Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo₂O₄

The compound NiCo₂O₄, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo₂O₄ of (Ni³⁺_0.1Co²⁺_0.9)_tet [Ni^3.5+_0.9Co^2.5+_1.1] _octO₄. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co³⁺ and Ni³⁺. The surface of NiCo₂O₄ was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co²⁺, Co³⁺, Ni²⁺, Ni³⁺ and, probably, Ni⁴⁺.