As a consequence of this Sn-lone-pair-fluoride-ion repulsion, c-BaSnF 4 exhibits a remarkable concentration of "interstitial" fluoride ions that occupy octahedral sites. In our simulations, 1/3 of fluoride ions occupy octahedral sites, making it equally likely that, on average, octahedral sites and tetrahedral sites are occupied by fluoride ions. This level of octahedral-site occupation exceeds that of previously reported "massively disordered" fluorites, such as RbBiF 4 , where 1/4 of fluoride ions occupy octahedral sites . In c-BaSnF 4 this extreme fluoride-ion disorder is a consequence of a high relative free energy of occupation for tetrahedral sites adjacent to tin centers and an associated low relative free energy of occupation for octahedral sites (Fig. ), which is a consequence of the mutual repulsion between Sn-lone-pairs and fluoride ions in adjacent tetrahedral sites.