Phase equilibria in the Y2O3-Nb2O5 system up to 2500 ℃: New high-temperature experiments and thermodynamic modeling

Abstract

The high-temperature phase transformation of the Y2O3-Nb2O5 system in the whole composition range was investigated up to 2500 ℃, based on the cooling trace tests using a combination of CO2 laser heater and high-speed pyrometer. Solidification characteristics of the as-cast oxides were clarified by using X-ray diffractometer and scanning electron microscopy. Three eutectic reactions were detected in this work: Liquid→Y3NbO7 +Y2O3, Liquid→Y3NbO7 +YNbO4 and Liquid→YNbO4 +Nb2O5. The Y3NbO7 and YNbO4 compounds were determined to form through the congruent reactions of Liquid→Y3NbO7 at 2487 ℃ and Liquid→YNbO4 at 2070 ℃, respectively. After that, thermodynamic optimization of the Y2O3-Nb2O5 system was performed by means of CALPHAD method based on the new experimental results and a systematic review of the literature. The ionic two-sublattice model was used to describe the liquid phase, and the fluorite-type Y3NbO7 compound was expressed by the formula (Y+3, Nb+5)1(O−2,Va)2 in order to explain its crystal structure. The comparisons between the calculated phase diagram and the new experimental data exhibit that our thermodynamic parameters are reasonable and self-consistent. The results emerging in this work can aid in future efforts for the development of yttrium niobates and further researches of phase equilbria and thermodynamic database of the related multicomponent systems.