We have also directly probed fluoride ion dynamics and the effect of cation disorder using variable-temperature 19 F MAS NMR experiments and additional analysis of our AIMD data. Our NMR data show that fluoride ions in c-BaSnF 4 can be broadly categorised as residing in either "Ba-rich" or "Sn-rich" environments, with fluoride ions in Sn-rich environments more mobile than fluoride ions in Ba-rich environments. This picture of cationenvironment-dependent fluoride-ion dynamics is corroborated by our AIMD simulations: calculated site-site transition frequencies are higher for sites with a higher proportion of coordinating tin, showing a direct relationship between the local cation configuration and local anion dynamics.