Novel high pressure monoclinic Fe 2 O 3 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 groupP21/n, a=4.588(3),b=4.945(2),c=6.679(7) Å and β=91.31(9)°) and refined toR1∼11%. It belongs to the cryolite double-perovskite structure type and consists of corner-linked FeO6octahedra and FeO6trigonal 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 theCmcmCaIrO3-type (post-perovskite (PPv)) phase. The PPv-Fe2O3crystal structure was refined by means of single-crystal XRD at ∼65 GPa. On decompression the PPv-Fe2O3phase fully transforms back to hematite at pressures between ∼25 and 15 GPa. [ABSTRACT FROM PUBLISHER]