Using ab initio molecular dynamics (AIMD) simulations, we have shown that the fluorine substructure in c-BaSnF 4 is highly inhomogeneous and depends strongly on the local cationic composition (Fig. and Fig. ). In Ba-rich regions, the fluoride ions occupy fluoritelike tetrahedrally-coordinated sites that form [F8] cubes around barium. In Sn-rich regions, in contrast, the fluoride-ion substructure is highly diffuse, with fluoride ions displaced from tetrahedral sites adjacent to tin into octahedral "interstitial" sites.