Lattice dielectric properties of rutile $\mathrm{TiO}_2$: First-principles anharmonic self-consistent phonon study

TL;DR

This study employs a self-consistent anharmonic lattice dynamics approach to accurately compute the dielectric properties of rutile TiO₂, highlighting the significance of higher-order anharmonic effects and phonon interactions.

Contribution

It introduces a modified self-consistent method including third and fourth-order anharmonicity for precise phonon and dielectric property calculations.

Findings

Better agreement with experimental phonon frequencies and linewidths.

Four-phonon scattering significantly affects phonon linewidths.

Two-phonon processes explain unidentified dielectric peaks.

Abstract

We calculate the lattice dielectric function of strongly anharmonic rutile $\mathrm{TiO}_2$ from ab initio anharmonic lattice dynamics methods. Since an accurate calculation of the $\Gamma$ 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 $\Gamma$ point much better agree 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. Furthermore, incorporating the frequency dependence of phonon linewidth reveals that experimentally known but unidentified peaks of the dielectric function are due to two-phonon process. This work emphasizes the importance of a self-consistent approach in predicting the optical properties of highly anharmonic materials.