The minimum energy path for the sodium migration in the low vacancy limit is illustrated in Figure . The high vacancy limit is not studied as the R 3 phase only persists in the material as long as a high sodium content is present. Otherwise the material reverts back to its cubic phase. The removal of one sodium ion from the unit cell results in a material with the formula Na 1.66 Fe[Fe(CN) 6 ]. The migration path that is studied in R-PW still remains similar to the diffusion pathways and transition states seen in cubic PW. Sodium ions are placed in a vertex of one of the small cubic structures (cubes) within a unit cell of R-PW, migrating to one of the neighboring cubes. Due to the distortions that occur by forming the rhombohedral phase, these cubes are slightly distorted, but the general structural feature persists. The transition state for this diffusion corresponds to the face-centered site, which is spanned by a square of Fe-CN-Fe units on each of the square edges. Due to the shift of the sodium atoms into the vertices of the cubes, there is one position that is close to the connecting/separating face-centered plane and one father into the small cubic structure. Here, we particularly studied the migration, where a sodium ion is starting from a place far from the separating plane to an equal place on the other side, as illustrated in Figure . A comparative analysis of both phases reveals that Na + diffusion in the cubic phase exhibits a reduced energy barrier, as depicted in Figure .