Concentration and Diffusivity of Oxygen Interstitials in Niobia-Doped Ceria

We studied the behavior of oxygen interstitials in donor-doped ceria using equilibrium conductivity measurements, conductivity relaxation experiments, and (18O2/16O2) isotope exchange experiments. Equilibrium conductivities and conductivity relaxation experiments were performed in the oxygen activity range 10–6 ≤ aO2 ≤ 10–1 at temperatures of 923, 973, and 1023 K. Oxygen isotope exchanges were carried out at 673 ≤ T/K ≤ 1073 at an oxygen activity of aO2 = 0.2, and oxygen isotope profiles in the solid were obtained by secondary ion mass spectrometry. Analysis of the measured equilibrium conductivities with a defect-chemical model yielded the incorporation enthalpy and entropy of oxygen interstitials. Values of the chemical diffusion coefficient of oxygen (from relaxation experiments) and of the tracer diffusion coefficient of oxygen (from isotope exchange experiments) were converted into diffusion coefficients of oxygen interstitials using a defect-chemical model. Based on these data, the activation enthalpy of oxygen-interstitial migration in Ce0.99Nb0.01O2+δ was found to be 1.28 ± 0.13 eV. Strong enrichment of Si, Ca, and Al at the surfaces of the ceramic samples prevented the determination of surface exchange coefficients that refer to donor-doped ceria.