POST-RUTILE HIGH-PRESSURE PHASES IN TIO2

The crystal structures of rutile (TiO2) and its high-pressure polymorp hs 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 baddel eyite (ZrO2) structure. Upon decompression, this phase transforms at 7 GPa to another phase with the alpha-PbO2 structure. At ambient condit ions, the alpha-PbO2-type phase is 2.1 (3)% denser than rutile and the baddeleyite-type phase is 11.3(9)% denser than rutile. In the pressur e range of therutile-to-baddeleyite transition, the difference in dens ity 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 2 90(20)GPa for rutile, the alpha-PbO2-type phase and the baddeleyite-ty pe phase, respectively.