Probing Cs2OsX6 (X = Cl, Br, I) double perovskites via DFT: prospects for photocatalytic water splitting and CO2 reduction

This study presents a comprehensive theoretical investigation of the structural, electronic, mechanical, and optical properties of Cs2OsX6 (X = Cl, Br, I) vacancy-ordered double perovskites using density functional theory (DFT) within the WIEN2k computational framework. The structural stability of Cs2OsX6 compounds was systematically evaluated through calculations of total ground state energy, cohesive energy, and formation energy, revealing remarkable stability under ambient conditions and favorable synthesis conditions. Mechanical property assessments, including Cauchy pressure and Poisson's ratio, indicated predominantly ductile behavior, suggesting excellent mechanical durability. Electronic structure calculations, performed using the Wu and Cohen generalized gradient approximation (WC-GGA) and the Tran–Blaha modified Becke–Johnson (TB-mBJ) methods, revealed semiconducting behavior with direct bandgaps of 1.98 eV (Cs2OsCl6), 1.68 eV (Cs2OsX6), and 0.91 eV (Cs2OsI6). Optical property analysis demonstrated strong absorption in the visible spectrum, with Cs2OsI6 exhibiting superior light-harvesting capabilities. Exciton binding energy calculations showed a decreasing trend with increasing halide atomic size, indicating enhanced charge carrier separation and reduced recombination rates. Band edge alignment suggested that Cs2OsCl6 and Cs2OsX6 are suitable for water oxidation, while Cs2OsI6 shows potential for CO2 reduction. These findings provide a robust theoretical foundation for the design and optimization of Cs2OsX6 perovskites in next-generation optoelectronic and photocatalytic devices, advancing sustainable energy technologies. Graphical abstract