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

Abstract

High-pressure and high-temperature phase relations of FeTiO 3 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 FeTiO 3 ilmenite transforms into the following phase(s) with increasing pressure: FeTiO 3 (perovskite) at 20-30 GPa, Fe 2 TiO 4 (Ca 2 TiO 4 -type) + TiO 2 (OI-type) at 30-44 GPa and high temperature, FeO (wüstite) + TiO 2 (OI) at 30-44 GPa and low temperature, and wüstite + FeTi 3 O 7 (orthorhombic phase) above 44 GPa. Among these dense high-pressure polymorphs, FeTi 3 O 7 is a new compound and its structure analysis was tried using particle swarm optimization simulation. This method successfully found a new high-density FeTi 3 O 7 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 FeTi 3 O 7 structure consists of polyhedra for monocapped FeO 7 prisms, bicapped TiO 8 prisms, and tricapped TiO 9 prisms, which develop on the b-c plane and stack along the a axis. The dense compound assemblage found in FeTiO 3 is promising for investigating the behavior of ABX 3 compounds under ultrahigh pressures.