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

Abstract

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.