We now have gained a comprehensive understanding of the Y→B and the reverse B→Y phase transitions in CsPbI 3 (Fig. ). The nucleation and propagation processes of the B↔Y phase transition both follow the C-A-C two-step mechanism . During the thermally induced Y→B phase transition, the yellow phase destabilizes at a high temperature, forming an amorphous nucleus, which then recrystallizes into the black α-CsPbI 3 (Fig. , upper panels). The propagation process of this transition (Fig. , lower panels) is similar to the nucleation process, whereby the yellow phase melts and transforms into the amorphous interface, which then recrystallizes in the black phase, leading to the growth of black-CsPbI 3 . Conversely, the reverse B↔Y transition follows similar mechanisms but necessitates the aid of water molecules to diminish the nucleation barrier and facilitate the diffusion of interfacial ions (Fig. ). The C-A-C transition mechanism has been well-established in metallurgy . It has also been observed in the nucleation process of protein crystallization , as well as in solid-solid phase transitions in colloidal crystals and PbTiO 3 . This mechanism has been theoretically verified from both kinetic and thermodynamic perspectives . However, prior to this work, there has been no specific validation or comparison of the applicability of the C-A-C and C-C mechanisms in MHP systems.