Electron density distribution of FeTiO3 ilmenite under high pressure analyzed by MEM using single crystal diffraction intensities

Many of ilmenites ABO(3) compounds bearing transition elements have semiconductive, ferroelectric and antiferromagnetic properties. The high-pressure diffraction studies of FeTiO3 have been conducted up to 8.2 GPa using synchrotron radiation in KEK at Tsukuba with diamond anvil cell. The compression mechanism of FeTiO3 ilmenite has been investigated by the structure refinements converged to the reliable factors R = 0.05. The deformations of the FeO6 and TiO6 octahedra were reduced with increasing pressure. In order to elucidate the electric conductivity change with pressure, electron density distribution of ilmenite have been executed by maximum entropy method (MEM) using single-crystal diffraction intensity data. MEM based on F-obs(hkl) of FeTiO3 clearly shows electron density in comparison with the difference Fourier synthesis based on F-obs(hkl) - F-calc(hkl). The radial distribution of the electron density indicates electron localization around the cation positions. The bonding electron density found in bond Fe-O and Ti-O is lowered with pressure. The isotropic temperature factors B (iso) become smaller with increasing pressure. Nevertheless the thermal vibration is considerably restrained by the compression, the electric conductivity is enhanced with pressure. Neither charge transfer nor electron hopping between Fe and Ti along the c axis in FeTiO3 is plausible under high pressure. But the electric conductivity due to electron super-exchange in Fe-Fe and Ti-Ti has been clarified by the MEM electron density distribution. The anisotropy in the electric conductivity has been clarified.