Post-Rutile High-Pressure Phases in TiO₂

The crystal structures of rutile (TiO₂) and its high-pressure polymorphs have been studied by X-ray powder diffraction in the pressure range up to 60 GPa. At 12 GPa, rutile transforms to a phase with the baddeleyite (ZrO₂) structure. Upon decompression, this phase transforms at 7 GPa to another phase with the α-PbO₂ structure. At ambient conditions, the α-PbO₂-type phase is 2.1 (3)% denser than rutile and the baddeleyite-type phase is 11.3 (9)% denser than rutile. In the pressure range of the rutile-to-baddeleyite transition, the difference in density between the two phases is 9.75 (15)%. The zero-pressure bulk moduli, as determined from the equation of state, are 230 (20), 260 (30) and 290 (20) GPa for rutile, the α-PbO₂-type phase and the baddeleyite-type phase, respectively.