We attribute the displacement of fluoride ions from tinadjacent tetrahedral sites into octahedral interstitial sites to the presence of the stereoactive lone pair on the tin cations. The tin cations sit relatively close to their ideal fluorite positions, and exhibit highly eccentric charge distributions that are characteristic of a stereoactive lone pair, in agreement with our 119 Sn Mössbauer data. This lone-pair charge density destabilises fluoride ions occupying adjacent tetrahedral sites, in effect pushing these fluoride ions into octahedral sites, thereby strongly disrupting the fluoride-ion substructure. This effect is clearly seen in the Sn-lone-pair-fluoride-ion polar spatial distribution function (Fig. ), where fluoride ions on the back face of Sn sites-i.e., the opposite side from the lobe of the lone pair-are well structured, while fluoride ions on the front face of Sn sites-in the direction the lone pair is oriented-are strongly repelled and highly disordered.