The 19 F MAS NMR and AIMD data presented above show that the local mobility of fluoride ions in c-BaSnF 4 is strongly dependent on the local cation composition: fluoride ions in Sn-rich environments are significantly more mobile than those in Ba-rich environments. An obvious partial explanation for this behaviour is that the stereoactive lone pairs on tin cations somehow promote the motion of fluoride-ions in adjacent tetrahedral and octahedral sites. Our calculated time-average fluorideion density (Fig. ) shows that the fluoride-ion substructure is highly diffuse in Sn-rich regions, which further suggests a possible direct interaction between the Sn lone pairs and the mobile fluoride ions.