Geometrical consideration of the crystallography of the transformation from α-Fe2O3 to Fe3O4

A mechanism is proposed for the transformation from α-Fe2O3 to Fe3O4 using the rigid-sphere model. By the shear slipping in every other {0001}hem plane, the h.c.p.-type stacking sequence of oxygen layers in α-Fe2O3 changes to the f.c.c.-type stacking sequence of Fe3O4. Since the octahedral sites in the α-Fe2O3 lattice change to tetrahedral sites after the restacking, the lattice is expanded by the restacking. Then the Coulomb force between the iron ion and the oxygen ion in restacked α-Fe2O3 becomes weaker. By the reduction, the iron/oxygen ratio in the restacked α-Fe2O3 increases, and during the reduction, Fe3+ ions in the “kagome” lattice turn into Fe2+ ions to maintain electroneutrality. Then the excess iron ions migrate in the restacked α-Fe2O3 in the state of Fe3+, resulting in an increase in the iron/oxygen ratio of the oxide. Then iron ions are rearranged and the Fe3O4 lattice is formed. This proposed mechanism can explain the experimental observation that the orientation relationship between α-Fe2O3 and Fe3O4 lattices satisfies the Shoji-Nishiyama relationship.