The Na + ion conductivity, σ, is calculated from the diffusion coefficient, D (the slope of the MSD near the diffusive region), according to Eq. 4. Calculation of the conductivity is also possible using the Green-Kubo relation instead of the Nernst-Einstein eq. 3. In principle, both approaches are equivalent. However, the correlation function involved in the Green-Kubo formula shows Inset shows the MSD of Na + ions along the c-axis direction (MSD-z) at 600 K. These results demonstrate that the framework atoms are immobile and the Na + ions are restricted along the ab-plane, rendering the diffusion to be highly anisotropic.