Strain-Tunable Electronic and Optical Properties of KSnI₃ Perovskite Polymorphs: From Structural Stability to...

The interplay between crystal symmetry and strain provides a powerful, yet underexplored, route for tailoring the optoelectronic properties of lead-free KSnI₃ perovskites. In the present work, a comprehensive structure-property-strain framework is established through systematic first-principles analysis, revealing how symmetry and mechanical deformation govern the electronic structure and optical response across multiple polymorphs. Only the orthorhombic (Pnma-1 and Pnma-2) and monoclinic (P2 ₁/m) phases are found to be dynamically, thermally, and mechanically stable, whereas the tetragonal (P4/mmm and P4/mbm) phases exhibit lattice instabilities. Electronically, Pnma-1 and P2 ₁/m exhibit indirect semiconducting behavior, while Pnma-2, P4/mmm, and P4/mbm possess direct band gaps, enabling a symmetry-driven optoelectronic functionality. All stable phases exhibit strong polarization-dependent visible-UV absorption with Pnma-2 extending into the infrared, yielding a broadband optical response. Notably, the P2 ₁/m phase is distinguished by a y-polarized absorption coefficient of 1.06 × 10⁸ cm^-1 at 1.88 eV and a reflectivity of up to 82%, highlighting its potential for high-performance optical coating applications. Going beyond static properties, biaxial and triaxial strain emerge as efficient routes for tuning the band gap and optical response of KSnI₃ phases over a wide energy range. Overall, our findings demonstrate KSnI₃ as a highly tunable lead-free perovskite platform where symmetry and strain serve as key design parameters for optoelectronic applications.