The argumentation that the sodium atoms themselves are the cause for the rhombohedral transformation would also directly solve the dilemma of why no rhombohedral phase exists for Prussian Yellow. As this compound does not contain any sodium ions, which would be there to cause the distortions and stabilize the rhombohedral structure, this phase is never thermodynamically favorable. To confirm this assumption we calculated the empty rhombohedral phase and compared it to its cubic counterpart. It can be shown that the Prussian Yellow structure is not thermodynamically stable in R 3 symmetry, as it is destabilized by 0.62 eV per formula unit over the cubic one.