Dénès and co-workers have previously proposed a structural model for fluorite Ca 1-x Sn x F 2 , on the basis of experimental XRD and Mössbauer data similar to those reported here . In this model, the presence of a tin stereoactive lone pair causes each tin cation to be displaced towards one face of its enclosing [F8] cube, giving square-pyramidal SnF 4 E coordination with a reduced nearest-neighbour Sn-F distance. This structural model at first appears to be consistent with our XRD and Mössbauer data and hence to provide an explanation for the deviation from the ideal fluorite structure evident in the short-range PDF data described above. The position of the first peak in our PDF data, however, at r = 2.08 Å, is too short to be explained by square-pyramidal SnF 4 E coordination within an ideal cubic array of fluoride ions: the shortest possible Sn-F distance from this model is a √ 2/2 = 2.19 Å. We therefore interpret the PDF feature at r = 2.08 Å as indicative of a significant degree of distortion to the fluorine substructure away from the reference simple-cubic structure. The structural model of Dénès et al. also predicts equivalent SnF 4 E coordination for all tin cations, which is inconsistent with the apparent variation in bonding and coordination geometry for tin cations evidenced by the Mössbauer data.