Ab Initio Investigation of the Structural, Electronic and Optical Properties of the Li2In2XY6 (X = Si, Ge; Y = S, Se) Compounds

The structural, electronic and optical properties of the Li2In2XY6 (X = Si, Ge; Y = S, Se) compounds, which are scarcely studied by theoretical methods previously, have been investigated by ab initio calculations based on the density functional theory (DFT) in this article by using the full potential linearized augmented plane wave method. The equilibrium structural ground state properties of the Li2In2XY6 (X = Si, Ge; Y = S, Se) compounds such as the lattice parameters were obtained from the structural optimization process (with the Perdew-Burke-Ernzerhof generalized gradient approximation), and they are in close agreement with the experimental lattice parameters. Conversely, calculations by the modified Becke Johnson exchange potential indicates that the Li2In2XY6 (X = Si, Ge; Y = S, Se) compounds are semiconductors with direct energy band gaps. It is clearly observed from the DFT-calculated partial density of states, that there are significant contributions of the S-s and S-p states in the Li2In2SiS6 and Li2In2GeS6 compounds as well as the Se-s and Se-p states in the Li2In2SiSe6 and Li2In2GeSe6 compounds, respectively. The calculated band gaps ranging from 1.92 eV to 3.24 eV of the Li2In2XY6 (X = Si, Ge; Y = S, Se) compounds are in good agreement with the experimental results, where the calculated band gap values are positioned in the visible region of the electromagnetic spectrum; therefore, these materials can be efficiently used for opto-electronic and optical applications. Furthermore, some general trends are observed in the optical responses of the compounds, which are possibly correlated to the energy band gaps when the X cations changes from Si to Ge and the Y anions changes from S to Se in the Li2In2XY6 (X = Si, Ge; Y = S, Se) compounds, respectively.