Experimental investigation and thermodynamic modelling in the ZrO2–La2O3–Y2O3 system

Phase equilibria in the La2Zr2O7–Y2O3 join were studied in the temperature interval between 1250 and 1600 °C using the X-ray diffraction and thermal analysis. It was demonstrated that the maximal solubility of Y2O3 in the La2Zr2O7 (pyrochlore structure) was approximately 20 mol% of Y2O3. In mixtures containing more than 20 mol% Y2O3 the La2Zr2O7 reacted with Y2O3 forming the fluorite structure at increasing temperature. The compound energy formalism was applied to describe solubility of Y2O3 in pyrochlore structure assuming that Y3+ can occupy both La3+ and Zr4+ enriched sites in the crystal structure. The Gibbs energies of compounds with pyrochlore structure were assessed. Ternary mixing parameters were included in thermodynamic description of the fluorite phase to reproduce in calculations its wide homogeneity range extending from ZrO2–Y2O3 binary in the direction of LaYO3. Calculated phase diagrams reproduce general character of experimental data.