Effect of S and Se replacement on electronic and thermoelectric features of BaCu2GeQ4 (Q = S, Se) chalcogenide crystals

For the Quaternary Chalcogenides BaCu(2)GeQ(4) (Q = S, Se) optoelectronic, structural and transport properties are explored using accurate all electrons FP-LAPW [full potential linearized augmented plane wave] method. The lattice constants calculated theoretically are established to be in fine agreement with the values that are measured experimentally. It is established that all the titled compounds are direct band gap semiconductors and this energy band gap is situated Gamma- Gamma symmetry points. At the same time the band gap magnitude decreases during replacement of S by Se. Optical constant dispersion like imaginary and real components of dielectric functions, extinction coefficients, refractive indices, reflectivity's, absorption coefficients, have been also calculated for these compounds. High absorption and the direct band gap characteristics of these compounds in the UV-Visible energy range indicate that these perovskite structures might be used in optoelectronic and optical devices working in the UV -Visible range of the energy spectrum. The calculated birefringence (-0.012 and -0.083) enhances the suitability of BaCu2GeSe4 compared to BaCu2GeS4 usually applied as nonlinear optical materials. The computed transport coefficients exhibit the anisotropic nature of the materials, in agreement with their electronic states. The transport properties show stronger carrier concentrations along the Ge-s, Ba-p and Cu-d orbitals, confirming that these orbitals are principal for the electrical transport features. The values of effective mass of electrons are computed by curvature of the CBM band 124 and 184 for BaCu2GeS4 and BaCu2GeSe4 respectively. Investigating of the thermoelectric power factor shows that BaCu2GeS4 is much better than BaCu2GeSe4 over the entire temperature interval which make it suitable for future technological applications. (C) 2019 Elsevier B.V. All rights reserved.