We first discuss the mechanism of the B↔Y phase transition occurring in MHPs. Previous studies based on DFT calculations have proposed both one-step and multi-step transition pathways for the B→Y phase transition in CsPbI 3 . However, the solid-solid phase transition mechanism cannot rely solely on calculating the minimum energy barrier per atom for homogenous transitions; the nucleation process during the transition must also be considered. Additionally, these studies were based on the assumption that the transition states and metastable phases along the pathway are crystals (i.e., the crystal-crystal (C-C) transition mechanism). We argue that the B↔Y phase transition in CsPbI 3 or other inorganic perovskite materials follows a twostep C-A-C mechanism . During the B↔Y phase transition, the nucleation and propagation stages can be effectively distinguished , and both can be understood using classical nucleation theory (CNT) . If the transition occurs via the direct C-C transition pathway, the nucleation barrier would be significantly higher than the corresponding values for a feasible phase transition.