Structural, Elastic, Electronic, and Optical Properties of Mixed‐Halide CsSnCl _x I ₃₋

A comprehensive density functional theory (DFT) study of structural, electronic, elastic, and optical properties of the mixed halide inorganic perovskite CsSnCl _x I _{(3− x )} is presented in this work. Controlled tuning of the atomic ratio paves a way for bandgap engineering, enhancement of mechanical stability, and optimization of optical characteristics towards lead‐free optoelectronic applications. Structural analysis shows that the CsSnI ₃ structure has maximum lattice constants and highest total free energy, which means suitable rigidity and structural rigidity. Additionally, expanded lattices and a partial loss of stiffness were observed. Electronic structure calculations confirm the direct bandgaps, which vary from 0.41 eV for CsSnI ₃ to 1.28 eV for CsSnCl ₂ I, rendering these compounds of interest for optoelectronic and photovoltaic applications. Projected density of states (PDOS) and charge density maps also confirm that there are strong bonds between Sn─Cl atoms in Cl‐rich structures as well as larger Sn─I interactions in I‐rich structures. Optical characterizations show that CsSnI ₃ has strong absorption and a high refractive index in the visible region, while CsSnCl ₃ structure exhibits strong UV absorption. The mixed halide perovskite compounds were investigated and found to have highly tunable dielectric constants, susceptibility, conductivity, and polarizability, making them ideal contenders for use in multijunction photovoltaic and photonic devices. Among the investigated compositions, CsSnCl ₂ I exhibits a calculated bandgap of 1.28 eV, placing it favorably within the optimal single‐junction Shockley–Queisser window (1.1–1.4 eV). While higher‐level hybrid functional and spin–orbit coupling treatments are recommended for quantitative device modeling, these results position CsSnCl ₂ I as a promising lead‐free candidate for optoelectronic and photovoltaic absorber layers.