Characterization of α-(Fe,Al)2O3 solid-solution powders

Abstract

Monophasic powders constituting α-(Fe,Al)₂O₃ solid solutions have been prepared by a coprecipitation method and have been characterized, from the point of view of their solid-state and surface structures, by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR) and diffuse reflectance ultraviolet–visible (DR-UV–VIS) spectroscopy. BET surface area and porosity and adsorption of probe molecules. Their bulk and surface properties have been compared with those observed for pure α-Al₂O₃ and pure α-Fe₂O₃ as well as with biphasic materials constituting mixtures of two α-(Fe,Al)₂O₃ saturated solid solutions, with overall compositions external to the solubility limits of these phases. The maximum solubility of Fe₂O₃ in α-Al₂O₃ is near 11%, while the maximum solubility of Al₂O₃ in α-Fe₂O₃ does not exceed 7%, for samples calcined at 1173 K. Metastable solid-solution phases with higher solute concentrations can be obtained if the calcination temperature is limited to 673 K. Al³⁺ added to α-Fe₂O₃ tends to hinder sintering and stabilize the pore structure, while Fe³⁺ added to α-Al₂O₃ accelerates the γ→α phase transition and causes a reduction in the surface area. In spite of their structures, which forecast only octahedral cation coordination, tetrahedral Al³⁺ and Fe³⁺ rations are found, mainly at the surfaces of the solid solutions.