Optoelectronic, Photo-Catalytic and Thermodynamic Properties of Ba-Based Spinels (BaLu2X4, X = S, Se) for Energy Harvesting Applications Explored Using First-Principles Calculations

Background: Chalcogenide spinels are promising semiconducting materials for optoelectronic, photocatalytic, and renewable energy applications because of their structural stability and tunable electronic properties. This study systematically investigates the structural, electronic, optical, elastic, thermodynamic, and photocatalytic characteristics of BaLu2S4 and BaLu2Se4 using first-principles calculations. Methods: Density functional theory calculations were performed using the full-potential linearized augmented plane wave method within the Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol-GGA) framework. The Tran–Blaha modified Becke–Johnson potential was employed for accurate electronic and optical properties, while thermodynamic behavior was evaluated using the quasi-harmonic Debye model. Results: Both compounds exhibited negative formation energies and satisfied the Born stability criteria, thus being thermodynamically and mechanically stable. The indirect band gaps of BaLu2S4 and BaLu2Se4 were obtained as 3.1 and 2.8 eV, respectively. Selenium substitution expanded the lattice, decreased the band gap, enhanced visible-light absorption, and improved photocatalytic performance. BaLu2S4 exhibited a higher mechanical rigidity and thermal stability, whereas BaLu2Se4 showed greater anharmonicity, thermal expansion, and hydrogen evolution capability owing to its favorable band-edge alignment. Conclusions: BaLu2S4 and BaLu2Se4 are stable semiconducting spinels with excellent optoelectronic and thermodynamic properties. BaLu2Se4 is the more favorable candidate for visible-light-driven photocatalytic hydrogen production based on its more negative CBM and stronger reduction potential.