However, the growth of the yellow phase from hydrated amorphous CsPbI 3 is confirmed by MD simulations (supplementary material, Fig. ). Since both B↔Y phase transitions should follow the two-step C-A-C mechanism and water affects the transitions at the amorphous B/Y interface, the water-assisted mechanism in the Y→B phase transition process should be consistent with the low-temperature reverse B→Y transition. We constructed the B-Y nanowire systems with different numbers of water molecules (0, 500, 1000, and 1500) and performed MD simulations at 600 K for 1 µs (Fig. ). Our simulations show that the number of penetrating water molecules increases with the total number of water molecules (supplementary material, Fig. ). Fig. shows that Y→B phase transitions are taking place in the simulations, with a significant contraction of the yellow phase region and a notable expansion of the black phase region. Here, we define the transition rate v as