Given that c-BaSnF 4 exhibits such a high degree of fluoride-ion disorder, it is perhaps surprising that it does not have an even higher ionic conductivity. We observe greater fluoride-ion site-disorder (1/3 of fluoride ions occupying octahedral sites) than in the mixed-valence mixed-cation fluorite RbBiF 4 (1/4 of fluoride ions occupying octahedral sites), which would seem to predict a higher ionic conductivity for c-BaSnF 4 than for RbBiF 4 . The room-temperature ionic conductivity of RbBiF 4 , however, is ×10 2 greater than that of c-BaSnF 4 . This result can be explained by recognising that fastion transport in solid electrolytes requires not only that there is a small, or non-existent, energy gap between occupied and unoccupied sites, but also that these "frontier" sites form a contiguous percolating diffusion pathway through the material . In c-BaSnF 4 , the combined effects of cation disorder and lone-pair-fluoride-ion repulsion produce a large spread in tetrahedral site energies (Fig. ), causing tetrahedral sites with either high-Ba-coordination or high-Sn-coordination to be largely unavailable for long-range fluoride-ion diffusion. Highly-Ba-coordinated tetrahedral sites (e.g., Ba 4 ) have low site energies, are nearly fully occupied, and have low site-site transition frequencies, and fluoride ions occupying these sites are therefore largely immobile. As such, clusters of barium cations are expected to obstruct long-range fluoride ion diffusion. Highly-Sn-coordinated tetrahedral sites (e.g., Sn 4 ) have a similar blocking effect on diffusion, but for the opposite reason; these sites have high site energies and are therefore rarely occupied, despite having very high site-site transition frequencies. As a result, these Sn-coordinated sites obstruct long-range fluoride ion diffusion by acting as high-energy bottlenecks. The remaining mixed-coordination tetrahedral sites (e.g., Ba 2 Sn 2 ) then form a tortuous diffusion pathway, resulting in a lower macroscopic ionic conductivity than might be expected on the basis of local site-site transition frequencies or purely from the high level of fluoride ion disorder present in the structure.