Our XRD data show no superstructure reflections, indicating that Ba and Sn are disordered over the cation sites. From indexing the XRD data, we obtain a cell parameter of a = 6.1945(2) Å, which is close to the value for pristine BaF 2 of a = 6.1964(2) Å . This result is somewhat unexpected, given the smaller ionic radius of Sn 2+ compared to Ba 2+ , and suggests the possibility of local distortions in the cation substructure. Düvel et al. reported similar excess-volume behaviour in Ba 1-x Ca x F 2 solid solutions , where this was proposed as a con- tributing factor to enhanced fluoride-ion transport relative to the end-members. HRTEM data provide further evidence of local deviations from an ideal fluorite-type structure; these show visible changes in inter-reticular distances (Fig. , white arrows) that indicate regions of local strain. Additional structural information is given by our Xray total-scattering PDF data. For interatomic distances between 12 Å and 50 Å, the PDF data are relatively well described by a cubic fluorite F m 3m model (R w = 20 %; see Fig. ). At short range, however (between 1 Å and 12 Å; Fig. ), this high-symmetry structural model gives a poor fit to the PDF data (R w = 32 %), indicating that the local structure of c-BaSnF 4 deviates significantly from an ideal fluorite-type structure. The cubic model fails to predict the peak observed at r = 2.08 Å and the apparent splitting at r = 3.96 Å to 4.15 Å. In other fluorides, Sn adopts short Sn-F distances (e.g., 2.28 Å in tetragonal BaSnF 4 , and as short as 2.03 Å in SnF 2 ). We therefore provisionally assign the peak at r = 2.08 Å to short Sn-F bonds, which requires that Sn or F, or both species, are displaced from their ideal fluorite positions.