The mechanism of the water-assisted Y→B phase transition in CsPbI 3 strongly relates to the diffusional nature of this transition. In these typical non-martensitic phase transitions, the ability of ions to diffuse at the interface directly determines the rate of phase propagation. Fig. shows the diffusion coefficients (D) of ions at the B/Y interface at 710 K as a function of ρ (supplementary material, computational methods and Fig. ). The diffusion coefficient for each ion and the total diffusion coefficient both increase with ρ. In the absence of water, the diffusion coefficient for the anions (D I = 1.94 × 10 -10 m 2 /s) is larger than those for the cations (D Cs = 0.08 × 10 -10 m 2 /s; D Pb = 0.24 × 10 -10 m 2 /s.) This result agrees with DFT calculations reporting that I ions diffuse more readily than cations . To calculate the energy barriers of ion diffusion ∆E diff , we again applied the Arrhenius equation: