Density-functional-theory-predicted symmetry lowering from cubic to tetragonal in nickel hexa­cyano­ferrate

Density-functional-theory (DFT) computations on a Prussian blue analogue (PBA), nickel hexa­cyano­ferrate, Ni²⁺₃[Fe³⁺(CN)₆]₂·nH₂O, predict the existence of a tetragonal (P4m2) crystal structure that is energetically degenerate with the previously reported cubic (F43m) structure for this PBA. The proposed tetragonal structure satisfies observations, such as X-ray diffraction and magnetic measurements, that have been reported previously. A van der Waals corrected exchange-correlation functional is used in the DFT+U computations for an improved description of hydrogen bonding. The results provide strong support for a revised and simplified crystallographic description of Ni²⁺₃[Fe³⁺(CN)₆]₂·nH₂O, and show how H₂O molecules stabilize the crystal structure and affect its magnetic and electronic properties. The symmetry lowering in nickel hexa­cyano­ferrate is attributed to the hydration shell of the interstitial nickel cation. Calculations strongly suggest a maximum of n = 7 interstitial H₂O molecules per formula unit for nickel hexa­cyano­ferrate at room temperature, and a higher water content at temperatures below T ≃ 200 K. Since the symmetry lowering relies on the presence of interstitial H₂O molecules, this revised crystallographic description may be applicable more generally to the large class of F43m-structured PBAs.