In BaSnF4 only a small fraction of F ions are present within the Sn-Sn layers (F1 sites) and Sn-Ba layers (F4 interstitial site, around 25 % occupancy) due to the repulsive interaction between a lone pair of electrons from the Sn 2+ ions and F -ions, and those remaining anions experience exchange with F2 ions leading to a conduction process in the a-b plane. On the contrary, in PbSnF4 no indication of F -being allocated within the Sn-Sn layers was given by neutron diffraction studies, instead, a small fraction of F -ions coexists with vacancies at the F4 interstitial site appearing within Sn-Pb layer. As a result, increased mobile anion density in the Sn-Pb layers improve ionic conductivity of PbSnF4 up to 10 -3 S cm -1 at RT. Also here, the lower activation volume for BaSnF4 in comparison to PbSnF4 is in accordance with the reported activation energies of BaSnF4 (0.34 eV ) and PbSnF4. (0.43 eV ). These findings are counterintuitive considering that Pb 2+ is a much more polarizable ion than Ba 2+ , showing the importance of small structural differences on the overall conductivity.