Dynamic Observation of Anisotropic Chainlike Structures during Nonclassical Two-Step Nucleation in Solid-State Iron Oxide Crystallization.

The demonstration of the self-crystallization nucleation process from an amorphous precursor in a solid is crucial for understanding of interactions between atoms. We report a study of dynamic crystallization process of iron oxides by virtue of in situ measurement of transmission electron microscopy. At first, semiordered chainlike structures are observed with the increase of concentration, and when sufficient chains form, the crystalline lattice begins to grow. The two-step nucleation pathway has also been confirmed by performing a molecular dynamics simulation, where Lennard-Jones and magnetic dipole-dipole interaction potentials are both taken into account and take effect individually predominantly in different ranges of distance between atoms. Furthermore, the total free energy profile in the crystallization nucleation process is calculated to evidence the stabilization of intermediate state. This work advances our understanding of nonclassical nucleation theory.