CHEMISTRY AND STRUCTURE OF MIXED MAGNESIUM FERRIC-OXIDE AEROGELS

Fine particles of mixed oxides having the composition MgxFe2-2xO3-2x (x between 0 and 1.0) were prepared via the sol-gel supercritical drying route (aerogels). Samples were then postprocessed by heating in air at 773 K and 1073 K. The structure and chemistry of the samples were deduced through x-ray diffraction, thermal gravimetric and differential thermal analysis, infrared, and specular UV-visible measurements. Surface areas were measured by the BET method. The volume saturation magnetization and particle volume distribution of MgFe2O4 samples were deduced from vibrating sample magnetometer measurements. The samples with x = 0 and x = 0.5 were well-crystallized alpha-Fe2O3 (hematite) and MgFe2O4 (magnesioferrite), respectively. Samples with 0 < x < 0.5 consisted of disordered lacunar spinels containing an excess of Fe3+ with respect to the spinel stoichiometry and a small amount of Fe2+. Samples heated above 773 K showed progressive segregation of alpha-Fe2O3. The sample with x = 0.66 was a mixture of a spinel phase and a hydrotalcite-like mixed Fe-Mg hydroxy compound containing methoxy and acetate ions. By heating in air, a monophasic Mg-excess spinel-type structure was obtained. The samples with 0.66 < x < 0.86 were mixtures of a spinel-type phase and another mixed Mg-Fe compound. When heated in air, the samples gave a mixture of MgO (probably containing Fe ions) and MgFe2O4. The saturation magnetization of MgFe2O4 samples is comparable to those previously reported, with the as-produced sample having a mean particle radius of 5.5 nm.