In a conventional fluorite, the tetrahedral sites are low energy, and the octahedral "interstitial" sites are higher energy. Moving an anion from a tetrahedral site to an octahedral site increases the total system energy, and forming Frenkel pairs is therefore a thermally activated process. Fig. (c) illustrates how this conceptual model breaks down in c-BaSnF 4 , where the relative free energy of tetrahedral sites increases with increasing Sn coordination. For sites with two or more coordinating Sn, some proportion of these sites are spontaneously depopulated, with the corresponding fluoride ions instead preferentially occupying octahedral sites. For Ba 1 Sn 3 -and Sn 4coordinated tetrahedral sites, this effect is large enough that these sites are nearly fully depopulated, contributing to the high octahedral-site occupation. This behaviour is consistent with a model where Sn lone pairs repel fluoride ions from adjacent tetrahedral sites, forcing these ions to instead occupy octahedral sites. The analysis presented here also indicates that this effect is additive; the more Sn cations coordinating a given tetrahedral site, the stronger the effective repulsion, and the greater the bias to spontaneously depopulate these sites.