It was recalled that these materials were both plastic crystals and super-ionic conductors. The reason for the high conductivity was that the mobility of the cations was strongly enhanced by rotational motion of the translationally static sulfate ions. This was true not only of the abundant cations, but also of other cations present (monovalent as well as polyvalent) and of monovalent anions. Both bulk diffusion, and migration along high-diffusivity paths, were affected. In addition, it was possible to distinguish between various contributions to the bulk diffusion. It was noted that the ionic radius was a very important parameter since it determined the solid solubility and the distribution of the ions between the various available sites of the lattice; thus affecting the relative importance of competing diffusion mechanisms. These factors provided a qualitative explanation for an anomalous behavior of monovalent ions (cations or anions), such that both the diffusion coefficients and the activation energies decreased when the ionic radius was increased.

R.Tärneberg, A.Lunden: Solid State Ionics, 1996, 90[1-4], 209-20