Phase evolution in the YO1.5–TiO2–ZrO2 system around the pyrochlore region

Abstract The evolution of metastable phases in the YO 1.5 –TiO 2 –ZrO 2 system was investigated, with emphasis on compositions of general formulae Y 2 Ti 2 − y Zr y O 7 and Y 2 − z Ti 2 Zr z O 7 + z /2 , centered around the pyrochlore phase. The study focused on powders synthesized from precursors. The competing phases in this system can all be envisaged as derivatives of the fluorite structure. The results provide insight into the thermodynamics and kinetics governing phase selection. Pyrolysis of most compositions led to an amorphous oxide that was subsequently heat-treated in a stepwise manner at progressively higher temperatures to explore the phase evolution sequence. When substituting Zr 4+ for Ti 4+ , the first crystallization product was a defect fluorite structure. Pyrochlore formed at temperatures increasing from 900 to 1400 °C with increasing ZrO 2 content (0 ⩽ y ⩽ 1.4). When substituting Zr 4+ for Y 3+ , a two-phase microstructure of pyrochlore and orthorhombic ZrTiO 4 developed at 1300 °C. This is believed to be the equilibrium structure, in disagreement with the existing phase diagram.