Oxidation of Magnetite(100) to Hematite Observed by in Situ Spectroscopy and Microscopy

We study where and how hematite (α-Fe2O3) nucleates and grows during the oxidation of magnetite(100) single crystals. Hematite inclusions grow along ⟨110⟩ directions of the magnetite (Fe3O4), leading to a biaxial array of hematite slabs in an electrically conducting matrix of magnetite. The slab arrays form in both bulk single crystals and thin films of magnetite. Atomic force microscopy reveals that the surface growth of magnetite that accompanies hematite formation is faster adjacent to the hematite slabs. In situ X-ray photoelectron and X-ray absorption spectroscopies at 600 °C in an oxygen environment reveal that the conversion of the Fe2+ in magnetite to Fe3+ in hematite occurs without the formation of the metastable phase maghemite (γ-Fe2O3). We offer an explanation of why Fe3O4(100) oxidizes faster than Fe3O4(111).