Thermodynamics of rare earth sesquioxides

Experimental thermochemical and related data for rare earth sesquioxides, R₂O₃ (R = Sc, Y, La-Lu) are critically reviewed to provide a consistent set of thermodynamic functions covering the temperature range from at least 298 K up to above the melting point taking the polymorphism into account. Discrepancies in the available data are discussed and further studies to resolve them and to fill the gaps in the current knowledge are suggested. The best values for the temperatures of the reversible polymorphic transformations are recommended. Using high-pressure and molar volume data as well as ionic radius of a trivalent rare earth cation as an independent parameter the relative stabilities of different structures (the cubic C-form, the monoclinic B-form, the hexagonal A-form, the hexagonal H-form, and the cubic X-form) are estimated quantitatively for all rare earth sesquioxides. Based on the empirical relationship between the entropy of fusion and the corresponding fractional volume change, the entropies and enthalpies of fusion of the lanthanide sesquioxides are estimated. The reliability of the assessed functions and transition data is tested through the sample calculations of phase diagrams, where R′ and R″ represent different rare earth cations.