Lattice dielectric properties of rutile TiO2: First-principles anharmonic self-consistent phonon study

We calculate the lattice dielectric function of strongly anharmonic rutile TiO2 from ab initio anharmonic lattice dynamics methods. Since an accurate calculation of the I' point phonons is essential for determining optical properties, we employ the modified self-consistent approach, including third-order anharmonicity as well as fourth-order anharmonicity. The resulting optical phonon frequencies and linewidths at the I' point agree much better with experimental measurements than those from a perturbative approach. We show that the four -phonon scattering process contributes as much as the third-order anharmonic term to phonon linewidths of some phonon modes. Furthermore, incorporating the frequency dependence of phonon linewidth reveals that experimentally known but unidentified peaks of the dielectric function are due to the two-phonon process. This work emphasizes the importance of the self-consistent approach in predicting the optical properties of highly anharmonic materials.