DFT study of structural, electronic and elastic properties of two polymorphs of monoclinic CsGaQ2 (Q = S, Se)

In this paper, we report an ab-initio calculations of the structural, electronic and elastic properties of monoclinic CsGaQ2 (Q = S, Se) crystals in two polymorphs CsGaQ2-mC64 and CsGaQ2-mC16 (Q = S, Se). The investigation is done using the pseudo-potential plane-wave (PP-PW) method combined to the generalized gradient approximation (GGA) within the density functional theory (DFT). The calculated equilibrium lattice constants (a, b and c), angle β are in good agreement with the available experimental data. We have calculated and analyzed the energy gap, band structure and density of states. The electronic structure calculation demonstrates that crystals are direct-gap semiconductors. The single-crystal elastic constants Cij of CsGaQ2-mC16 are predicted, for the first time, using the stress-strain method. The polycrystalline bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν, and elastic anisotropy AU are determined based on the predicted Cij. Our results indicate that CsGaQ2 (Q = S, Se) can be classified as brittle materials.