Mechanochemical transformation of haematite to magnetite: Structural investigation

The reaction products resulting from extended low energy wet milling of haematite, alpha-Fe2O3 in vacuum have been investigated in detail by time of flight neutron diffraction. About 14% of the alpha-Fe2O3 R(3) over barc$ remains in a relatively undistorted state but the remaining fraction (similar to 86%) is found to have transformed to magnetite, Fe3-vO4 <Fd(3)over bar m>, of vacancy concentration v = 0.182. The optimal mal refinement indicates that the vacancies are distributed primarily (similar to 75%) over the more energetically favourable octahedral sites with the remaining similar to 25% in the tetrahedral sites. Good agreement is found with the Mossbauer measurements which show evidence of increased relaxation time for electron hopping (between the Fe2+ and Fe3+ which share the octahedral sites) in off stoichiometry defect Fe3-vO4 compared with Fe3O4. The reduction process is due mainly to breaking of oxygen bonds in oxide layers and accompanying release of oxygen.