First-Principles Insights into the Electronic Structure, Optoelectronic, and Thermoelectric Properties of X₂SrS₄ (X = Y, La) Chalcogenides for Energy Generation Applications

A first-principles investigation of the chalcogenide compounds Y₂SrS₄ and La₂SrS₄ is carried out to explore their structural, electronic, optical, and thermoelectric properties with a view toward photovoltaic and energy-conversion applications. Density functional theory (DFT) calculations are performed using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code. The electronic band structures are evaluated using both the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential to obtain reliable band-gap estimates. The calculated band gaps for Y₂SrS₄ are 1.744 eV (GGA) and 2.481 eV (TB-mBJ), while those for La₂SrS₄ are 1.861 eV and 2.479 eV, respectively. In both compounds, the band gaps are direct in nature and fall within the visible energy range, confirming their semiconducting behavior with dominant p-type conduction. The density of states analysis reveals that the primary electronic transitions originate from S- p states in the valence band to Y- d states in the conduction band for Y₂SrS₄, and from S- p to La- d states for La₂SrS₄. Optical properties, including the complex dielectric function, refractive index, absorption coefficient, energy-loss function, and reflectivity, are systematically examined. The static reflectivity at zero photon energy is found to be approximately 20% for Y₂SrS₄ and 21% for La₂SrS₄, indicating moderate surface reflection and favorable light-harvesting characteristics. Strong optical absorption in the visible and ultraviolet regions further supports their suitability for optoelectronic and photovoltaic applications. In addition, thermoelectric transport calculations reveal promising performance at elevated temperatures, with the dimensionless figure of merit (ZT) reaching 0.74 for Y₂SrS₄ and 0.70 for La₂SrS₄ at 300 K, and increasing to 0.79 and 0.82, respectively, at 800 K. Overall, the combined electronic, optical, and thermoelectric characteristics identify X₂SrS₄ (X = Y, La) compounds as attractive multifunctional materials for photovoltaic solar cells, optoelectronic devices, and high-temperature thermoelectric applications.