Monoclinic distortion and magnetic transitions in FeO under pressure and temperature

Fe1-xO, although chemically simple, possesses a complex structural and magnetic phase diagram. The crystal structures of Fe1-xO and its magnetic properties at high-pressure and high-temperature conditions are still a matter of debate. In this work, we performed a systematic investigation on Fe0.94O up to 94 GPa and 1700 K using synchrotron X-ray diffraction and synchrotron Mössbauer source spectroscopy. We observe a transition of Fe1-xO to the monoclinic phases above 40 GPa and at high temperatures and use the group theory analysis of the observed phases to discuss their properties and their relation to the ambient pressure phases. The Mössbauer spectra of the rhombohedral (rB1) and the room temperature monoclinic C2/m phase (mC4) contain a component attributed to Fe2.5+, which is caused by the electron exchange between the Fe3+ defect and neighboring Fe2+ atoms. Our results present a clear structural and magnetic transitional pressure-temperature diagram of Fe1-xO and show the complex physicochemical properties of simple Fe1-xO binary oxide under extreme conditions.