It was found that the compositional dependence of the activation energies decreased slightly with increasing Br concentration. The lattice constant decreased upon increasing the Br concentration. A reduction in the bottle-neck size in the conduction path of the mobile ion, which arose from the decreasing lattice constant, was expected to lead to an increase in the activation energy. However, this prediction was contradicted by the present observations. The behavior was instead attributed to a distortion of the crystalline lattice. The heat of transport which was obtained from measurements of the thermoelectric power was slightly higher than the activation energy for ionic conduction in the superionic phase. This relationship could be qualitatively explained by the Fokker-Planck equation.

M.Kusakabe, M.Arai, Y.Ito, S.Tamaki: Solid State Ionics, 1999, 121[1-4], 295-9