HIGH-PRESSURE TRANSFORMATIONS OF CORUNDUM-TYPE TRANSITION METAL OXIDES

On the basis of seismic observations and compositional studies of meteorites, the Earth's outer core is believed to be composed of liquid iron alloyed with a small amount of a lighter component (e.g. Merrill and Bassett, 1974). Among the possible alloying elements are S, Si, H and O. It is, therefore, obvious that an understanding of the crystal chemistry, as well as electronic and magnetic structure and properties, of iron oxides at high pressure and high temperature can be considered as one of the primary challenges of geophysics. Transition metal oxides constitute a fascinating class of materials of great importance in solid state physics, as well as materials science, with a uniquely wide range of electronic properties. The diversity of electronic properties and the richness of unique phenomena exhibited by TMO result mostly from incomplete filling of the shells of d orbitals, and these materials are prototypes of strongly correlated electron systems, whose structure-property relationships remain a forefront topic in condensed matter research. The majority of trivalent TM oxides crystallizes in the hexagonal corundum structure (Cox 1995). The substances assuming this structure type include Fe2O3, Co2O3, Cr2O3, Rh2O3, and Ti2O3, among others. The behavior of corundum-type oxide structures at higher pressure has attracted a significant deal of attention over the years. The most interesting and widely studied transformation in Fe2O3 occurs at a pressure of about 50 GPa. It was first suggested more than 30 years ago, when Reid and Ringwood (1969) predicted the existence of perovskite-type Fe2O3 at pressures 60-120 GPa in shock wave experiments. A variety of different experimental techniques, including Mössbauer spectroscopy (Suzuki et al., 1985; Syono et al., 1984; Nasu et al., 1986; Kurimoto et al., 1986), powder diffraction (Yagi and Akimoto, 1982; Suzuki et al., 1985) electrical conductivity measurements (Endo and Ito 1980; Goto et al. 1980; Kondo et al. 1980), and X-ray emission spectroscopy (Badro et al. 2002) have been used to better characterize the phenomenon. We present results of single-crystal x-ray diffraction experiments on Cr2O3 up to 55 GPa and Fe2O3 up to 70 GPa that shed new light on the crystal structure of high-pressure post-corundum phases.