A closer inspection of the ion and water motion at the B/Y interfaces reveals that the water molecules are able to bind with Cs and Pb ions, thus releasing I ions that are originally bound to the cations (Fig. and supplementary material, Movie S1). In the pure black and yellow phases, Cs is coordinated with 12 and 9 I, respectively, and Pb is coordinated with 6 I. We sampled the coordination numbers of ions at the B/Y interface (supplementary material, computational methods). In the zero-water system, the average coordination number of Pb (i.e., the number of I surrounding Pb) at the interface is 6.1, and that of Cs is 10.2. As shown in Fig. , the coordination numbers of both Cs and Pb slightly decrease as the water content increases. We calculated the probabilities of different binding situations (I n-x W x , where n is the total number of I and water molecules bound to a cation, x is the number of water molecules bound to the cation) for cations at the interface. Figs. 4c-d compare the probability distribution of cation binding situation in the zero-and 150-water systems. Both systems show similar probability distributions of the total coordination numbers (n) of Pb (Fig. ) and Cs (Fig. ). However, the numbers of I ions surrounding cations in the 150-water system (pink bars in Fig. ) are smaller than those in the zero-water system (gray bars in Figs. ), and the differences are compensated by 1and 2-water bindings (blue and green bars in Figs. ). The statistical analysis of the binding situations is consistent with the observation that water molecules occupy the positions of I and bind to the cations. The release of I in the interface region facilitates the motion of the ions, thus enhancing the ion diffusion. This mechanism also suggests that other small polar molecules (e.g., DMSO) or anions (e.g., OH -) may also accelerate the diffusional phase transitions in perovskite materials .