First-principles study of the mechanical properties and phase stability of TiO₂

We performed a density-functional theory study of the mechanical properties, phonon and phase stability of TiO₂ in the structures of rutile, anatase, columbite, baddeleyite, OI, cotunnite, fluorite, and pyrite. Six exchange-correlation functionals were used to evaluate the structural and elastic properties of TiO₂. The calculated bulk and shear moduli of TiO₂ confirm that the cotunnite and fluorite phases are not as hard as traditional ultrahard materials, such as diamond. The predicted phonon spectra of the cubic phases of TiO₂, i.e., the fluorite and pyrite phases, show that they are dynamically unstable at ambient conditions. However, the fluorite structure can be stabilized as a metastable phase at high pressures. The pressure-induced phase transitions of TiO₂ are found to depend on the starting material. The predicted pressure-induced phase transition pressures and sequence are consistent with previous experimental and theoretical studies. From the calculated Gibbs energies, we investigated the pressure-temperature phase diagram of TiO₂. The calculated phase equilibria are in good agreement with the available experimental results. The currently predicted phase diagram is expected to provide helpful guidance for the future synthesis of high-pressure phases in TiO₂.