Nanocrystalline manganese oxide powders were synthesized by an inert gas condensation technique. The manganese oxide powders, prepared from vaporized metallic manganese in a helium pressure of 10 mbar, then oxidized under the oxygen pressures of 50 and 100 mbar, exhibited a mixture of β-Mn, MnO and Mn3O4 phases. X-ray diffractometry (XRD) analysis identified the predominant oxide formation of the as-prepared powder to be the phase of MnO. In situ synchrotron XRD and differential scanning calorimetry (DSC) data showed that synthesized manganese powder converts from β-Mn to MnO in the temperature range of 150-250 °C and subsequently converts to Mn3O4 above 250 °C. Transmission electron microscopy examinations confirmed that oxidation starts from the surface of the condensed β-Mn particles. Detailed valance variations of manganese of the resulting powders were investigated by synchrotron X-ray absorption spectroscopy techniques.
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