Structural complexity of simple Fe2O3 at high pressures and temperatures

Although chemically very simple, Fe₂O₃ is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe₂O₃ at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO.mFe₂O₃. Decomposition of Fe₂O₃ and Fe₃O₄ observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe₅O₇ and Fe₂₅O₃₂ phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.