Structural changes induced by lattice–electron interactions: SiO₂ stishovite and FeTiO₃ ilmenite

The bright source and highly collimated beam of synchrotron radiation offers many advantages for single-crystal structure analysis under non-ambient conditions. The structure changes induced by the lattice–electron interaction under high pressure have been investigated using a diamond anvil pressure cell. The pressure dependence of electron density distributions around atoms is elucidated by a single-crystal diffraction study using deformation electron density analysis and the maximum entropy method. In order to understand the bonding electrons under pressure, diffraction intensity measurements of FeTiO₃ ilmenite and γ-SiO₂ stishovite single crystals at high pressures were made using synchrotron radiation. Both diffraction studies describe the electron density distribution including bonding electrons and provide the effective charge of the cations. In both cases the valence electrons are more localized around the cations with increasing pressure. This is consistent with molecular orbital calculations, proving that the bonding electron density becomes smaller with pressure. The thermal displacement parameters of both samples are reduced with increasing pressure.