On the time scale of the simulation, no long-range movement occurs along the a, b, or c axes, which would be needed for ionic conductivity. As a side remark, for pure Na2ZnCl4 not a single sodium ion jump was observed within the MD simulation time of 75 picoseconds at T < 800 K. Structure B has only Na2 vacancies. This is only a hypothetical structure, as it is energetically less stable than configuration A (Figure ) and we experimentally observe the Na1 site to be less occupied than Na2. Interestingly, however, Na + migration was observed in the MD simulation at temperatures as low as 475 K (as shown in the simulated density plots of Figures and). The main diffusion occurs between edge-sharing Na1 sites in the mixed Al/Zn layers along the b axis. This migration is akin to Li + migration in isostructural LiFePO4 . It is facilitated by the appearance of vacancies in the Na1(Zn/Al) layer through the spontaneous diffusion of Na1(Zn/Al) ions into the adjacent vacant Na2(Al) layer.