High-pressure structural studies of hematite (formula presented)

Structural studies and a full-profile refinement of the high-pressure phases of hematite (formula presented) were carried out to 76 GPa using x-ray synchrotron powder diffraction. It was found that pressure induces a progressive distortion of the corundum-like hematite structure (HP1), culminating in a structural phase transition (HP2) at 50 GPa. At first sight the powder diffraction data obtained for HP2 could be equally explained in terms of either an orthorhombic perovskite or a (formula presented) -type structure, but by a comparative analysis of the O-O bond length for both structures, recent Mössbauer spectroscopy results, and ab initio calculations allowed for the unambiguous assignment of the HP2 phase to the (formula presented) -type structure. As a result of the phase transition the following changes are observed: (i) a substantial decrease in the Fe-O distances with a slight increase in Fe-Fe distances which led to a reduced cell volume, (ii) a diminution of the Fe-O-Fe bond distortion, and, (iii) a reduction in the distortion of the (formula presented) octahedron. The structural transition coincides with a previously reported insulator-metal transition due to the electronic Mott transition. It is suggested that the unusual volume reduction of 10% is accounted by the combined crystallographic and electronic phase transition, the latter resulting into a substantial reduction of the ionic radii and consequently of the Fe-O bond lengths due to electron delocalization attributed to a charge-transfer gap closure. The mechanism of the combined structural, electronic, and magnetic transformations is discussed.