Density Functional Theory‐Based First‐Principles Analysis of A 2 WBr 6 Vacancy‐Ordered Double Perovskites: Band Tuning, Optical Response, and Photocatalytic Potential

This study explores vacancy‐ordered double perovskites A 2 WBr 6 (A = Cs, Rb, K, Tl) for photocatalytic hydrogen production and CO 2 reduction, offering sustainable solutions to the climate crisis. Using density functional theory, structural, mechanical, electronic, and optical properties are analyzed with the Wu–Cohen GGA and Tran–Blaha mBJ potential, including spin–orbit coupling. A remarkable synergistic effect between half‐metallic ferromagnetism and photocatalytic performance was identified, where spin‐polarized charge carriers generated by W 5d states enhance light absorption and charge separation efficiency. Direct bandgaps (2.18–3.06 eV for α‐spin, 2.48–4.80 eV for β‐spin) arise from W‐d and Br‐p hybridization at the valence band maxima and W‐d states at the conduction band minima. Cs 2 WBr 6 , Rb 2 WBr 6 , and K 2 WBr 6 support water splitting, while Tl 2 WBr 6 requires band engineering. Optical properties reveal high absorption (20–27 × 10 5 cm −1 , 370–480 nm), low reflectivity (17%–27%), and dielectric constants (3.59–5.95). Low exciton binding energies (23.7–56.7 meV) and tunable effective masses (electrons: 0.71–0.83 m 0 , holes: 2.15–2.41 m 0 ) enhance charge transport, making these perovskites promising for spin‐assisted solar‐driven photocatalysis.