Oxygen Surface Exchange and Tracer Diffusion in Differently Oriented Thin Films of Gd-Doped CeO₂

The exchange of ¹⁸O between gaseous molecular oxygen and thin-film samples of Ce_0.99Gd_0.01O_1.995 with two different, nominal surface orientations [(111) and (110)] was studied. Oxygen isotope exchange experiments were conducted in the temperature range of 573 ≤ T/K ≤ 673 at an oxygen activity of aO₂ = 0.2. Subsequently, secondary ion mass spectrometry (SIMS) measurements were performed on the thin-film samples to obtain ¹⁸O isotope depth profiles. All ¹⁸O diffusion profiles showed two features, suggesting spatially nonuniform oxygen tracer diffusion coefficients in the samples. A numerical solution to the diffusion equation was used to describe the experimental profiles and yielded oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Values of D* obtained were found, surprisingly, to be different for the two orientations and also orders of magnitude lower than values for ceramic samples in this temperature range. As possible explanations, we examine quantitatively the effect of halide anion impurities and the effect of ultrasmall columnar grains on oxygen tracer diffusion. Surface exchange coefficients for the (111) oriented surface were found to be roughly 1 order of magnitude higher than those for (110). We discuss two possible explanations for the observed behavior: the enrichment of anion impurities at the surface and the interaction between the surface and vapor water in the gas phase.