First-principles study of lattice dynamics and thermodynamics of TiO2 polymorphs.

The structural, phonon, and thermodynamic properties of six TiO(2) polymorphs, i.e., rutile, anatase, columbite, baddeleyite, orthorhombic I, and cotunnite, have been systematically investigated by density functional theory. The predicted volumes, bulk modulus, and Debye temperature are in good agreement with experiments. The phonon dispersions of the TiO(2) polymorphs were studied by the supercell approach, whereas the long-range dipole-dipole interactions were calculated by linear response theory to reproduce the LO-TO splitting, making accurate prediction of phonon frequencies for the polar material TiO(2). The calculated phonon dispersions show that all TiO(2) polymorphs are dynamically stable at ambient pressure, indicating the high-pressure phases might be quenched to ambient conditions as ultrahard materials. Furthermore, the finite temperature thermodynamic properties of TiO(2) polymorphs were predicted accurately from the obtained phonon density of states, which is critical in the future study of the pressure-temperature phase diagram of TiO(2). The calculated Gibbs energies reveal that rutile is more stable than anatase at ambient pressure. We derived the Gibbs energy and heat capacity functions for all TiO(2) polymorphs for use in thermodynamic modeling of phase equilibria.