High-pressure and high-temperature phase transitions in FeTiO3 and a new dense FeTi3O7 structure

High-pressure and high-temperature phase relations of FeTiO₃ were investigated up to a pressure of about 74 GPa and 2600 K by synchrotron X-ray diffraction and analytical transmission electron microscopy. We conclude that FeTiO₃ ilmenite transforms into the following phase(s) with increasing pressure: FeTiO₃ (perovskite) at 20–30 GPa, Fe₂TiO₄ (Ca₂TiO₄-type) + TiO₂ (OI-type) at 30–44 GPa and high temperature, FeO (wüstite) + TiO₂ (OI) at 30–44 GPa and low temperature, and wüstite + FeTi₃O₇ (orthorhombic phase) above 44 GPa. Among these dense high-pressure polymorphs, FeTi₃O₇ is a new compound and its structure analysis was tried using particle swarm optimization simulation. This method successfully found a new high-density FeTi₃O₇ structure, and Rietveld refinement based on this model structure gave an excellent fit with the experimentally obtained X-ray diffraction pattern. This new high-density FeTi₃O₇ structure consists of polyhedra for monocapped FeO₇ prisms, bicapped TiO₈ prisms, and tricapped TiO₉ prisms, which develop on the b-c plane and stack along the a axis. The dense compound assemblage found in FeTiO₃ is promising for investigating the behavior of ABX₃ compounds under ultrahigh pressures.