Abstract
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.
Highlights
Chemically very simple, Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures
In order to study the high-pressure high-temperature (HPHT) behaviour of ferric iron (Fe3 þ ) oxide we apply the complementary methods of single crystal X-ray diffraction in laser-heated diamond anvil cells (DACs) and synchrotron Mossbauer source (SMS) spectroscopy
Crystal chemical analysis of the new structures and known Fe(II, III) oxides reveals their fundamental relationships as members of the homologous series nFeO Á mFe2O3
Summary
Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. Recent single-crystal high-P,T diffraction data[12] were able to solve this challenge; they demonstrated that the Rh2O3-II-type phase of Fe2O3 (which we refer to below as i-Fe2O3, Fig. 1b) forms upon laser heating at pressures above B40 GPa; whereas, compression of hematite at ambient temperature to over B50 GPa results in the formation of a phase with distorted GdFeO3-perovskite-type, dPv z-Fe2O3, structure (Fig. 1c). In order to study the high-pressure high-temperature (HPHT) behaviour of ferric iron (Fe3 þ ) oxide we apply the complementary methods of single crystal X-ray diffraction in laser-heated DACs and synchrotron Mossbauer source (SMS) spectroscopy (see Methods section). Our results indicate that mixed-valence iron oxides may play a significant role in oxygen cycling between the earth’s atmosphere and mantle
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