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Novel high pressure monoclinic Fe2O3 polymorph revealed by single-crystal synchrotron X-ray diffraction studies

A novel high pressure polymorph of iron sesquioxide, m-Fe2O3, has been identified by means of single-crystal synchrotron X-ray diffraction (XRD). Upon compression of a single crystal of hematite, α-Fe2O3, in a diamond anvil cell, the transition occurs at pressure of about 54 GPa and results in ∼10% volume reduction. The crystal structure of the new phase was solved by the direct method (monoclinic space group P21/n, a=4.588(3), b=4.945(2), c=6.679(7) Å and β=91.31(9)°) and refined to R1 ∼11%. It belongs to the cryolite double-perovskite structure type and consists of corner-linked FeO6 octahedra and FeO6 trigonal prisms filling the free space between the octahedra. Upon compression up to ∼71 GPa at ambient temperature no further phase transitions were observed. Laser heating to ∼ 2100±100 K promotes a transition to the Cmcm CaIrO3-type (post-perovskite (PPv)) phase. The PPv-Fe2O3 crystal structure was refined by means of single-crystal XRD at ∼65 GPa. On decompression the PPv-Fe2O3 phase fully transforms back to hematite at pressures between ∼25 and 15 GPa.