High-pressure polymorphs of anatase TiO2

The equation of state of anatase TiO2 has been determined experimentally-us ing polycrystalline as well as single-crystal material-and compared with th eoretical calculations using the ab initio perturbed ion model. The results are highly consistent, the zero-pressure bulk modulus being 179(2) GPa fro m experiment and 189 GPa from theory. Single-crystal tetragonal anatase tra nsforms to the orthorhombic alpha-PbO2 structure at about 4.5 GPa. This tra nsition is suppressed in the polycrystalline material at room temperature, probably due to the presence of grain boundaries and other crystal defects. Polycrystalline anatase is found to transform to the monoclinic baddeleyit e structure at about 13 GPa. Upon decompression, the baddeleyite phase tran sforms to the alpha-PbO2 phase at about 7 GPa. The experimental zero-pressu re bulk moduli an 258(8) GPa for the alpha-PbO2 phase and 290(10) GPa for t he baddeleyite phase.