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

Monophasic powders constituting alpha-(Fe,Al)(2)O-3 solid solutions ha ve been prepared by a coprecipitation method and have been characteriz ed, from the point of view of their solid-state and surface structures , by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/F T-FIR) and diffuse reflectance ultraviolet-visible (DR-UV-VIS) spectro scopy, BET surface area and porosity and adsorption of probe molecules , Their bulk and surface properties have been compared with those obse rved for pure alpha-Al2O3 and pure alpha-Fe2O3 as well as with biphasi c 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-Al2O 3 is near 11%, while the maximum solubility of Al2O3 in alpha-Fe2O3 do es not exceed 7%, for samples calcined at 1173 K. Metastable solid-sol ution 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 transitio n and causes a reduction in the surface area. In spite of their struct ures, which forecast only octahedral cation coordination, tetrahedral Al3+ and Fe3+ cations are found, mainly at the surfaces of the solid s olutions.