Defect chemistry of pyrochlore Pr2O3-ZrO2 system: the relevant thermodynamic parameters

Materials of the system Pr₂O₃-ZrO₂, namely the Pr₂Zr₂O₇-based pyrochlores, have received considerable attention in the last decade, being a very interesting structure for defect chemistry because of its high solubility for various dopants, anti-site behaviour between A and B, and the multitude of possible combinations of A and B that are compatible in this type of structure. The compositions (Pr_2-xZr_x)Zr₂O_7+x/2 (x = 0.15), Pr₂Zr₂O₇, and Pr₂(Zr_2-xPr_x)O_7-x/2 (x = 0.1), were prepared in previous works through the coprecipitation method and were characterised by impedance spectroscopy as a function of the oxygen partial pressure. In the present work, a defect chemistry model is proposed, and, based on the previously obtained experimental conductivity data, the relevant thermodynamic parameters were obtained by fitting, using a non-linear optimisation numerical method. The mobility of oxygen vacancies and interstitials oxygen were accurately determined, as well as the equilibrium constant of the formation of anti-Frenkel defects. It was observed that deviations from the stoichiometry promote an increase in ionic conductivity, respectively, 1.3x10^-4, 1.4x10^-3 and 1.7x10^-2 S/cm, for the stoichiometric, excess of Pr and excess of Zr composition. The higher value obtained for the composition with an excess of Zr⁴⁺, suggests a higher interstitial oxygen mobility when compared with the oxygen vacancy mobility. It is also demonstrated that the novel applied methodology of fitting conductivity experimental data with an optimisation numerical method is suitable for determining the thermodynamically relevant parameters of defect chemistry models, allowing the prediction of material properties.