Dielectric properties and lattice dynamics of alpha-PbO2-type TiO2: The role of soft phonon modes in pressure-induced phase transition to baddeleyite-type TiO2

Dielectric tensor and lattice dynamics of alpha-PbO2-type TiO2 have been investigated using the density functional perturbation theory, with a focus on responses of the vibrational frequencies to pressure. The calculated Raman spectra under different pressures are in good agreement with available experimental results and the symmetry assignments of the Raman peaks of alpha-PbO2-type TiO2 are given for the first time. In addition, we identified two anomalously IR-active soft phonon modes, B1u and B3u, respectively, around 200 cm-1 which have not been observed in high pressure experiments. Comparison of the phonon dispersions at 0 and 10 GPa reveals that softening of phonon modes also occurs for the zone-boundary modes. The B1u and B3u modes play an important role in transformation from the alpha-PbO2-type phase to baddeleyite phase. The significant relaxations of the oxygen atoms from the Ti4 plane in the Ti2O2Ti2 complex of the baddeleyite phase are directly correlated to the oxygen displacements along the directions given by the eigenvectors of the soft B1u and B3u modes in the alpha-PbO2-type phase.