Cubic BaSnF 4 was synthesised by mechanically milling BaF 2 and SnF 2 in a manner similar to Ref. 94 (see Methods). The X-ray diffraction pattern (Fig. ) indexes to a face-centered cubic structure from the F m 3m (225) space group, consistent with an average fluorite-structure. The X-ray pattern shows no visible peaks for the parent SnF 2 (C2/c) phase, and Energy Dispersive X-ray analysis (EDX-mapping) shows homogeneous distributions for both Sn and Ba. Quantitative analysis of the EDXmapping data gives proportions of Ba and Sn of 49.6(7) % and 50.3(7) %, respectively, which is close to the nominal 1:1 Ba:Sn stoichiometry (see Fig. for the full mapping data). As a further check on the synthesised compound, we performed electrochemical impedance spectroscopy, and obtained an ionic conductivity at 30 • C of 4.6 × 10 -6 S cm -1 , with an activation energy of 0.56 eV. This ionic conductivity is consistent with previous literature values for c-BaSnF 4 , and is > 10 3 higher that that of fluorite-structured BaF 2 , illustrating the positive effect of cation mixing on fluoride-ion transport.