Effects of urea concentration and reaction temperature on morphology of gadolinium compounds prepared by homogeneous precipitation

Europium-activated Gd2O3 phosphor powders were prepared by homogeneous precipitation method, and the effects of urea concentration and reaction temperature on the morphology of precursor powders were investigated. The synthesized powders were characterized by means of XRD, SEM, TG/DTA and FT-IR. The synthesis of gadolinium carbonate compounds was carried out using this method from relatively high concentrations of gadolinium salt and urea for the first time. Two kinds of powders formed were the crystalline Gd2(CO3)3·2.5H2O and Gd2O(CO3)2·H2O phases, which were mainly affected by reaction temperature. Namely, the Gd2(CO3)3·2.5H2O phase was formed at 80–90 °C, and the Gd2O(CO3)2·H2O at 100 °C. The morphology of Gd2(CO3)3·2.5H2O phase was plate-like. And the powders of Gd2O(CO3)2·H2O phase showed the rectangular plate-like morphology. In order to synthesize the nanometer-sized precursor powders using the homogeneous precipitation technique, it was found that EDTA/[Gd + Eu] mole ratio should be larger than 7.5 × 10−3 M, corresponding to 15 mol% of total cation concentration. The nanometer-sized precursor showed the amorphous-like pattern. The thermal decomposition behaviors of the three kinds of gadolinium carbonate compounds were also investigated. The thermal stability had the following order: Gd2O(CO3)2·H2O > Gd2(CO3)3·2.5H2O > amorphous phase.