A Study of Ag + -Ion Dynamics in Ag 2 S-GeS 2 Glasses
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A STUDY OF Ag+-ION DYNAMICS IN Ag2S-GeS2 GLASSES
A.P. OWENS*, A. PRADEL+, M. RIBES+, AND S.R. ELLIOTT= * Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, U.K. + Laboratoire de Physico-chimie des Mat~riaux, Universit6 des Sciences et Techniques du Languedoc, Montpellier, France. ABSTRACT A quasi-elastic neutron scattering (QENS) study of the Ag+-ion diffusion coefficient, D, in glassy (Ag2S)0.5 (GeS2)0.5 has been performed. The results indicate that the magnitude of D is larger, and its activation energy is smaller, than the corresponding quantities inferred from measurements of the d.c. ionic conductivity. This behaviour is ascribed to the fact that QENS probes diffusive processes occurring at very high frequencies (~1011FHz) where the magnitude of the a.c. conductivity is correspondingly larger, and its temperature dependence is less, than that of the d.c. conductivity. INTRODUCTION A number of experimental techniques can be used to investigate ionic transport in solids, including electrical conductivity (impedance spectroscopy), nuclear spin relaxation and quasi-elastic neutron scattering (QENS). Here we report the results of a QENS study of the glassy superionic material (Ag2S)0.5 (GeS2)0.5, which has been synthesised and characterised previously [1]. This material has the advantages that it is highly ionically conducting with negligible electronic conductivity [1,2], and such a high ionic conductivity is obtained without the complicating incorporation of "doping salts", e.g. AgI; furthermore, the material appears, from small-angle neutron scattering measurements [3], to be structurally simpler (i.e. more homogeneous) than many other glasses (containing doping salts) with comparable ionic conductivities [4]. EXPERIMENTAL The glass was prepared by melt quenching [1]. Conductivity measurements were performed by impedance spectroscopy (using an HPI 4192A bridge), on pelletised powdered samples with evaporated Pt electrodes. QENS measurements were performed using the IN6 spectrometer at the ILL, Grenoble. The powdered glass (exactly the same material as used for conductivity measurements) was held in a standard thin-wall Al sample can; the thickness of the sample was 2mm. Counting times were typically 4h per run at each temperature. Analysis of the time-of-flight (TOF) data to obtain the QENS results was carried out using standard ILL packages. RESULTS The temperature dependence of the ionic conductivity at various frequencies for glassy (Ag2S)0.5 (GeS2)0.5 is shown in fig. 1. The d.c. conductivity, a(0) (extrapolated to wo=O) is thermally activated with an activation energy of E0 =O.34eV, in agreement Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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with previous work [1], although the absolute values of ( (0) were about one order of magnitude smaller than reported previously [1], (perhaps due to the powdered nature of the sample). In contrast, the temperature dependence of the total measured a.c. conductivity, a((co), is considerably weaker than that of the
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