A STUDY OF THE CRYSTAL-STRUCTURE OF FE2O3 IN THE PRESSURE RANGE UP TO 65 GPA USING SYNCHROTRON RADIATION

High-pressure X-ray powder diffraction has been performed on corundum-type alpha-Fe2O3 (hematite) in the pressure range up to 65 GPa using synchrotron radiation and a diamond anvil cell. A structural phase transformation has been observed with a transition pressure of about 55 GPa. The high-pressure form of Fe2O3 is orthorhombic. The lattice parameters at 60 GPa are a = 4.59(3) angstrom, b = 4.97(3) angstrom, c = 6.68(5) angstrom, Z = 4. The crystal structure is most probably a GdFeO3-type perovskite with space group Pbnm. The volume per formula unit in the orthorhombic phase is 10% smaller than in alpha-Fe2O3. It is suggested that the volume change is due to a combination of a valence change and a high-spin/low-spin transition. The bulk modulus, B0, and its pressure derivative, B'0, evaluated at zero pressure for alpha-Fe2O3 are B0 = 230(5) GPa and B'0 = 3.5(6).