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.