In the Ca case, the only difference is that the Na2 equivalent position is much less stable compared to the Na1 position, transforming it into a metastable intermediate state instead of a partially occupied site. Therefore, a full diffusion event takes place between two neighboring Na1 positions. Along the minimum energy path for diffusion, the migrating ion has to pass through a triangular area consisting of the three oxygen atoms in the PO 4 tetrahedron and the TiO 6 or ZrO 6 octahedron, respectively. It has been suggested to consider the area of this bottleneck that corresponds to the structure with the smallest opening along the diffusion pathway as a descriptor for the activation energy for diffusion. As shown in Figure , there are two such bottlenecks in the pathway from Na1 to the Na2 intermediate site.