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 dryin g route (aerogels). Samples were then postprocessed by heating in air at 773 K and 1073 K. The structure and chemistry of the samples were d educed through x-ray diffraction, thermal gravimetric and differential thermal analysis, infrared, and specular UV-visible measurements. Sur face areas were measured by the BET method. The volume saturation magn etization and particle volume distribution of MgFe2O4 samples were ded uced from vibrating sample magnetometer measurements. The samples with x = 0 and x = 0.5 were well-crystallized alpha-Fe2O3 (hematite) and M gFe2O4 (magnesioferrite), respectively. Samples with 0 < x < 0.5 consi sted of disordered lacunar spinels containing an excess of Fe3+ with r espect 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 hydrotalci te-like mixed Fe-Mg hydroxy compound containing methoxy and acetate io ns. By heating in air, a monophasic Mg-excess spinel-type structure wa s 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 MgFe2O 4. The saturation magnetization of MgFe2O4 samples is comparable to th ose previously reported, with the as-produced sample having a mean par ticle radius of 5.5 nm.