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. Vegard's 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 Vegard's 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 deg. 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. - Graphical abstract: The composition dependence of lattice parameter for compositions containing 20 mol% YO1.5 as a function of Ti-content in the system ZrO2-TiO2-YO1.5 showing contraction of the cubic defect fluorite-phase field upon slight temperature fluctuations or slow cooling rates