Ab initio study of some physical properties of the newly synthesized selenides ... (X = Ge, Sn)

Abstract

Motivated by the increasing need for high-performance semiconductor materials, we conducted a comprehensive investigation into the structural, elastic, electronic, and optical properties of two recently synthesized compounds, namely Tl2CdGeSe4 and Tl2CdSnSe4, using the full potential linearized augmented plane wave (FP-LAPW) and pseudopotential plane wave (PP-PW) employing density functional theory calculations. The calculations were carried out with the inclusion of relativistic effects, specifically accounting for spin-orbit coupling (SOC). The resulting equilibrium structural parameters obtained from the computations exhibit remarkable agreement with available measurements. It should be noted that the calculations for all the properties examined were carried out using the theoretic equilibrium lattice parameters. The obtained results for both monocrystalline and polycrystalline elastic constants indicate that the investigated compounds exhibit softness, ductility, mechanical stability, and significant structural and elastic anisotropy. By employing the Tran-Blaha modified Becke-Johnson potential and considering the inclusion of spin-orbit coupling (SOC), our calculations reveal that both Tl2CdGeSe4 and Tl2CdSnSe4 are direct bandgap semiconductors. Incorporating SOC leads to a reduction in 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 utilized to determine the individual contributions of each constituent atom to the electronic states within the energy bands. The upper valence subband predominantly arises from the Se-4p states, while the bottom of the conduction band primarily originates from the Se-4p and Ge-4p/Sn-5p states. Furthermore, frequency-dependent linear optical parameters, including the complex dielectric function, absorption coefficient, refractive index, reflectivity, and energy-loss function, were calculated across a wide energy range for electromagnetic waves polarized parallel and perpendicular to the c-axis. Efforts were made to elucidate the microscopic origins of the observed peaks and structures in the calculated optical spectra.