The Na2 octahedron is slightly smaller, so the area of its triangular faces correspond to the defining bottleneck. It is marked as A2 in the figure and has an area of 4.67 Å2 for the Ti phase and 5.98 Å2 for the Zr phase. The bottleneck in the Ti phase is thus significantly smaller and only 80% of the area in the Zr phase, suggesting that diffusion will be more hindered in the Ti phase, i.e. associated with a higher diffusion barrier than the Zr phase. Now we discuss the energy barriers for migration derived by Nudged Elastic Band (NEB) calculations. As a fist step we check the diffusion energy of only one cation, to exclude all influence of different occupational environments on the diffusion pathway. This calculation yielded for the Ca 1 Ti 12 (PO 4 ) 18 a barrier of 0.92 eV, for Ca 1 Zr 12 (PO 4 ) 18 a barrier of 0.71 eV. For both isostructural cells, the energy barriers were roughly the same between all six possible sites with a variance of 5 meV, showing them to be of the same symmetry. Also the prediction of the bottleneck size holds true, as the Zr barrier is only 77 % as high Figure : NEB pathway of the diffusion event for one Ca moving through the two bottlenecks and the metastable intermediate in the different phases with a representation of the diffusion pathway of Ca through the NASICON structures. Ca moves through the first octahedra plane A1, then A2 into the unoccupied meta stable Na2 equivalent site, then again through A2 and A1 into the next Na1 equivalent Ca site. The bottleneck of the diffusion is A2, because of its slightly smaller size. The yellow tetraeder and grey octahedra correspond to PO 4 and the TiO 6 units or ZrO 6 , respectively. as the Ti barrier. Figure shows that the diffusion pathway for one isolated Ca atom is symmetric with respect to the metastable Na2 site. This symmetry can become broken for higher Ca occupations in the simulation cell. For symmetric environments, only half of the total pathway was calculated and then mirrored to the other half to save calculation time. This is the case for most pathways with an occupation of three cations and for all pathways with only one cation present. The following systems with an asymmetrical environment of cations, were determined in two sections with the intermediate positions as an start and end point for ease of calculation.