where D is the diffusion constant extrapolated from the Arrhenius plot at temperature T and zi is the charge of the moving ion (for Na + zi = 1). F is the Faraday constant, and R is the ideal gas constant. Ni is the charge carrier molar concentration. For the calculation of Ni for structure B, it is assumed that all Na + in the Na1(Zn/Al) sites contribute to the conduction along the b axis and that the actual number of vacancies in Na1(Zn/Al) sites is negligible. The final concentration is based on this assumption and the volume of structure B shown in Figure . The corresponding charge carrier concentration is found to be 4.9 mol/l Na + , which yields an ionic conductivity of 1.7 mS/cm at room temperature. This is several orders of magnitude above the measured value for x = 0.75, indicating that the actual charge carrier concentration is lower than assumed, likely because structure B is thermodynamically significantly less stable than structure A. Comparing structures A to B, the main impediment for ion movement in structure A is expected to be, as in Na2ZnCl4, the lack of easily accessible vacant sites or interstitial sites in the structure. Structure B exhibits potential Na + migration pathways, whereas due to the competition with structure A, structure B is expected to be less likely to form due to its inferior thermodynamic stability depicted in Figures and. Finally, according to the XRD refinements in Figure , the vacancies are neither exclusively located at the Na1 site nor the Na2 site, which implies the co-existence of structures A and B to some extent. This leads to a much higher ionic conductivity of Na1.75Zn0.75Al0.25Cl4 composition than pure Na2ZnCl4, which is observable in Figure .