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-YO1.5-NbO2.5 system were experimentally studied at 1300 degrees C, and a comparison with the existing ZrO2-YO1.5-TaO2.5 (ZYTO) was conducted. Ternary fluorite phases on the Y:Nb > 1 side extend from binary ZrO2-YO1.5 to YO1.5-NbO2.5 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 YO1.5 and NbO2.5 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 39.2ZrO(2)-29.2YO(1.5)-31.6NbO(2.5) and 1813 degrees C. This study provides the basis for thermodynamic assessment of ZrO2-YO1.5-NbO2.5 and is valuable for the design and development of the ZrO2-YO1.5-NbO2.5-based ceramic materials.