First-principles study of Cs₂AgXBr₆ (X = In, Bi): A hierarchy of functional performance for renewable energy applications

In recent years, lead-free double halide perovskites have attracted significant attention due to their potential in renewable energy technologies. To explore their suitability for eco-friendly applications, we have investigated the structural, elastic, electronic, optical, and photocatalytic properties of Cs₂AgXBr₆ (X = In, Bi) double perovskites. These characteristics were calculated using the WIEN2k code with the full-potential linearized augmented plane-wave (FP-LAPW) method based on density functional theory (DFT). Using the strongly constrained and appropriately normed (SCAN) meta-GGA functional, we calculated lattice constants of 11.0501 Å and 11.2998 Å and band gaps of 0.208 eV and 1.277 eV for Cs₂AgInBr₆ and Cs₂AgBiBr₆, respectively. By joining the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential with the SCAN functional, we obtained improved band gap values of 1.796 eV (X = In) and 2.304 eV (X = Bi), showing better agreement with experimental data. The structural stability of both compounds is evaluated using modified Goldschmidt and octahedral tolerance factors. The elastic constants, obtained using the SCAN functional, suggest that the materials are mechanically stable and ductile. Both functionals are employed to study optical properties by calculating relevant optical parameters such as absorption coefficient, αω, energy loss function, Lω, extinction coefficient,kω, and optical conductivity, σω. Subsequently, the potential of these materials for photocatalytic hydrogen production via overall water splitting is systematically examined. Our study demonstrates that the SCAN and TB-mBJ functionals offer reliable results while significantly reducing computational cost compared to resource-intensive hybrid DFT methods.