Moessbauer studies of microscopic disorder in solid electrolytes

We implement for the first time Moessbauer spectroscopy (MS) to investigate short-range properties of disorder in solid electrolytes. MS in 129I and 119Sn was carried out in RbAg4I5 and as impurity in Ag2Se, respectively. Measurements were performed both in the superionic and the normal phases. It is shown that localized cation hopping is an inherent feature of the α-AgI-type solid electrolytes. In RbAg4I5, at temperatures far below Tc, a small fraction of Ag is still locally mobile and at T>Tc, its concentration increases exponentially. A strong linear temperature dependence of the point-charge electric field gradient is observed and explained in terms of local hopping. With 119Sn in Ag2Se we observe the onset of 'local melting' of the Ag surroundingt the SnSe4 cluster at 50 K below the bulk superionic phase transition. The characteristic features of MS related to microscopic studies of solid electrolytes are fully described. (orig.)