On the other hand, an amorphous structure as a transition/metastable state can effectively reduce the interfacial energy, thereby lowering the nucleation barrier. The calculated interfacial energy of the yellow/amorphous (Y/A) interface (Fig. , green box) is approximately 0.5 eV/nm 2 (supplementary material, computational methods, force field validation, Table and Fig. ), corresponding to a nucleation barrier of about 6 eV (green line in Fig. ). This value is close to the maximum barrier allowed for nucleation at the B↔Y phase transition point (4 eV≈80 k B T , T = 580 K). By introducing water molecules into the amorphous region (Fig. , blue box), σ is further reduced to 0.3 eV/nm 2 (supplementary material, computational methods, force field validation, Table and Fig. ), corresponding to a nucleation barrier of 1.4 eV (blue line in Fig. ).