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.

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