. Jansson, H. Knape, A. Lundén, L. Nilsson, C.-A. Sjöblom, L.

Torell

To cite this version:

R. Aronsson, B. Jansson, H. Knape, A. Lundén, L. Nilsson, et al.. Fast ion conductors with rotating sulphate ions. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-35-C6-37.

�10.1051/jphyscol:1980609�. �jpa-00220004�

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Colloque C6, supplkment au no 7 , Tome 41, Juillet 1980, page C6-35

Fast ion conductors with rotating sulphate ions

R. Aronsson, B. Jansson, H. E. G. Knape, A. Lundtn, L. Nilsson, C.-A. Sjoblom and L. M. Torell

Department of Physics, Chalmers University of Technology, S-412 96 Gothenburg, Sweden

RBsumk. - Des ttudes de diffraction des rayons X et des neutrons par Li2S04 c.f.c. (entre 575 OC et 860 OC pour Li2S04 pur) montrent la grande facilite des ions sulfates a prendre toutes les orientations. Ces mouvements de rotation fortement couplte entrainent un changement permanent de la position des cations. Cela explique pourquoi tous les cations mono- et divalents ont, jusqu'a maintenant, montre une grande mobilitt dans plusieurs phases sulphates, contrairement au cas d'autres types d'tlectrolytes solides. Ces phases sulfates sont plastiques avec des proprittts qui ressemblent a celles d'autres types de mattriaux plastiques. Des Btudes de diffusion BrilIouin montrent des difftrences notables entre les phases plastiques c.f.c. et liquide Li2S04 alors que l'indice de refraction change a peine au point de fusion.

Abstract. - X-ray and neutron diffraction studies of fcc Li2S04 ⁽⁵⁷⁵to 860 OC for pure Li2S0,) show a high degree of spherical delocation of the sulphate ions. This strongly coupled rotation causes the dimensions of the cation positions to be varying all the time. This explains why all mono- and divalent cations studied so far have a high mobility in a number of sulphate phases, in contrast to the situation in other types of solid electrolytes.

The sulphate phases in question are plastic with properties that resemble those of other types of plastic materials.

Brillouin scattering studies show marked differences between the plastic fcc phase and molten Li,S04, while the refractive index hardly changes at the melting point.

I. Introduction. - During slightly more than a decade, it has been discovered that a very large number of ionic solids have a high ionic conductivity in a region, which can cover several hundreds of degrees. Such compounds are called solid electrolytes provided that the electronic conductivity is negligible in comparison with the ionic, while both are comparable in mixed conductors. These materials can be divided into classes which differ in certain aspects. Thus the sulphate phases studied in our laboratory are characterized by having a high mobility for all mono- and divalent cations for which diffusion studies have been made El], see table I, while for other classes the number of very mobile s ~ e c i e s is much more restricted. Our recent studies

Table I. ^-DifSusion coeficients ( D x 10' cm2 s- l)

for some cations in solid su[phates.

fcc Li2S04 bcc LiNaS04

750 OC 550 ^{O C}

- -

Li 4.2 1 .O

Na 3.7 0.93

Ag 2.8 0.67

K 2.1 0.44

Mg 0.22 0.13

Zn 0.22

Ca 0.51 0.08

Pb 0.87 0.20

of su1phates have aimed at the is in progress. Characteristic of all phases is that the mechanism and at of properties in X-ray diffraction patterns show a very low number order to find out what is characteristic of a solid .flines which, however, are sharp (see ~ i1). ~N~~~~~~ . electrolyte. We here rep0rt on and neutron diffraction studies have also been made for the fee diffraction studies as well as on Brillouin scattering phase, using the high-flux powder digractometer D2 (inelastic scattering of light) and on rheology studies. at Grenoble, France, and the DIDO triple-axis instrument at ~ a r w e l l , England, for pure L~,SO, [6]

and the triple-axis instrument TAS 4 at Ris0, Den- 2. X-ray and neutron diffraction studies. - Several mark, for a sample containing 20

%

Ag2S04. The authors have studied the structure of sulphate phases Bragg peaks are now superimposed on a high back- by means of X-ray diffraction [2-51. In addition to fcc ground consisting of at least three wide humps (see Li,S04 there are bcc phases with the approximate Fig. 2). The diffraction patterns are essentially the compositions LiNaSO,, LiAgSO, and Li,Ag4(SO,),I same for pure Li2S04 and the mixture, indicating and a non-cubic phase Li,Zn(SO,), on which work that the location and the transport mechanism is

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980609

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C6-36 R. ARONSSON e t a / .

I N T . Q ?

Fig. 1. - X-ray diffraction pattern of fcc lithium sulphate at 610 OC.

I N T

Fig. 2. - Neutron diffraction pattern of fcc lithium sulphate at 635 O C obtained with the D2 dsractometer of ILL, Grenoble.

about the same for both Li+ and Ag+ ions in this phase. This is expected from the diffusion data, table I.

The use of the energy-resolving triple axis spectro- meters rules out the possibility that the background might be due to inelastic scattering. The high intensity of the background can only be due to oxygen atoms.

Least-squares refinement methods were used to analyse the pure Bragg-reflexion part of the diffracto- gram [6]. The sulphate ion is situated at the origin of the face-centered-cubic structure with the oxygen atoms essentially rotationally disordered around the sulphur. The isotropic temperature factors are large, which suggests that the cations occupy a statistical distribution of sites instantaneously displaced from (114, 114, 1/41 in short range correlation with the instantaneous orientations of the surrounding sulphate ions.

