Optoelectronic and thermoelectric characterization of MgX2S4 (X= Ga, In) Spinels: A DFT approach for energy Applications

Spinel chalcogenides are attracting considerable interest due to their potential applications in advanced technologies such as solid-state lighting, thermoelectric systems, and photovoltaic devices. In the present work, the physical properties of spinel chalcogenides MgX${}_2$S${}_4$ (X = Ga, In) are systematically investigated using a combination of density functional theory (DFT) and semi-classical transport theory. The research focuses on the electronic, optical, and transport properties of these materials, with an emphasis on their suitability for optoelectronic and solar energy conversion applications. Exchange and correlation potentials are considered within the framework of the generalized gradient approach (GGA), including spin–orbit coupling effects (SOC). The thermodynamic stability of both compounds is confirmed by the computed negative formation energies. The materials exhibit direct band gaps of approximately 1.47 eV for MgGa${}_2$S${}_4$ and 1.81 eV for MgIn${}_2$S${}_4$. Optical properties as follows the dielectric function, refractive index,reflectivity and absorption coefficient were evaluated, revealing strong absorption ($10^5{\mathrm{cm}}^{-1}$ ) in the visible region, which supports their potential in optoelectronic applications. Transport properties, Including the Seebeck coefficient, figure of merit (ZT), electrical and thermal conductivity, and others, were evaluated. The estimated ZT values are $\sim$ 0.98 for MgGa${}_2$S${}_4$ and $\sim$ 1 for MgIn${}_2$S${}_4$ near room temperature, indicating good thermoelectric performance. Furthermore, the transport data suggest that MgX${}_2$S${}_4$ (X = Ga, In) exhibits p-type semiconducting behavior, reinforcing its promise for thermoelectric applications.