The activation volume ΔV was determined from the slope of the logarithm of the ionic conductivity lnσ against pressure p in the linear high-pressure regime , according to equation (1) with R = 8.314 J K -1 mol -1 and T being the measurement temperature. We acknowledge that this is a simplified form, disregarding two terms which take into account the pressure dependence of the pre-exponential factor 𝜎 0 and the jump frequency ν have minimal impact on the result. The determined activation volumes are shown in Table 1 and graphically represented in place via jumps between the tetrahedral site and the partly unoccupied octahedral site. Therefore, the bottleneck of the jump is the equilateral triangle formed by three metal cations in the cubic close packing of the fluoride structure. La0.9Ba0.1F2.9 has a tysonite structure, in which the fluoride ions occupy three different sites, one of them also showing a tetrahedral coordination (F1). Among the possible jumps between those different sites, the F1-F1-pathway shows the lowest activation energy and therefore likely has the biggest contribution to the ionic conductivity. The comparatively lower activation volume for Ba0.6La0.4F2.4 indicates that the jump through a bottleneck in the fluorite structure does not require as large of a deformation of the unit cell as is required for ionic conduction by jumps between F1-sites in the tysonite structure of LaF3. This is also reflected by the difference in activation energies of the two materials, where Ba0.6La0.4F2.4 shows a lower activation energy (0.62 eV ) than La0.9Ba0.1F2.9 (0.73 eV ). Remarkably, BaSnF4 and PbSnF4 investigated in this work show a big difference in activation value, (refer to Figure ), even though both materials are structurally similar and crystallize in a tetragonally distorted modification of the fluorite type structure which originates from cation ordering crystal structure. In this structure, the cations stack in Sn-Sn, Sn-X, X-X (X: Ba, Pb) layers along the c axis, with F -ions residing in the X-X and the Sn-X layers at F3 sites (immobile F ions) and F2 sites coexisting with vacancies, respectively (see Figure S 9).