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 ZrO 2 –YO 1.5 –NbO 2.5 system were experimentally studied at 1300°C, and a comparison with the existing ZrO 2 –YO 1.5 –TaO 2.5 (ZYTO) was conducted. Ternary fluorite phases on the Y:Nb > 1 side extend from binary ZrO 2 ‐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‐YNbO 4 , fluorite, and m‐ZrO 2 were in equilibrium. On the Y:Nb < 1 side, two three‐phase equilibrium regions were measured, namely M‐YNbO 4 +m‐ZrO 2 +O‐Nb 2 Zr 6 O 17 and h‐Nb 2 O 5 +O‐Nb 2 Zr 6 O 17 +M‐YNbO 4 . The tetragonal YNbO 4 and ZrO 2 phases both extend nearly along with Y:Nb = 1. The t‐ZrO 2 phases with slightly lower stabilizer content were found to transform into their monoclinic polymorph during the cooling process. However, in the ZrO 2 –YNbO 4 subsystem, the tetragonal ZrO 2 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 YNbO 4 phase, even with the maximum ZrO 2 content, is transformable to monoclinic form. The maximum solubility of ZrO 2 in YNbO 4 is determined to be about 20.7 mol%. Finally, the high‐temperature liquidus in the ZrO 2 –YNbO 4 system was first determined based on the cooling traces, and a vertical section diagram was proposed for further speculation. A eutectic reaction of Liquid→YNbO 4 +ZrO 2 was discovered, and the eutectic composition and temperature were measured as 39.2ZrO 2 –29.2YO 1.5 –31.6NbO 2.5 and 1813°C. This study provides the basis for thermodynamic assessment of ZrO 2 –YO 1.5 –NbO 2.5 and is valuable for the design and development of the ZrO 2 –YO 1.5 –NbO 2.5 ‐based ceramic materials.