A more quantitative analysis of the c-BaSnF 4 structure is presented in Fig. . Fig. (a) shows the probability distributions of cation distances from their closest 4a site, P [r(M -4a)], for Ba and Sn. Both cation species are, on average, displaced from their corresponding ideal fluorite cation positions, indicating how the cation substructure is locally distorted from a perfect fcc lattice. On average, Sn is displaced further from the nearest 4a position than Ba, which is consistent with the smaller size of Sn. In general, however, the two probability distribution functions have similar shapes, indicating no qualitative difference between Ba positions and Sn positions relative to their formal crystallographic sites. The calculated Sn-F coordination number, however, rises continuously after the first step, reaching ∼ 8 at 4 Å: the average coordination environment around Sn includes four neighbouring F that occupy 8c-type positions, with four more F in some diffuse disordered arrangement at distances of 2.3 Å to 4.0 Å. Our data therefore suggest an alternative model for the Sn coordination environment, where the Sn lone pair is accommodated not by Sn being displaced significantly from the 4a position, but instead by a significant disruption of the fluoride ions on the lone-pair-adjacent face of the nominal [F8] coordination environment, which presumably reduces the mutual electrostatic repulsion expected between these fluoride ions and the proximate lone pair . Additional evidence for significant disordering of fluoride ions comes from the F-F RDF (Fig. ), which shows a very weak second peak, more typical of an amorphous glassy phase than a regular crystalline array of atoms. ever, the fluoride-ion density is highly diffuse, which is consistent with the proposal above that Sn lone pairs are associated with significant disorder in the local fluoride substructure. These fluoride-ion density data also suggest that the dynamic behaviour of the fluoride ions is strongly dependent on the identity of the nearby cation species: fluoride ions in Ba-rich regions of c-BaSnF 4 appear to be relatively immobile, while fluoride ions in Snrich regions appear to be much more mobile, and we return to this point below.