First-principles study of lead-free A3InX6 (A = Rb, Cs; X = Cl, Br, I) defect -perovskites: structural, electronic, optical, and mechanical properties.

The development of environmentally benign and stable alternatives to lead-based perovskites remains a key challenge for next-generation photovoltaic materials. In this work, a comprehensive first-principles investigation of lead-free alkali indium halide defect-perovskites A3InX6 (A = Rb, Cs; X = Cl, Br, I) is performed to elucidate the influence of cation and anion substitution on their structural, electronic, optical, and mechanical properties. Density functional theory calculations using GGA-PBE and PBEsol functionals confirm that all compositions crystallize in a stable cubic phase with negative formation energies and favorable Goldschmidt tolerance factors, where substitution of the larger Cs+ cation enhances lattice stability compared to Rb+. Systematic halide substitution from Cl- to Br- to I- induces lattice expansion and a pronounced reduction in the direct band gap at the Γ-point, enabling effective band gap tunability. Electronic structure analysis reveals that the valence-band maximum is dominated by halogen p states, while the conduction band is primarily governed by In-derived states, underscoring the decisive role of anion chemistry in controlling electronic transitions. Optical calculations demonstrate enhanced dielectric response and significantly improved visible-light absorption for iodide-rich compositions, whereas chloride-based compounds retain wide band gaps and stronger structural rigidity. Mechanical and elastic analyses indicate that both cation and anion substitution modulate lattice stiffness, ductility, and hardness, with Cs- and I-based compounds exhibiting increased mechanical softness and improved machinability.