Phase equilibria in the ZrO2-YO1.5-TaO2.5 system at 1500 °C

The ZrO2-YO1.5-TaO2.5 (ZYTO) system is of interest in phosphors, fuel cells, photocatalysis, and thermal barrier coatings. However, there is inadequate information on the ternary phase equilibria, especially in the ZrO2-lean compositions. Precursor derived ZYTO compacts were heat treated at 1500 °C to elucidate the relevant phase relations. Chemical compositions measured using TEM/EDS and crystallographic information via x-ray diffraction, Raman spectroscopy and selected area electron diffraction were used to construct the isothermal section at 1500 °C, which was found to vary significantly from previous reports. It is shown for the first time that the fluorite field extends from the YO1.5-ZrO2 binary to the YO1.5-TaO2.5 binary, revealing a large YTaO4 + fluorite phase field with potential engineering implications. The mutual solubility of ZrO2 and YTaO4 was quantified, and the presence of both YTa3O9 and YTa7O19 at 1500 °C was demonstrated. The emerging understanding helps identify compositions with potentially attractive properties for further investigation.