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 1T and 2H phases of TaS2 and TaSe2 using the full-potential linear augmented plane wave (FPLAPW) method. In the 1T and 2H 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 1T phase, in the 2H phase the occupied/unoccupied bands move towards higher/lower energies with respect to EF. In the 1T 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 0.5eV. The single peak at 6eV in ε2(ω) of the 1T phase is split into two peaks in the 2H 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