Phase transition between iron oxides practically defines their functionalities in both physical and chemical applications. Direct observation of the atomic rearrangement and a quantitative description of the dynamic behavior of the phase transition, however, are rare. Here, we monitored the structure evolution from a rod-shaped hematite nanoparticle to magnetite during H reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with FeO nucleation on the surface defective sites, followed by laterally growing into a FeO film until fully covering the particle surface. The FeO phase then propagated toward the bulk particle via a FeO/α-FeO interface with the relationship α-FeO(0001)//FeO(111) in an aligned orientation of [112]||[112̅0]. Upon this FeO/α-FeO interface, the Fe-O octahedra in FeO(111) (as layer A) matches that of α-FeO(0001) at a rotation angle of 30°, and the reduction proceeds in such a pattern that two-thirds of the Fe in the adjacent layer (layer B) is transformed into Fe.
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