Atomic Structure of the Fe 3 O 4 /Fe 2 O 3 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 H 2 reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with Fe 3 O 4 nucleation on the surface defective sites, followed by laterally growing into a Fe 3 O 4 film until fully covering the particle surface. The Fe 3 O 4 phase then propagated toward the bulk particle via a Fe 3 O 4 /α-Fe 2 O 3 interface with the relationship α-Fe 2 O 3 (0001)//Fe 3 O 4 (111) in an aligned orientation of [112] Fe 3 O 4 ||[112̅0] α-Fe 2 O 3 . Upon this Fe 3 O 4 /α-Fe 2 O 3 interface, the Fe-O octahedra in Fe 3 O 4 (111) (as layer A) matches that of α-Fe 2 O 3 (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 .