Full-potential calculations of the electronic and optical properties for 1T and 2H phases of TaS2 and TaSe2

The band structure, density of states and anisotropic frequency-dependent optical properties have been calculated for the 1 T and 2 H phases of TaS2 and TaSe2 using the full-potential linear augmented plane wave (FPLAPW) method. In the 1 T and 2 H phases, when S is replaced by Se, the unoccupied Ta-5d and chalcogen-p bands move closer to the Fermi energy EF and the bandwidth of the chalcogen-s group decreases. Compared to the 1 T phase, in the 2 H phase the occupied/unoccupied bands move towards higher/lower energies with respect to EF. In the 1 T phase, when S is replaced by Se, the peak positions in the imaginary part of the frequency-dependent dielectric function ε2(ω) move towards lower energies by 0.5 eV. The single peak at 6 eV in ε2(ω) of the 1 T phase is split into two peaks in the 2 H phase. We make a detailed comparison of the frequency-dependent reflectivity and absorption coefficient with the available experimental data. The linear muffin tin orbital method within the atomic sphere approximation (LMTO-ASA) shows poor agreement with the experimental data while our FPLAPW results give excellent agreement with the experimental data suggesting that a better representation of the potential is essential for calculating optical properties accurately.