The Structure of Salt Doped Superionic Oxide Glasses by Neutron Diffraction
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THE STRUCTURE OF SALT DOPED SUPERIONIC OXIDE GLASSES BY NEUTRON DIFFRACTION L BORJESSON Chalmers University of Gothenburg, Dept. of Physics S- 412 76 Gothenburg, Sweden
ABSTRACT Comparative neutron diffraction experiments have been performed on metal-halide MX (M = Ag, Li, Na; X = CI, Br, I) doped oxide glasses and their corresponding low conducting host glasses, e.g. M20-2B203 and MPO3. The experiments reveal large changes in the intermediate range structure as the dopant salt is introduced, whereas the short range order of the host matrix is virtually unaffected by the dopant salts. An extra peak at anomalously low 0values (0 = 0.7-0.8 A-I) appears in the structure factors of the silver-halide glasses which indicates the building up of a new type of intermediate range ordering with a characteristic length of 8-10 A. The observation is tentatively ascribed to a-microscopic biphase system consisting of an expanded hosi glass network, unaffected on a microscopic scale, in which the silver-halide tends to form microclusters within the voids. In contrast, sodium- and lithiumhalides do not give rise to distinct new features in the structure factor. Instead a considerable smearing out of the first sharp diffraction peak of the host glaiss is observed. This indicates a partial breakdown of the intermediate range ordering of the host matrix. The results are discussed in relation to structure-conductivity models suggested for superionic glasses. 1 INTRODUCTION During the past decade much attention has been focussed on fast Ion conducting glasses since they offer extraordinary high room temperature ionic conductivities, up to 10-2 Scm-1, and the possibility to vary their physical properties over large intervals by changing the chemical composition[1-4]. Particular materials can therefore be designed for specific applications, such as solid state micropower devices[5], as well as for studies of fundamental properties of disordered systems, e.g. fracton properties[6]. A class of superionic conductors has attracted particular attention, namely the metal-halide salt doped oxide glasses. These glasses are commonly formed by an oxide glassformer (e.g. B203, SiO2, P205 or GeO2) which together with network modifiers (e.g. Ag20, Li20 or Na20) can dissolve metal halide salts as dopants (e.g. AgX, LiX or NaX where X = CI, Br or I). The ionic conductivity increases rapidly as the glassformer is modified by an increasing amount of a metal oxide. It is the cations which are the mobile species. In this way, room temperature conductivity of 10-5 Scm-1 can be obtained. An even more rapid increase of the conductivity occurs as the metal halide salt is added to the modified glass and conductivities as high as 10-2 Scm-1 have been reached [15]. Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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A continous unresolved problem exists as to the microscopic picture of the cationic motion in metal-halide doped glasses. It is focused on possible structure-conductivity relations. In crystalline electrolytes, there is a fram
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