Thermodynamic assessment of the ZrO₂-LaO_1.5 system

Two optimal sets of thermodynamic functions for the ZrO₂-LaO_1.5 system are obtained by coupling the CALPHAD (CALculation of PHAse Diagrams) technique, the regularity between thermodynamic properties and crystal structure parameters within the family of rare earth oxides, and the Van't Hoff equation. The phases are modelled with the substitutional solution model (liquid, cubic, tetragonal, monoclinic, and hexagonal solid solutions), as the stoichiometric compounds (Zr₁₉La₄₂O₁₀₁, Zr₂La₂O₇), and with the sublattice model (Zr₂La₂O₇). Two optimizations are performed. In the first optimization, the Zr₂La₂O₇ phase is treated as a stoichiometric compound, while in the second one it is described by the Wagner-Schottky model. The eutectoid reaction, high-temperature hexagonal LaO_1.5 solid solution low-temperature hexagonal LaO_1.5 solid solution + Zr₂La₂O₇, is predicted from the calculation. The calculated phase diagrams and thermodynamic quantities agree well with the experimental data.