First-principles calculation of M₃Ta₆Si₄O₂₆ (M = Ba, Sr) using Tran-Blaha exchange potential

Abstract

The first-principles energy band calculation for M₃Ta₆Si₄O₂₆ (M = Ba, Sr) with optimized structure is performed using the Tran-Blaha (TB) exchange potential combined with correlation potential from the local density approximation (LDA). The theoretical minimum band gaps of Ba₃Ta₆Si₄O₂₆ and Sr₃Ta₆Si₄O₂₆ are estimated to be 4.39 and 4.46 eV, respectively. These results are in large contrast to the underestimation based on the generalized gradient approximation calculation. The band gaps of M₃Ta₆Si₄O₂₆ (M = Ba, Sr) hardly changed with the spin-orbit interaction. The main conduction band can be divided into two energy regions near 7.6 eV due to the octahedral crystal-field splitting based on the interaction between Ta-d and O-p states. On the other hand, the main valence band in the energy range of −5.3 to 0 eV is constructed from O-p states, where the Ta-d states hybridize strongly with the O-p states. The optical properties of M₃Ta₆Si₄O₂₆ (M = Ba, Sr) are predicted from the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω). Polarized peaks in the z-direction appear at 5.5 and 6.4 eV for Ba₃Ta₆Si₄O₂₆, 5.6 and 6.6 eV for Sr₃Ta₆Si₄O₂₆ in the ε₂(ω) function, respectively. The absorption coefficient estimated from the TB-LDA is very similar to the experimental result obtained in the literature.