Spin-orbit coupling driven A2WCl6 perovskites: DFT exploration of electronic, optical, and photocatalytic potential for H2 generation and CO2 conversion

The global climate crisis necessitates sustainable energy solutions, driving research into photocatalytic materials for hydrogen production and CO₂ reduction. This study employs density functional theory (DFT) within the WIEN2k framework to investigate vacancy-ordered double perovskites A₂WCl₆ (A = Cs, Rb, K, Tl) for photocatalytic and photovoltaic applications. Using the FP-APW + lo method with Wu–Cohen GGA, TB-mBJ corrections, and spin-orbit coupling (SOC), we analyze their structural, mechanical, electronic, and optical properties. The A-site modulation yields direct band gaps of 3.02 eV (Cs), 2.78 eV (Rb), 2.45 eV (K), and 2.22 eV (Tl), with half-metallic ferromagnetic behavior confirmed via spin-polarized calculations (α-spin: metallic, β-spin: semiconducting). Mechanical stability is affirmed by bulk moduli (38.25–42.98 GPa) and B/G ratios (1.97–2.52), ensuring durability in aqueous environments. Optical analyses reveal high visible-light absorption (19.5–33 × 10⁵ cm⁻¹), low reflectivity (18–25 %), minimal energy loss (<0.5), and tunable exciton binding energies (22.1–42.8 meV), ideal for photocatalysis. Band edge alignments indicate Cs₂WCl₆ and Rb₂WCl₆'s suitability for dual water splitting and CO₂ reduction, while K₂WCl₆ and Tl₂WCl₆ require band engineering. These findings position A₂WCl₆ as promising, stable photocatalysts, with A-site engineering offering precise tunability for solar-driven applications, contributing to sustainable energy advancements.