Application of the bond valence method to reverse Monte Carlo produced structural models of superionic glasses

Abstract

The reverse Monte Carlo RMC method has shown to be a useful tool for extracting structural properties from diffraction data of disordered systems, such as ion-conducting glasses. In this paper we investigate ion conduction in Ag-based superionic glasses by simple random-walk simulations based on the bond-valence information present in the RMC-produced structural models. Using this method we are able to explore the ion-conduction pathways and to calculate the ionic conductivity on a quantitative basis. The migration pathways are assumed to be the regions of the structural models where the valence mismatch for the mobile ion remains below a threshold value. The results for the AgI-doped glasses show that there are no long-range migration pathways for Ag sites in an entire iodine environment. Rather, the ${\mathrm{Ag}}^{+}$ ions are generally moving between sites with a mixed oxygen-iodine coordination. The method is able to predict the ionic conductivity of highly AgI-doped superionic glasses, but tends to overestimate the conductivity of undoped glasses (with $\ensuremath{\sigma}<{10}^{\ensuremath{-}5}{\ensuremath{\Omega}}^{\ensuremath{-}}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1})$ and underestimate the conductivity of highly conducting crystalline \ensuremath{\alpha}-AgI. The discrepancies for these materials are discussed, as well as the possibility and limitations of using a similar approach to study the frequency dependence of the conductivity.