Structural, elastic, electronic and optical properties of the newly synthesized selenides Tl2CdXSe4 (X = Ge, Sn)

Motivated by the growing demand for new performant semiconducting materials, we investigated in detail the structural, elastic, electronic and optical properties of two newly synthesized compounds, namely Tl2CdGeSe4 and Tl2CdSnSe4, using density functional theory calculations. The calculations were performed relativistically, including the spin-orbit coupling (SOC). The computed equilibrium structural parameters are in excellent agreement with available measurements. Note that the calculations of all the considered properties were performed with the theoretically obtained equilibrium lattice parameters. The predicted monocrystalline and polycrystalline elastic constants reveal that the studied compounds are soft, ductile, mechanically stable and substantially structurally and elastically anisotropic materials. Our calculations using the Tran-Blaha modified Becke-Johnson potential with the inclusion of SOC show that Tl2CdGeSe4 and Tl2CdSnSe4 are direct bandgap semiconductors. The inclusion of SOC is found to reduce the fundamental bandgap of Tl2CdGeSe4 from 1.123 to 0.981 eV and that of Tl2CdSnSe4 from 1.097 to 0.953 eV. The L-decomposed atom-projected densities of states were calculated to identify the contribution of each constituent atom to the electronic states in the energy bands. The upper valence subband predominantly comes from the Se-4p states, while the bottom of the conduction band mainly originates from the Se-4p and Ge-4p/Sn-5p states. The frequency-dependent linear optical parameters, viz., the complex dielectric function, absorption coefficient, refractive index, reflectivity and energy-loss function, were calculated for electromagnetic waves polarized parallel and perpendicular to the c-axis in a wide energy window. An attempt was made to identify the microscopic origin of the peaks and structures observed in the calculated optical spectra.