Reactive Molecular Dynamics Study of Cyclic Deformation and Lattice Instability in Cesium Lead Iodide

In this study, molecular dynamics simulations were performed to investigate the mechanical response and phase evolution of CsPbI 3 in its cubic (α), tetragonal (β), orthorhombic (γ), and nonperovskite (δ) phases under cyclic deformation. The simulations reveal a deformation‐induced phase transformation sequence from the cubic to the orthorhombic and finally to the nonperovskite structure. During early loading cycles, a reversible α ↔ γ transition is observed. With increasing cycle number, this transition becomes irreversible, leading to the formation of the δ phase and indicating mechanical fatigue of the perovskite lattice. Stress–strain analysis shows negligible hysteresis for single‐phase cubic and orthorhombic CsPbI 3 , suggesting limited resistance to cyclic deformation. In contrast, significant hysteresis develops when perovskite and nonperovskite phases coexist, reflecting enhanced energy dissipation associated with structural instability. This work provides atomistic insight into fatigue‐induced phase instability in CsPbI 3 and demonstrates the capability of reactive molecular dynamics to capture deformation‐driven transformations.