New High-Pressure Phase in Fe2O3
Abstract
Hematite Fe2O3, a prototype of trivalent transition metal oxides, crystallizes in the antiferromagnetic (AFM) insulating phase with the corundum structure at ambient conditions. Extensive studies have been carried out to clarify its structural, magnetic, and electronic evolutions under high pressure due to the broad interests in hematite from condensed matter physics to geosciences. The high-pressure phase relation in Fe2O3 is also substantial to understand geophysically important MgSiO3-Fe2O3 phase equilibria. Those are however still yet to be clarified as for example, some in situ X-ray diffraction measurements using the diamond anvil cell (DAC) reported a phase change from Rh2O3(II) (or orthorhombic Pv) to the CaIrO3-type structure over 60 GPa, while an experiment using the Kawai-type apparatus with sintered diamond (SD) anvils suggested to stabilize a different phase with an unidentified orthorhombic structure at much lower pressure of 40~45 GPa. On the other hand, recent theoretical and experimental investigations of non-magnetic sesquioxide compounds have revealed an emerging systematics of their high-pressure phase sequence (Tsuchiya et al., 2005; Tsuchiya et al., 2007; Yusa et al., 2008; Yusa et al., 2009). While the CaIrO3-type phase with six and eight disproportionate coordination polyhedra was found to stabilize in Al2O3 and Ga2O3 at megabar pressure, several other compounds such as In2O3 and Sc2O3 were reported to transform directly to a further denser phase with the α-Gd2S3 structure composed only of high eight-fold coordination polyhedra at much lower pressure. Similarly to these studies, we searched for a stable form of Fe2O3 under pressure theoretically by means of the density-functional structurally consistent LDA+U method and succeeded to discover a new phase transformation from Rh2O3(II) at the pressure fairly close to that reported by the SD experiment. The high-pressure phase however has different lattice constants suggested experimentally and a structure more than 7.5%, 5% and 2.5% denser than hematite, Rh2O3(II) and CaIrO3 respectively with a strong AFM-preferential ground state. A large drop in band gap was also found across the phase change, suggesting a significant increase in the electric conductivity. We also successfully identified this phase by in situ X-ray experiments using SD. Although the details are now under analysis and will be provided in the presentation, the new phase change would affect the high-pressure solid solution mechanism of ferric iron into geophysically important MgSiO3. Research supported by Ehime Univ. Global-COE program “Deep Earth Mineralogy” and Grants-in-Aid for Scientific Research from JSPS 19740331 and 1840001. Tsuchiya, Tsuchiya, Wentzcovitch (2005) Phys. Rev. B 72, 020103(R). Tsuchiya, Yusa, Tsuchiya (2007) Phys. Rev. B 76, 174108. Yusa, Tsuchiya, Tsuchiya, Sata, Ohishi (2008) Phys. Rev. B 78, 092107. Yusa, Tsuchiya, Sata, Ohishi (2009) Inorg. Chem. 48, 7501.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMMR43C1885T
- Keywords:
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- 3924 MINERAL PHYSICS / High-pressure behavior