Phase equilibria in the ZrO2–YO1.5–NbO2.5 system at 1300°C and high-temperature experiments in the ZrO2–YNbO4 subsystem

Phase equilibria in the ZrO₂–YO_1.5–NbO_2.5 system were experimentally studied at 1300°C, and a comparison with the existing ZrO₂–YO_1.5–TaO_2.5 (ZYTO) was conducted. Ternary fluorite phases on the Y:Nb > 1 side extend from binary ZrO₂-YO_1.5 to YO_1.5–NbO_2.5 system, generating a continuous solid solution region. Furthermore, it was observed that three phases of M-YNbO₄, fluorite, and m-ZrO₂ were in equilibrium. On the Y:Nb < 1 side, two three-phase equilibrium regions were measured, namely M-YNbO₄+m-ZrO₂+O-Nb₂Zr₆O₁₇ and h-Nb₂O₅+O-Nb₂Zr₆O₁₇+M-YNbO₄. The tetragonal YNbO₄ and ZrO₂ phases both extend nearly along with Y:Nb = 1. The t-ZrO₂ phases with slightly lower stabilizer content were found to transform into their monoclinic polymorph during the cooling process. However, in the ZrO₂–YNbO₄ subsystem, the tetragonal ZrO₂ was nontransformable due to larger YO_1.5 and NbO_2.5 co-dopants, and the maximum solubility is about 18.1 mol%. Upon cooling, the tetragonal YNbO₄ phase, even with the maximum ZrO₂ content, is transformable to monoclinic form. The maximum solubility of ZrO₂ in YNbO₄ is determined to be about 20.7 mol%. Finally, the high-temperature liquidus in the ZrO₂–YNbO₄ system was first determined based on the cooling traces, and a vertical section diagram was proposed for further speculation. A eutectic reaction of Liquid→YNbO₄+ZrO₂ was discovered, and the eutectic composition and temperature were measured as 39.2ZrO₂–29.2YO_1.5–31.6NbO_2.5 and 1813°C. This study provides the basis for thermodynamic assessment of ZrO₂–YO_1.5–NbO_2.5 and is valuable for the design and development of the ZrO₂–YO_1.5–NbO_2.5-based ceramic materials.