Experimental Investigation of Phase Relations in the ZrO2-La2O3-Yb2O3 System

The phase equilibria in the La 2 O 3 -Yb 2 O 3 system were investigated using differential thermal analysis, and the phase transformations temperatures were verified. Microstructure investigation using scanning electron microscopy confirmed the presence of eutectoid reactions in the system. The heat capacity of LaYbO 3 with a perovskite structure was measured in the temperature range from 250 to 1250 K using differential scanning calorimetry. The experimental results of the present work as well as reliable literature data were used to derive a thermodynamic description of the La 2 O 3 -Yb 2 O 3 system. Isothermal sections of the ZrO 2 -La 2 O 3 -Yb 2 O 3 system were constructed based on x-ray and microstructural investigations. The tie-lines change from Pyrochlore + Yb 2 O 3 _ C to LaYbO 3 _ Perovskite + Fluorite was explained by a solid-state reaction occurring in the range between 1673 and 1873 K. The heat effect observed at 1858 K using DTA confirms the phase transformation, and its temperature was established more precisely at 1808 ± 15 K using stepwise heat treatment.