Energetic insights into the crystallization of lanthanum carbonate amorphous precursors

Abstract

Lanthanide carbonates form during the use of rare earth (RE) oxides in moist air. They are interesting as functional materials for catalysis and luminescence. In addition, they are related to minerals. Knowing their crystallization conditions is relevant both for minimizing their degradation in use and for the synthesis of RE carbonates of tailored composition, crystallinity and particle size. In this paper, we explore the thermodynamic control of the synthesis of amorphous and crystalline lanthanum carbonates. Compounds were prepared via direct precipitation and urea hydrolysis routes. Their characterization by XRD, DSC, and TG-MS showed amorphous samples to have a lower CO2 content than the hypothetical stoichiometric crystalline La2(CO3)3·8H2O, therefore we call them amorphous precursors rather than amorphous carbonates. High temperature oxide-melt solution calorimetry in molten sodium molybdate solvent at 700 °C was used to obtain their formation enthalpies. Enthalpies of formation from oxides become more negative in the sequence La2O2CO3 - amorphous precursor - 2hex-(LaOHCO3) - 2ortho-(LaOHCO3) - La2(CO3)3·8H2O. We describe an energy landscape defining the phase transformation in the ternary system La2O3 - H2O - CO2 and conclude all reactions to be sensitive to the partial pressure of water and carbon dioxide. Higher CO2 pressure will favor La2(CO3)3·8H2O over LaOHCO3 in an aqueous environment where the activity of water is essentially constant.