x M ′′ 1-x F 2 . The highest ionic conductivity materials in this class are those with different valence cations, such as RbBiF 4 , where cation disorder induces high levels of anion disorder . A significant increase in ionic conductivity compared to analogous single-cation fluorites is also observed for mixed-cation systems where both cations have a formal 2+ oxidation state, such as Ba 1-x Ca 1+x F 2 : again, the presence of cation disorder causes considerable fluoride disorder where fluoride ions displaced significantly from their ideal crystallographic positions to form "pseudovacancies" . Some fluorite materials exhibit both stereoactive lone pairs and cation mixing . Of particular relevance to the present study is the work of Dénès et al. on Ca 1-x Sn x F 2 (x = 0.27) . For this system, X-ray diffraction (XRD) data give a cubic fluorite average structure, consistent with a solid solution of Ca and Sn distributed randomly over the cation positions, and 119 Sn Mössbauer data show a large quadrupole doublet, characteristic of a stereoactive tin lone pair.