Phase equilibria in the ZrO2-GdO1.5-TaO2.5 system at 1500 °C

Phase equilibria in the ZrO2-GdO1.5-TaO2.5 (ZGTO) system are experimentally investigated, and the differences between ZGTO and the existing ZrO2-YO1.5-TaO2.5 (ZYTO) are discussed. On Gd:Ta> 1 side, binary fluorite Gd3TaO7 is measured to be stable at 1500 degrees C, and ternary fluorite phase forms continuous solid solution from binary ZrO2-GdO1.5 to GdO1.5-TaO2.5. On Gd:Ta< 1 side, there is a large difference between ZGTO and ZYTO. Firstly, GdTa7O19 is unstable at 1500 degrees C, and GdTa3O9 shows large ZrO2 solubility and extends along Gd1/4Ta3/4O9/4-ZrO2 connecting line without introducing vacancies. Secondly, non-transformable tetragonal ZrO2 and GdTaO4 are in equilibrium with GdTa3O9 in ZGTO. Thirdly, the tetragonal ZrO2 in equilibrium with Ta2Zr6O17 and GdTa3O9 is transformable and has lower stabilizer content. For ZrO2-GdTaO4 section, tetragonal ZrO2 and GdTaO4 extend nearly along with Gd:Ta= 1 and exhibit smaller solubilities. This study is useful for thermodynamic assessment of ZGTO, and can help guide the design of novel thermal barrier coatings materials.