Optical properties of BaTiO₃ at room temperature: DFT modelling

The optical properties of the tetragonal phase of BaTiO₃ have been studied using the density functional method. In the study of a static lattice, we employed the generalized gradient approximation functional combined with on-site Hubbard correlation (GGA + U) and a hybrid functional. To account for the thermal motion of the atoms, we performed ab initio molecular dynamics calculations using the GGA + U method. We calculated the optical absorption spectra both for the static lattice and along the molecular dynamics trajectory. The results show that considering the motion of atoms leads to a significant decrease in the calculated value of the threshold energy for optical absorption. This effect occurs for two main reasons. First, changes in the atomic configuration due to thermal motion make electronic transitions that were previously dark for a static lattice become bright. Second, the optical absorption threshold decreases due to fluctuations in the energy of electronic transitions caused by the motion of atoms. The calculations were performed separately for different k-points of the Brillouin zone. The dispersion of electron energy in the reciprocal space may explain some features observed in the photoluminescence spectra.