11:00 AM - 11:15 AM
[SMP26-02] Magnetite oxidation investigated by synchrotron X-ray diffraction and X-ray absorption spectroscopy
Keywords:magnetite, maghemite, oxidization, XRD, XANES
The oxidation process from magnetite to hematite through maghemite was investigated by synchrotron X-ray diffraction and X-ray absorption spectroscopic analyses. The XRD pattern of magnetite heated at 100 oC showed a small peak corresponding to maghemite with partial Fe vacancy disorder structure (space group P4132.) Diffraction peaks of hematite occurred from magnetite heated at 250 oC, in which maghemite and hematite coexisted with magnetite. Diffraction peaks of magnetite subsequently disappeared at 300 oC. Instead, maghemite and hematite dominated the XRD pattern, but the amount of maghemite reduced from 300 oC. Maghemite completely disappeared at 500 oC, and hematite finally dominated the XRD pattern. Rietveld fitting results clearly showed that the site occupancy of Fe at the octahedral sites in the oxidized magnetite continuously decreased from 1.000 to 0.3992(4) at the temperature range between 25 oC and 300 oC. The XANES result showed that the Fe3+/ΣFe increased up to 300 oC and remained constant until 500 oC, indicating that the Fe2+ in magnetite was completely oxidized to Fe3+ at 300 oC. Furthermore, peaks derived from octahedrally coordinated Fe continuously reduced up to 300 oC. The fitting results of the first shells indicated that the coordination number and site occupancy factor at the octahedral site were continuously decreased at the temperature range between 25 oC and 300 oC, which were approximately consistent with those of Rietveld fitting analysis. It is therefore clearly confirmed that during oxidation from magnetite to maghemite Fe is continuously removed from the octahedral sites with keeping degree of vacancy ordering in the space group P4132.