Effects of firing schedule on solubility limits and transport properties of ZrO2–TiO2–Y2O3 fluorites

The low Y/high Zr edge of the cubic defect fluorite solid solution in the system ZrO2–TiO2–Y2O3 in air is reassessed, as it is these compositions which have been suggested to offer the highest levels of mixed conductivity. Vegardʹs law is obeyed for values of x which lie within the cubic defect fluorite phase in Zr1−x−yYyTixO2−δ for values of y=0.2 and 0.25. Measured lattice parameters show good agreement with those calculated from the Kim relation. Deviation from Vegardʹs law places the limit of the solid solution at x=0.18 and 0.20 for values of y=0.2 and 0.25, respectively, at 1500 °C. Discrepancies in current literature data can be shown to be due to differences in firing schedule such as slight temperature fluctuations and/or different cooling rates. A high level of care of sintering temperature and cooling profile is essential to form the most promising single-phase materials which contain maximum Ti-contents with low Y-contents. Contraction of the phase limit as a result of poor synthesis control leads to erroneously high values of bulk ionic conductivity while values of electronic conductivity are shown to be less affected.