Critical evaluation of thermodynamic properties of rare earth sesquioxides (RE = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y)

In order to develop the thermodynamic database containing rare earth sesquioxides (RE2O3), accurate thermodynamic descriptions of all stable and metastable RE2O3 phases are required. Rare earth sesquioxides have five polymorphs, which are (from low to high temperature) the cubic phase (C), the monoclinic phase (B), the hexagonal phase (A), the high temperature hexagonal phase (H), and the high temperature cubic phase (X). However, the thermodynamic property data of all polymorphs available in the literature are insufficient. In particular, the thermodynamic data for the phases stable only at high temperatures and for metastable phases are not well investigated. In this study, all ∆H298Ko, S298Ko, CP (or heat content) and phase transition temperature (Ttr) data available in the literature for each rare earth sesquioxide were collected and critically evaluated based on the sample preparations, experimental procedures and characterization techniques. Relationships between ∆H298Ko, S298Ko, and Ttr against the ionic radii of the entire rare earth cations were then established and missing thermodynamic information was predicted based on the general trends. In these ways, the accurate and consistent Gibbs energies of all stable and metastable RE2O3 phases (RE = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc) were prepared. These Gibbs energy data can be readily used for the development of a comprehensive thermodynamic database containing rare earth oxides.