Structure re-refinement of this tetragonal phase was carried out by using new data. It was found that the root mean square amplitude in the M2(Sn) site was larger in the (001) plane than along the c-axis, and that the thermal vibrations of F3 fluorine ions were very large. The M2F8 polyhedra formed a distorted cube. The F ions on the F3 site were located between the M2 ion layers, and their weak bonds with Sn ions played an important role in conduction. The ionic conductivity in the (001) plane was higher than that in the <001> direction.

Y.Ito, T.Mukoyama, S.Yoshikado: Solid State Ionics, 1995, 80[3-4], 317-20