Abstract
<h2>Summary</h2> All-inorganic photoactive CsPbI<sub>3</sub> perovskites easily transform into a photo-inactive non-perovskite phase, but the transition kinetics at the atomic level are currently unknown. In this study, we used first-principle-based stochastic surface walking (SSW) pathway sampling to resolve the phase evolution of the CsPbI<sub>3</sub> transition from γ to δ. The lowest-energy pathway of γ (3D) → <i>Pm</i> (3D) → <i>Cmcm</i> (2D) → <i>Pmcn</i> (1D) → δ (1D) was found to have a transition barrier as low as ∼31 meV/atom. The γ-to-<i>Pm</i> transition was identified as the performance-controlling step. Furthermore, volcano-shaped transition barriers were obtained depending on the ionic size of the substitution dopants, as explained by Pauling's rule and the cohesive energy. The [010] plane has the largest strain variation during phase transition, implying that the strain involving the [010] axis has a more significant effect on increasing the transition barriers. These results provide rational suggestions and guidance for achieving stable, long-term, all-inorganic halide CsPbI<sub>3</sub> perovskites.
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