GEOMETRICAL CONSIDERATION OF THE CRYSTALLOGRAPHY OF THE TRANSFORMATION FROM ALPHA-FE2O3 TO FE3O4

A mechanism is proposed for the transformation from alpha-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 alpha-Fe2O3 changes to the f.c.c.-type stacking sequence of Fe3O4. Since the octahedral sites in the alpha-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 alpha-Fe2O3 becomes weaker. By the reduction, the iron/oxygen ratio in the restacked alpha-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 alpha-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 alpha-Fe2O3, and Fe3O4 lattices satisfies the Shoji-Nishiyama relationship.