Atomic Structure of the Fe₃O₄/Fe₂O₃ Interface During Phase Transition from Hematite to Magnetite

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 H₂ reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with Fe₃O₄ nucleation on the surface defective sites, followed by laterally growing into a Fe₃O₄ film until fully covering the particle surface. The Fe₃O₄ phase then propagated toward the bulk particle via a Fe₃O₄/α-Fe₂O₃ interface with the relationship α-Fe₂O₃(0001)//Fe₃O₄(111) in an aligned orientation of [112]_Fe3O4||[112̅0]_α-Fe2O3. Upon this Fe₃O₄/α-Fe₂O₃ interface, the Fe-O octahedra in Fe₃O₄(111) (as layer A) matches that of α-Fe₂O₃(0001) at a rotation angle of 30°, and the reduction proceeds in such a pattern that two-thirds of the Fe_Oh in the adjacent layer (layer B) is transformed into Fe_Te.