Mechanochemical transformation of α-Fe₂O₃ to Fe_3-xO₄-microstructural investigation

The effects of wet-milling α-Fe₂O₃ in vacuum for up to 144 h have been investigated by neutron diffraction measurements at room temperature and in situ at ∼950 K. Rietveld refinements show that the main product is iron-deficient magnetite of approximate stoichiometry ∼Fe_2.8O₄. Comparison of the phases derived from the neutron data with results of the Fe²⁺/Fe³⁺ oxidation states as determined by chemical analysis reveals that a significant fraction of the unreacted α-Fe₂O₃ occurs in an amorphous-like or disordered state. The wet-milled products are also found to contain ∼8% γ-Fe₂O₃. The transformation from the α-phase to the γ-phase occurs as a result of the shearing during the low-energy milling, with the further collisions and impacts leading to defect magnetite on extended milling. While other contributing effects take place, the transformation process from α-Fe₂O₃ to Fe_3-xO₄ occurs mainly as a result of rupturing the oxide surface layers of α-Fe₂O₃ and releasing oxygen with consequent reduction to Fe_3-xO₄.