The measured room-temperature ionic conductivity of c-BaSnF 4 (4.6 × 10 -3 S cm -1 ) is significantly higher than that of, for example, fluorite-structured BaF 2 , which is consistent with the general observation that within structurally-related families of solid electrolytes, hostframework disorder is often correlated with increased ionic conductivities . In other materials, this relationship between host-framework disorder and ionic conductivity has been explained as a consequence of a concomitant disordering of the mobile ion species that promotes ion transport , or of a reduction in differences in site-occupation energies between mobile-ion sites that flattens the mobile-ion potential energy surface . Our results for c-BaSnF 4 are consistent with both of these conceptual models: in Sn-rich regions of the structure, the fluoride-ion density is highly diffuse (Fig. ), indicating significant local fluoride-ion disorder-which is also evident from our calculated F-F radial distribution function, Fig. (d)-while our site-occupation analysis shows a destabilisation of Sn-coordinated tetrahedral sites and a stabilisation of octahedral sites that gives overlapping tetrahedral and octahedral site energies (Fig. ).