Migration pathways in Ag-based superionic glasses and crystals investigated by the bond valence method

Abstract

The bond valence technique has been applied on reverse Monte Carlo produced structural models of Ag-based superionic glasses and crystals. The results for the ${(\mathrm{AgI})}_{0.75}{\ensuremath{-}(\mathrm{A}\mathrm{g}}_{2}{\mathrm{MoO}}_{4}{)}_{0.25}$ and ${(\mathrm{AgI})}_{0.6}{\ensuremath{-}(\mathrm{A}\mathrm{g}}_{2}{\mathrm{O}\ensuremath{-}2\mathrm{B}}_{2}{\mathrm{O}}_{3}{)}_{0.4}$ glasses show the importance of including Ag sites with a high oxide coordination for the long-range mobility. The majority of the ${\mathrm{Ag}}^{+}$ ions belonging to the long-range conduction pathways is coordinated to both O and ${\mathrm{I}}^{\mathrm{\ensuremath{-}}},$ and the pathways including only iodide coordinated sites are restricted to very local regions of a few \AA{}. The most important finding of the present study is that the ionic conductivity as well as its activation energy can be determined directly from the ``pathway volume'' (i.e., the volume fraction of the percolating pathway cluster) of the structural models. The conductivity pathway volume relation is found to hold for glassy and crystalline conductors (including their temperature dependencies), whose conductivities differ by more than 11 orders of magnitude.