The role of disconnections in redox-induced phase transformation of metal oxides

Detailed atomic-level understanding of the phase transformation between magnetite ($\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$) and haematite ($\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$) is lacking, despite widespread interest in redox reactions of metal oxide systems. While the magnetite-to-haematite transformation entails the addition of oxygen to the system and thus a net diffusive flux, haematite formations have been reported to grow along well-defined habits on the close-packed $\{0001{\}}_{\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}\mathrm{/}\mathrm{/}\{111{\}}_{\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4}$ planes. The propagation of the phase interface in the bulk is thus thought to maintain the oxygen sublattice fixed up to a shear, and to require long-range diffusion of iron ions. In this letter we propose interfacial steps with dislocation character, i.e. disconnections, as the elementary defects propagating the transformation. We use the topological model of interfacial defects to predict three disconnection modes for the system, and we employ scanning transmission electron microscopy to ascertain their presence in a haematite/magnetite interface of a partially oxidised magnetite powder. Using atomistic simulations, we determine the equilibrium structures of the disconnections, and study their motion and how they accomplish the transformation, obtaining excellent agreement with atomic-resolution experimental observations.