The valence and site occupancy of substituting metals in magnetite spinel structure Fe3−xMxO4 (M = Cr, Mn, Co and Ni) and their influence on thermal stability: An XANES and TG-DSC investigation

Four series of substituted magnetite (Fe3−xMxO4, M = Cr, Mn, Co and Ni) were characterized by X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) spectroscopy, and thermogravimetry and differential thermal analysis (TG-DSC). XRD patterns confirmed the formation of samples with the spinel structure. XANES showed the valence and site occupancy of the substituting cations. Cr cations are in the valence of +3 and occupy the octahedral sites. The valences of Mn cation are +2 and +3. Mn2+ and Mn3+ cations preferentially entered the tetrahedral and octahedral sites, respectively. Both Co and Ni cations have a valence of +2 and mainly occupy the octahedral sites. The introduction of Cr and Ni cations obviously increases the amount of superficial hydroxyl groups. The incorporation of Mn, Co and Ni enhances the oxidation temperature of magnetite. All the four kinds of substituting cations in this study increase the temperature of phase transformation from maghemite (γ-Fe2O3) to hematite (α-Fe2O3). The mechanism of substitution changing the temperatures of oxidation and phase transformation was also discussed.

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► The valence and coordination environment of the doping metals like Cr, Mn, Co and Ni in the magnetite.
► The influence of doping metals on the amount of surface hydroxyl.
► The influence of doping metals on the temperature of phase transformation.
► The mechanism of the doping metals influencing the phase transformation temperature.