Study on magnetite oxidation using synchrotron X–ray diffraction and X–ray absorption spectroscopy: Vacancy ordering transition in maghemite (γ–Fe2O3)

The oxidation process from magnetite to hematite through maghemite was investigated by X–ray diffraction (XRD) and X–ray absorption spectroscopic techniques. The XRD pattern of magnetite heated at 100 °C for 3 h showed small reflections of maghemite with partially ordered distribution of vacancy (space group P4132 or P4332). Thereafter, the XRD pattern of magnetite heated at 250 °C for 3 h exhibited extra reflections corresponding to the tetragonal maghemite with fully ordered distribution of vacancy (space group P41212 or P43212). Diffraction peaks of hematite occurred from the magnetite heated at 250 °C, in which maghemite and hematite coexisted with magnetite. Diffraction peaks of magnetite subsequently disappeared at 300 °C. Instead, maghemite and hematite dominated the XRD pattern, but the amount of maghemite reduced from 300 °C. The maghemite completely disappeared at 500 °C, and hematite finally dominated the XRD pattern. Rietveld fitting results clearly showed that the a lattice parameter and site occupancy factor of Fe at the octahedral site continuously decreased at the temperatures from 25 to 300 °C. The X–ray absorption near edge structure (XANES) result showed that the Fe3+/ΣFe increased up to 300 °C and remained constant until 500 °C, indicating that Fe2+ in oxidized magnetite was completely oxidized to Fe3+ at 300 °C. Furthermore, the intensities of radial structure function (RSF) peaks at 1.7 and 3.1 Å corresponding to the Fe–O bonds in octahedral site and the Fe–Fe interaction between the octahedral sites reduced continuously from 25 to 300 °C. The fitting results of the first shells indicated that the coordination number and site occupancy factor at the octahedral site continuously decreased at the temperature range from 25 to 300 °C, which were approximately consistent with those of Rietveld fitting analysis. The a lattice parameter of the oxidized magnetite displayed a linear trend between stoichiometric magnetite and stoichiometric maghemite with a relationship of a = 0.0985x + 8.3397 (x = Fe2+/Fe3+). It was clearly confirmed that during the magnetite oxidation, Fe was continuously removed from the octahedral sites, which resulted in the formation of maghemite with partially ordered distribution of vacancy. Just after magnetite oxidation was completed, the vacancy ordering further progressed by the diffusion of Fe3+ within the structure, leading to the formation of maghemite with fully ordered distribution of vacancy.