Small angle, quasielastic and inelastic neutron scattering from 0.85 AgPO 3−0.15 PbI 2 glass

Abstract

A small angle neutron scattering (SANS) experiment and a quasielastic neutron scattering (QENS) experiment were performed on the fast ionic conductor 0.85 AgPO 3−0.15PbI 2, which is a vitreous electrolyte. The SANS data show that the scattering obeys Guinier's law for Q<7 × 10 −2 Å; dispersed heterogeneities are present in the glass with a mean radius of gyration of 20 Å. The QENS spectra show a quasielastic broadening of the elastic peak and a long tail up to 40 meV which is due to an inelastic distribution. The mechanism of the conductivity is a translational diffusion with a coefficient of self-diffusion of the mobile silver ions D=3.7 × 10 −6 exp(−8.778/ RT) (activation energy being in J-mol −1). The value of the activation energy is very close to that of α-AgI and 0.5AgPO 3−0.5AgI. The value of the proportion of purely elastic scattering: 0.925 indicates that the observed quasielastic scattering is due to silver ions which can be coordinated with the iodide ions. The two peaks observed in the inelastic frequency distribution function can be related to vibrations of the AgPO 3 host glass and of AgI 4 structural units respectively. These three last results and the SANS result seem to confirm our hypothesis on the structure of this glass: small “clusters” of AgI with tetrahedral coordination are dispersed in the AgPO 3 host glass. In order to build these clusters, an exchange between Ag + and Pb 2+ is proposed. These hypotheses assume that ionic conduction in the glass is almost entirely within the cluster phase and that the ionic conductivity of the host glass varies with the cluster concentration and with the nature of this host glass.