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

Abstract

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 (P 4 m2) crystal structure that is energetically degenerate with the previously reported cubic (F 43m) 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 F 43m-structured PBAs.