The electrical conductivity of FeyMn1−yO1+xsingle crystals in equilibrium with various partial pressures of oxygen was measured in the temperature range 900–1150°C. When the conductivity data were plotted as functions ofxory, sigmoidally shaped curves were obtained. This composition dependence is consistent with a region of solid‐solid immiscibility within the (Fe,Mn)O phase field. The visible and near‐infrared absorption spectra of (Fe,Mn)O crystals exhibited crystal‐field peaks characteristic of both the Mn2+and Fe2+cations. The position of Fe2+peaks was found to shift with MnO content although the peaks corresponding to the Mn2+ion did not shift. It was concluded that the manganese ions are located on undistorted lattice sites which may be grouped into phase‐separated regions. Thus, both the electrical and optical results reflect a miscibility gap in the pseudobinary phase field.
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