Hydrothermal Synthesis of LaCO3OH and Ln3+-doped LaCO3OH Powders under Ambient Pressure and Their Transformation to La2O2CO3 and La2O3

Orthorhombic and hexagonal lanthanum(III) hydroxycarbonate (LaCO3OH) and Ln(3+)-doped LaCO3OH (LaCO3OH:Ln(3+), where Ln = Ce, Eu, Tb, and Ho) powders were prepared by a hydrothermal reaction under ambient pressure and characterized by thermogravimetry, powder X-ray diffraction, infrared and luminescence spectroscopy, and field-emission scanning electron microscopy. The polymorph of LaCO3OH depended on the reaction temperature, inorganic salt additive, species of Ln(3+). dopant, and solvent. The calcination of orthorhombic LaCO3OH:Ln(3+) (2 mol %) powers at 600 degrees C yielded a mixture of hexagonal and monoclinic La2O2CO3:Ln(3+) powders. The relative quantity of the latter increased with decreasing ionic radius of the Ln(3+) dopant ion and increasing doping concentrations. On the other hand, the calcination of hexagonal LaCO3OH:Ln(3+) (2 mol %) powders at 600 degrees C resulted in a pure hexagonal La2O2CO3:Ln(3+) powder, regardless of the species of Ln(3+) ions (Ln = Ce, Eu, and Tb). The luminescence spectra of LaCO3OH:Ln(3+) and La2O2CO3:Ln(3+) were measured to examine the effect of their polymorph on the spectra.