Nanostructured (CoxFe1-x)(3-y)O-4 spinel - mechanochemical synthesis

The effects of wet-milling mixtures of alpha-Fe2O3 and cobalt hydroxide Co(OH)(2) over a range of Co/Fe ratios for 215 h have been investigated by neutron diffraction and Mossbauer spectroscopy. The starting materials were mixed according to the stoichiometric formula (CoxFe1 -x)(3)O-4 for values of x = 0.037, 0.071, 0.133, 0.234 and 0.380 (i. e., from approximate to Co0.1Fe2.9O4 to the cobalt spinel CoFe2O4). These studies reveal the formation of a nanostructured, mixed Co-Fe spinel phase with non-stoichiometric composition (CoxFe1- x)(3 -y)O-4; 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 approximate to 0.1 -0.2. Both the spinel phase and un-reacted alpha-Fe2O3 are found to occur in the neutron diffraction patterns and Mossbauer 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 Mossbauer 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.