Sub-solidus phase equilibria in the YO1.5-TaO2.5 system

The phase equilibria in the YO1.5-TaO2.5 binary system were studied in air from 1250 degrees C to 1700 degrees C across the entire composition range, clarifying discrepancies in previous investigations. Materials were synthesized by precursor routes and phases identified by X-ray diffraction and Raman spectroscopy. Solubility limits were determined using TEM/EDS and EPMA. The fluorite phase field was shown to extend down to at least 1250 degrees C. Ordering phenomena in two closely related phases near Y3TaO7 were investigated, suggesting a second-order ordering relationship between them. The order-disorder transformation to fluorite was observed to occur through formation of a superstructure. The homogeneity range for YTa7O19 was expanded based on chemical analysis, while YTa3O9 was shown to be a line compound. The emerging understanding provides valuable insight into related ternary systems as well as inputs for thermodynamic assessments.