Resolving the early-stage nucleus structure and evolution in atomic systems

Nucleation underpins a vast range of phase-transition phenomena in many disciplines. Critical to revealing nucleation thermodynamics and kinetics is the understanding of the nucleus structure at its early stage. Typically, it is assumed that nucleation is a sudden local structural transition from one phase to another. Here, we are able to access fundamental steps in the nucleation from amorphous phase by a combination of molecular simulations and experimental observation. We discover a surprising pathway of semicrystalline nucleation where one of the materials components crystallizes and another remains amorphous between the crystalline planes in the nuclei. The early-stage crystallization nucleus is robustly evidenced to undergo a gradual ordering and densification, originating from the presence of diffuse interfaces, and renders an ultralow interfacial energy that is orders of magnitude lower than those typically used in various formulations of nucleation. Our study provides critical information and insight for the early stages of nucleation that determine how crystallization is initiated and benefits controllable synthesis of materials.