Formation and energetics of amorphous rare earth (RE) carbonates in the RE₂O₃–CO₂–H₂O system

Amorphous materials are of interest in the genesis of crystalline solids. They are often the first to precipitate from supersaturated solutions and are in the nanometer size range, transforming into a nanocrystalline metastable phase, and then to thermodynamically stable phases. One must seek knowledge of the composition, structure, thermodynamics, kinetics, and reaction pathways that relate the amorphous and crystalline phases. The subject of this study is the amorphous phases in the system RE₂O₃−CO₂–H₂O (RE- La, Nd, Dy,Yb). They were synthesized via direct precipitation and urea hydrolysis, and characterized by powder XRD, DTA with mass spectrometric analysis of the evolved gases. Phases have the non-stoichiometric composition RE₂O₃·xCO₂·yH₂O (1 < x < 3, 3 < y <7), which is different from that of crystalline simple carbonates and hydroxycarbonates. Thus we consider them to be amorphous precursors, rather than amorphous carbonates. High temperature oxide melt solution calorimetry in molten sodium molybdate solvent was used to derive their enthalpies of formation from oxides and elements. Increase of energetic stability per mole of RE₂O₃ compounds occurs in the order: RE₂O₃ → 2RE(OH)₃→ 2REO(OH) → RE₂O₂CO₃ → Amorphous precursor → 2(REOHCO₃) → RE₂(CO₃)₃·yH₂O. We calculated the enthalpies of possible transformations of amorphous precursors to crystalline phases and conclude that thermodynamics determines the dependence of the crystallizing products on the temperature and partial pressure of CO₂ and H₂O. Amorphous precursors are clearly intermediate in the synthesis, and ternary phases containing both H₂O and CO₂ compete with each other in terms of thermodynamic stability. The energy landscape obtained here will allow one to directly synthesize specific products and control their functionality.