Magnetic structure of molecular magnet Fe[Fe(CN)₆]·4H₂O

Abstract

We have studied the magnetic structure of Fe[Fe(CN)₆]·4H₂O, prepared by precipitation method, using neutron diffraction technique. Temperature dependent DC magnetization study down to 4.2 K shows that the compound undergoes from a high temperature disordered (paramagnetic) to an ordered magnetic phase transition at 22.6 K. Rietveld analysis of neutron diffraction pattern at 60 K (in its paramagnetic phase) revealed a face centred cubic structure with space group Fm3m. The structure contains three-dimensional network of straight Fe³⁺-C≡N-Fe³⁺ chains along the edges of the unit cell cube. Fe3+ ions occupy 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2) positions. Fe³⁺(0, 0, 0) is surrounded octahedrally by six nitrogen atoms and Fe³⁺ (1/2, 1/2, 1/2) is surrounded octahedrally by six carbon atoms. Magnetic Rietveld refinement of neutron diffraction data at 11 K shows a ferromagnetic coupling between the two inequivalent Fe³⁺ sites. Refinement yielded an ordered moment of 4.4(6) and 0.8(6) μ_B per Fe ion located at (0, 0, 0) and (1/2, 1/2, 1/2), respectively. Ordered moments are found to align along the face diagonal. The observed net moment from low temperature neutron diffraction study is consistent with DC magnetization results.