The observation that fluoride ion exchange between Sn-rich environments is much faster than that between Ba-rich environments or between Ba-rich and Sn-rich environments is further supported by the observation of motional narrowing of the Sn-rich peak with increasing temperature, indicating that the so-called fast-exchange regime is reached. This picture of locally inhomogeneous fluoride-ion dynamics is also qualitatively consistent with the time-average fluoride-ion density obtained from AIMD (Fig. ), where Ba-coordinated regions show highly localised fluoride-ion density, indicative of significantly less mobile ions, while Sn-coordinated regions show diffuse interconnected fluoride-ion density, suggesting more facile fluoride-ion motion between these sites. To validate this model of faster fluoride-ion motion in Sn-rich regions, we performed additional analysis of our AIMD data to calculate site-site transition frequencies for each type of tetrahedral and octahedral site. To estimate the degree to which these fluoride-ion site-site transitions contribute to long-range diffusion, rather than simple back-and-forth motion between adjacent sites, we also calculated frequencies of "non-returning" transitions; these are transitions between two sites, 1 → 2, where the next transition made by the mobile ion takes it to a third site, 1 → 2 → 3, rather than returning it to the original site, 1 → 2 → 1.