Phase equilibria in the Gd2O3-Ta2O5 system: New high-temperature experiments and thermodynamic modeling

A CO₂ laser heating apparatus combined with a high-speed pyrometer was employed to conduct new high-temperature experiments on the phase transformation of the Gd₂O₃-Ta₂O₅ system. The solidification features of the as-cast samples were examined by means of an X-ray diffractometer and scanning electron microscopy. Four eutectic reactions were found in this study, namely Liquid→F-Gd₃TaO₇ +B-Gd₂O₃, Liquid→F-Gd₃TaO₇+T-GdTaO₄, Liquid→T-GdTaO₄+O-GdTa₃O₉ and Liquid→O-GdTa₃O₉+α-Ta₂O₅. Their eutectic compositions and temperatures were updated based on our experimental results. The congruent reaction temperatures of Liquid→T-GdTaO₄ and Liquid→O-GdTa₃O₉ were determined to be at 2055 °C and 2010 °C, respectively. Moreover, phase stability of h-GdTa₇O₁₉, which can form between 1400 °C and 1500 °C, was discussed. Finally, the CALPHAD method was applied to conduct a thermodynamic optimization of the Gd₂O₃-Ta₂O₅ system. The liquid phase was characterized by the ionic two-sublattice model, while the thermodynamic models of intermediate compounds were formulated based on their crystal structures. The comparisons between the calculated results and the accessible experimental data show that our thermodynamic modeling is sound. The findings of this research can promote the development of gadolinium tantalate materials and offer understandings regarding phase equilibrium and the development of the thermodynamic database for relevant multicomponent systems.