Structural complexity of simple Fe(2)O(3) at high pressures and temperatures

Although chemically very simple, Fe(2)O(3) 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 Fe(2)O(3) 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·mFe(2)O(3). Decomposition of Fe(2)O(3) and Fe(3)O(4) observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe(5)O(7) and Fe(25)O(32) phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.