Nanostructured (Co_xFe_1-x)_3-yO₄ spinel - Mechanochemical synthesis

The effects of wet-milling mixtures of α-Fe₂O₃ and cobalt hydroxide Co(OH)₂ over a range of Co/Fe ratios for 215 h have been investigated by neutron diffraction and Mössbauer spectroscopy. The starting materials were mixed according to the stoichiometric formula (Co_xFe_1-x)₃O₄ for values of x = 0.037, 0.071, 0.133, 0.234 and 0.380 (i.e., from ≈ Co_0.1Fe _2.9O₄ to the cobalt spinel CoFe₂O₄). These studies reveal the formation of a nanostructured, mixed Co-Fe spinel phase with non-stoichiometric composition (Co_xFe_1-x) _3-yO₄; the defect spinels have refined values x _c = 0.04, 0.08 and 0.14 for the mixtures with the lowest Co content (x = 0.037, 0.071 and 0.133) and defect concentrations in the range y ≈ 0.1-0.2. Both the spinel phase and un-reacted α-Fe₂O ₃ are found to occur in the neutron diffraction patterns and Mössbauer spectra for the high Co content mixtures x = 0.234 and x = 0.380. Rietveld refinements of the neutron data indicate that the Co atoms predominantly occupy the octahedral B sites with vacancies also found to be located on the octahedral B sites. Analyses of the Mössbauer spectra of the milled samples confirm the existence of vacancy defects in the B sites and reveal that the vacancies cause similar effects to those of the Co ions, leading to a higher average charge state per iron atom.