Molecular dynamics study of mechanical deformation in cesium lead halide perovskites

Mechanical response of single-crystal ${\mathrm{CsPbI}}_{3}, {\mathrm{CsPbBr}}_{3}, {\mathrm{CsPbIBr}}_{2}$, and ${\mathrm{CsPbI}}_{2}\mathrm{Br}$ is investigated under uniaxial tension and compression using molecular dynamics (MD) simulations. Stress-strain curves for these metal-halide perovskites are determined and mechanisms that strengthen and weaken them under plastic deformation are revealed. Our study finds the mechanical response of these crystals to differ considerably under uniaxial tension and compression. While ${\mathrm{CsPbI}}_{3}$ and ${\mathrm{CsPbBr}}_{3}$ exhibit a smooth elastic to plastic transition under uniaxial tension, distinct elastic and plastic regimes are observed under uniaxial compression. Only ${\mathrm{CsPbI}}_{3}$ is noted to exhibit strain hardening under uniaxial compression, which is deciphered to arise from generation of defects in the plastic regime. These defects, which take the form of deformation-induced distortion of the Pb-I octahedra, are shown to manifest as superlattice reflections in optical diffraction patterns derived from fast Fourier transform analysis of MD simulations. While a perfectly plastic postyield response is observed in ${\mathrm{CsPbIBr}}_{2}$ and ${\mathrm{CsPbI}}_{2}\mathrm{Br}$ under tension, a monotonic reduction in stress occurs after yielding under compression. Formation and growth of bands of high von Mises shear strain is revealed as an important plastic deformation mechanism in ${\mathrm{CsPbIBr}}_{2}$ and ${\mathrm{CsPbI}}_{2}\mathrm{Br}$ under uniaxial tension and compression. The evolution of defect microstructure with strain discussed here is expected to provide insights into plausible mechanisms that operate in metal-halide perovskites under experiments such as nanoindentation, while the defect-induced strengthening demonstrated here points to the possibility of fabricating robust perovskite films by engineering defects into the material.