CHARACTERIZATION OF ALPHA-(FE,AL)(2)O-3 SOLID-SOLUTION POWDERS

Monophasic powders constituting alpha-(Fe,Al)(2)O-3 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 alpha-Al2O3 and pure alpha-Fe2O3 as well as with biphasic materials constituting mixtures of two alpha-(Fe,Al)(2)O-3 saturated solid solutions, with overall compositions external to the solubility limits of these phases. The maximum solubility of Fe2O3 in alpha-Al2O3 is near 11%, while the maximum solubility of Al2O3 in alpha-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 alpha-Fe2O3 tends to hinder sintering and stabilize the pore structure, while Fe3+ added to alpha-Al2O3 accelerates the gamma --> alpha 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+ cations are found, mainly at the surfaces of the solid solutions.