Conduction mechanisms in the high-temperature phases of these, and related, compounds were considered. It was recalled that 2 different models had been suggested. One of these, the so-called paddle-wheel model, assumed that cation transport was closely related to rotation of the SO4 tetrahedra. The other, so-called percolation, model suggested that there was no direct correlation between ionic conduction and SO4 rotation and that the rotations and vibrations indirectly produced conduction pathways. Here, the total structure factors for Bragg and diffuse scattering were measured by means of powder neutron diffraction. The data were modelled by using the reverse Monte Carlo method. From a detailed analysis of 3-dimensional models, it was deduced that ionic conduction in the present materials involved a combination of paddle wheel and percolation processes.

L.Karlsson, R.L.McGreevy: Solid State Ionics, 1995, 76[3-4], 301-8