Monodispersed Sc2O3 precursor particles were synthesized by the urea-based homogeneous precipitation method, with an investigation into the effects of supporting anions (NO3−, Cl−, and SO4 2−) on powder properties. Characterizations of the powders were achieved by elemental analysis, x-ray diffractometry, Fourier transform infrared, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and the Brunauer-Emmett-Teller method. Unlike other rare earths, Sc3+ does not precipitate as basic carbonate but instead forms hydrated γ-ScOOH from either nitrate or chloride solution. Particles of the hydrated γ-ScOOH are pumpkin-shaped (approximately 1.0 μm) and are made up of thin-platelike crystallites emanating from a common axis. The presence of complexing SO42− changes the reaction chemistry toward Sc2O3 powders, leading to basic sulfate [Sc(OH)1.6(SO4)0.7 η H2O] precursor particles having hexagonal morphology (approximately 10 μm in diameter and 0.5 μm in thickness). The hydrated γ-ScOOH directly converts to Sc2O3 by calcination at 400 °C or above, while the basic sulfate transforms to oxide at temperatures ≥ 900 °C via an amorphous state and a Sc2(SO4)3 intermediate. The effect of SO42− on powder morphologies and reaction chemistry is discussed. Nanocrystalline Sc2O3 powders comprising monodispersed particles were obtained via thermal decomposition of the precursors.

A carbonate coprecipitation method was used for the facile synthesis of highly reactive 10 mol%-Y2O3-doped CeO2 (20YDC) nanopowders, employing nitrates as the starting salts and ammonium hydrogen carbonate (AHC) as the precipitant. The AHC/RE3+ (RE = Ce + Y) molar ratio (R) and the reaction temperature (T) significantly affect the final yield and precursor properties, including chemical composition and particle morphology. Suitable processing conditions are T = 60 °C and R = 2.5 to 10, under which precipitation is complete, and the resultant precursors show ultrafine particle size, spherical particle shape, and good dispersion. The thus-processed precursors are basic carbonates with an approximate formula of Ce0.8Y0.2(OH)CO3 · 2H2O, which directly yield oxide solid solutions upon thermal decomposition at a very low temperature of approximately 400 °C. The 20YDC solid-solution powders calcined at 700 °C show excellent reactivity and were densified to >99% of theoretical via pressureless sintering at a very low temperature of 950 °C for 6 h.

Ammonium hydrogen carbonate was used as the precipitant to synthesize yttrium aluminate garnet (YAG) precursors from a mixed solution of aluminum and yttrium nitrates via coprecipitation. The carbonate precursor, with an approximate composition of NH4AlY0.6(CO3)1.9(OH)2 · 0.8H2O, transformed to pure YAG at 900 °C without the formation of intermediate phases. Reactive YAG powder was produced by calcining the precursor at 1100 °C. The YAG powder densified to 99.8% of the theoretical density by vacuum sintering at 1500 °C for 2 h, and the sintered body showed transparency. Less agglomeration of the precursor and good dispersity of the resultant YAG powder were responsible for the excellent sinterability.