3. Brillouin scattering. - Brillouin scattering, i.e.

inelastic scattering of light due to interactions with fonons (thermal sound waves), is a well-known method for structional studies of liquids and solids.

Since the solid electrolyte phases have several properties that resemble liquids rather than solids, we considered it of interest to extend our previous studies of molten salts to cover both solid and molten lithium sulphate [7]. Figure 3 shows that the Brillouin spectra of the two phases differ considerably. The very intense central line in the solid phase is due to elastic scattering of light by defects. Furthermore, a transverse mode typical of a solid appears in the

L Liquid

' 1

^phase

T=922 O C

L L

1 1 1 1 1 1 1 1 1 1 ~ ~ ~ ~ ~ ' ~ ~ ~

-8 -6 -4 -2 0 2 4 6 8

Brillouln s h ~ f t (GHz) Fig. 3. - Brillouin spectra of Li,SO, m the l~quld and fcc phases obtained at a scattering angle of 40 OC. The longitudinal (L) and transverse (T) modes are indicated.

fcc phase. The longitudinal mode is shifted further away from the central peak in the solid phase than in the liquid. This means that the hypersonic velocity is lower in the melt than in the solid. In both phases the velocity decreases smoothly with increasing temperature, but with a sharp drop of about 35

%

at the melting point. On the other hand, the refractive index, another optical property studied by us, decreases only 0.3

%

when the salt melts.

In the experiments performed so far the measurements started in the melt, and the temperature was then lowered slowly through the freezing point, whereupon the measurements were continued with the sample holder in the same position through the whole range down to the transition to the monoclinic phase at 575 ^{O C .}We also made an experiment where the sample holder was rotated at constant temperature. It was found that the frequency shift varied periodically with the rotation angle, which indicates that the scattering volume is comparable with, or less than the volume of a single crystal [7].

In order to obtain additional information it is neces- sary to work with oriented single crystals and a high resolution interferometer. Work is in progress along these lines.

4. Rheology. - The transition at 575 O C between the monoclinic and the fcc phases is characterized by an extremely high latent heat, 23.5 kJ/mol, while

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FAST ION CONDUCTORS WITH ROTATING SULPHATE IONS C6-37

the heat of melting is only 7.4 kJ/mol [6]. A first order phase transition with a high heat of melting is typical not only of a large number of solid electrolytes, but also of another class of substances called plastic crystals. We have found that bcc AgI and fcc Li2S04 show plastic crystal behaviour. Thus, they are both thixotropic, and the relaxation time for Li2S04 is 8.4 hrs at 781 OC [8].

Stress relaxation studies show that AgI, Li2S04 and Li,S04-K2S04 mixtures follow the general pattern of most plastic substances which has been established by a great number of investigators : The activation volume for flow is inversely pro- portional to the initial shear stress. This proportionality is improved if a constant term, the internal stress, is subtracted from the applied stress. In most work reported up till now the proportionality constant is of the order of 10, while in our work on Li,S04 mixtures the constant is about 5. The theoretical work considering this observed proportionality [9]

uses a statistical approach and cannot include specific properties of the different relaxing substances which should be responsible for the different values of the proportionality constant.

The internal stress mentioned above is often considered to be constant. Careful analysis of the

Refer

11 BENGTZELIUS, A., Thesis, Gothenburg, 1973.

21 F ~ R L A N D , T., KROGH-MOE, J., Acta Chem. Scand. 11 (1957) 565.

31 F$RLAND, T., KROGH-MOE, J., Acta Cryst. 11 (1958) 224.

41 @YE, H. A., Thesis, Trondheim, 1963.

51 SCHROEDER, K., Thesis, Gothenburg, 1975.

61 ARONSSON, R., HEED, B., JANSSON, B., LUNDEN, A., NILSSON, L., SCHROEDER, K., SJOBLOM, C.-A., THOMAS, J. o . ,

Li,S04 data shows, however, a weak temperature dependence which can be described by a linear equation in T in the range 1 010-1 110 K.

5. General conclusions. ^-fcc Li2S04 is the first example of a rotator phase found among solid electrolytes, and there is reason to assume that the other sulphate phases also are rotator phases. In such a phase the high degree of spherical delocation of the sulphate ions can be considered the cause of all the unusual properties, including the high cation mobi- lities. Further comparisons with other types of substances, including other solid electrolytes, should give a better understanding of how physical properties depend upon structure.

Acknowledgments. - The neutron diffraction studies of pure lithium sulphate are performed in cooperation with Dr. J. 0 . Thomas, University of Uppsala, and Dr. B. C. Tofield of AERE, Harwell, while those of the mixture with silver sulphate are done together with Dr. J. Kjems, RisG. Our work has been supported by the Swedish Natural Science Research Council, Chalmerska Forskningsfonden and Wilhelm och Martina Lundgrens Stiftelse.

TOFIELD, B. C., Fast Ion Transport in Solids, Eds Vashishta, P., Mundy, J. N., and Shenoy, G. K. (North-Holland Publ. Co) 1979, p. 471.

[7] ARONSSON, R., KNAPE, H. E. G., TORELL, L. M., Phys. Lett.

73A (1 979) 2 10.

[8] JANSSON, B., SJOBLOM, C.-A., Rheol. Acta 16 (1977) 628.

[9] KUBAT, J., SELDEN, R., Muter. Sci. Eng. 36 (1978) 65.