In M ′ x M ′′ 1-x F 2 mixed-cation fluorites with no stereoactive lone pair, such as Ca x Ba 1-x F 2 , 19 F MAS NMR spectra show five distinct features corresponding to tetrahedral fluorine environments with different combinations of neighbouring cation species, i.e. FM ′ 4-x M ′′ x (x = {0, 1, 2, 3, 4}) . The 19 F MAS NMR spectrum for c-BaSnF 4 instead shows only two distinct contributions at -14 ppm and -45 ppm (Fig. ). The first of these peaks has a δ iso value close to that of BaF 2 (-14.2 ppm), where fluoride ions have occupy Ba 4coordinated tetrahedral sites. The second peak aligns with the average δ iso value of α-SnF 2 (-46 ppm) , in which fluorine is triply coordinated with short Sn-F distances . Based on these comparisons, we assign these features at -14 ppm and -45 ppm to broadly Ba-rich and Sn-rich fluorine environments, respectively. The assignment of fluorine environments into broadly two types is qualitatively consistent with the computational fluorideion density data (Fig. ), where we observe quite different fluoride-ion densities in Ba-rich versus Sn-rich regions of our simulation model.