Composition and size effect on electronics and optical properties of lead-free halide double perovskite Cs2Ag(InxBi1-x)Cl6 by first-principles calculation

The electronic structure and optical properties of the lead-free halide double perovskite Cs₂Ag(InₓBi₁₋ₓ)Cl₆ were investigated by using first-principles calculations with inclusion the effect of spin-orbit coupling (SOC). Varying indium composition ratio (x = 0.0–1.0) revealed tunable structural, electronic, and optical properties with potential applications in optoelectronic devices. Key findings include a linear decrease in lattice constants adhering to Vegard's law, a composition-driven transition from an indirect band gap to a direct band gap and a non-monotonic variation in band gap energy across the alloy range, as determined by HSE06 functional calculations. SOC effects exhibit pronounced composition dependence by reducing band gaps for Bi-rich compositions (x ≤ 0.25). This due to relativistic effects on heavy Bi atoms. However, this negligible for In-rich systems (x ≥ 0.75). Thermodynamic stability analysis via formation energy calculations revealed favorable negative values for all compositions, confirming energetic feasibility. Quantitative partial density of states analysis demonstrated that the indirect-to-direct band gap transition originates from systematic evolution of conduction band character from Bi 6p-dominated states at the X-point to In 5 s-dominated states at the Γ-point, with SOC-induced orbital mixing playing a crucial role in Bi-containing compositions. Optical calculations confirmed the tunability of the absorption edge, with HSE06+SOC results reveal agreement with experimental data. This study highlights the critical importance of relativistic effects in accurate electronic structure predictions for heavy-element double perovskites and demonstrates the potential of compositional engineering in Cs₂Ag(InₓBi₁₋ₓ)Cl₆ for designing lead-free materials with tailored optoelectronic characteristics.