Vacancy-ordered A2RuBr6 double perovskites as dual-functional visible-light photocatalysts and half-metallic spintronic materials

Abstract

This study explores the vacancy-ordered double perovskites A₂RuBr₆ (A = Cs, Rb, K) as novel photocatalysts and spintronic materials, addressing the global climate crisis through sustainable oxidation processes. Employing density functional theory (DFT) with the Wu-Cohen Generalized Gradient Approximation (GGA-WC), Tran-Blaha modified Becke-Johnson (TB-mBJ) potential, and spin-orbit coupling (SOC) within the WIEN2k package, we investigate their structural, electronic, optical, and mechanical properties. Structural analyses reveal a stable cubic Fm-3 m lattice with lattice constants of 10.23–10.58 Å, bulk moduli of 32.06–37.46 GPa, and negative formation energies (–2.61 to − 3.57 eV), confirming thermodynamic and mechanical stability. Electronic properties exhibit direct band gaps of 1.50–1.68 eV (λ ≈ 738–826 nm), enabling efficient absorption deep into the visible-red region and covering ~ 30–35% of the solar spectrum, significantly higher than TiO₂ (Eg ≈ 3.2 eV, λ ≈ 388 nm, ~ 5%) and CdS (Eg ≈ 2.4 eV, λ ≈ 517 nm, ~ 17%). Half-metallic ferromagnetic behavior driven by Ru 4d electrons enhances spin-polarized transport. Optical characteristics include high absorption coefficients (4.25–7.34 × 10⁵ cm⁻¹), low reflectivity (10–14%), and excitonic binding energies of 139–167 meV, promoting efficient charge separation. Band edge positions (EVB: 1.69–1.75 eV; ECB: 0.01–0.25 eV) favor water oxidation, surpassing TiO₂, CdS, and WO₃ in visible-light responsiveness and spin selectivity, though reduction is limited by positive ECB. Mechanical ductility (B/G > 1.88) supports durability. Future research on stability enhancements and experimental validation is essential to realize A₂RuBr₆’s potential in sustainable energy applications.