Chemistry and structure of mixed magnesium ferric oxide aerogels

Abstract

Fine particles of mixed oxides having the composition Mg_xFe_2−2xO_3−2x (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 α–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 α–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.