First-principles investigation of Raman and infrared spectra in MgTa2O6

The Raman and infrared spectra of MgTa₂O₆ have been studied using density functional perturbation theory. A partial symmetry reassignment of the available experimental data has been proposed, using the theoretical results. We found that some infrared-active modes of the A_2u and E_u symmetries exhibit an unusual negative LO-TO splitting. Furthermore, the lattice dynamical properties such as Born effective charges, oscillator strengths, and dielectric permittivity tensors are also provided. In both directions, parallel to the tetragonal c-axis and perpendicular to it, a low-frequency mode provides the primary ionic contribution to the dielectric permittivity tensor. The calculated static dielectric constant is in excellent agreement with the experimental value.