Structural phase transition and mechanical properties of TiO₂ under high pressure

A systematic examination of structural phase transitions and mechanical properties for TiO₂ under high pressure has been performed by using first-principles calculations. First, we show that the orthorhombic Pca21 structure becomes stabilize at certain pressure and temperature, whereas the cubic fluorite and pyrite structures are not energetically viable in the whole pressure range of 0-200GPa. These findings support that the experimentally assumed cubic TiO₂ should be the Pca21-type TiO₂. Secondly, our calculated equations of state for various TiO₂ polymorphs are consistent with previous experimental and theoretical results. The only exception is the baddeleyite phase for which we find a significantly lower bulk modulus of 149GPa than the measured value 290-304GPa. Finally, our calculations reveal that the recently synthesized Fe₂P-type TiO₂ exhibits semiconducting features and has the potential to be a superhard material under ultrahigh pressure. It is shown that the high pressure could open a valid avenue for new hard or superhard materials.