The defect chemistry of Ce(Pr, Zr)O2−δ

The influence of composition upon resultant mixed conductivity is analysed for the fluorite-type compositions Zr0.1Ce0.7Pr0.2O2−δ, and Ce0.8Pr0.2O2−δ. Measurements of oxygen concentration cell e.m.f. combined with impedance spectroscopy at elevated temperatures reveal the materials to be predominantly ionic conductors in oxidising conditions and to show a decrease in both ionic and electronic conductivities with decreasing pO2. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t0=0.8 and 0.9, respectively, for the two compositions at 950 °C. The combination of this information with results of coulombic titration, facilitates analysis of the pO2 dependence of total conductivity measured by a steady-state electrochemical technique between air and reducing conditions. In more oxidising conditions, depletion of total conductivity with decreasing pO2 results predominantly from a decreasing ionic conductivity. This is one of first examples in the literature where a fluorite-type material is shown indisputably to exhibit such behaviour. These materials show significant levels of mixed conductivity in both reducing and oxidising conditions. In more reducing conditions, an increase in total conductivity is related to increased n-type conductivity upon the reduction of cerium.