Anisotropic mechanical behavior of cesium tin iodide perovskite subjected to uniaxial tension

Lead-based metal halide perovskites (MHPs) have wide-ranging applications as solar cells, field-effect transistors, diodes, and photodetectors. However, their poor stability and concerns about toxicity have enabled lead-free tin-based MHPs to emerge as a promising alternative. We utilize molecular dynamics (MD) simulations to investigate the anisotropic mechanical behavior of single-crystal cubic $\mathrm{CsSn}{\mathrm{I}}_{3}$, a promising lead-free MHP, under uniaxial tension. Among the three investigated crystal orientations, [111] is found to be the strongest and to exhibit the highest ultimate strain while [100] is the weakest. While shear strain localization and amorphization precede fracture along [100], fracture directly follows strain localization along [110] and [111]. We also investigated the influence of a crystal defect, in the form of an embedded rectangular crack, on the anisotropic mechanical behavior of cubic $\mathrm{CsSn}{\mathrm{I}}_{3}$. The presence of crystal defects is found to substantially reduce the anisotropy in mechanical properties, with very similar crack growth behavior and almost identical stress-strain response noted along starkly different crystal orientations of loading. Finally, the ultimate strengths and ultimate strains of cubic $\mathrm{CsSn}{\mathrm{I}}_{3}$ determined here are comparable to or higher than those of cubic $\mathrm{CsPb}{\mathrm{I}}_{3}$ and $\mathrm{MAPb}{\mathrm{I}}_{3}$ determined in prior MD-based investigations. Thus, our study supports the applicability of cubic $\mathrm{CsSn}{\mathrm{I}}_{3}$ as a lead-free alternative to commonly used cubic MHPs, while the sensitivity to crystal defects revealed here underlines the importance of defect control for obtaining robust devices with reliable properties.