Investigation of the oxygen transport properties in acceptor- and donor-doped cerium oxide

The oxygen transport properties of different ceria compositions were studied. The two fundamental questions of this work were: 1) Is there a fundamental difference in the diffusion properties of the two oxygen point defects; namely oxygen vacancies and oxygen interstitials? 2) How is the oxygen surface exchange coefficient influenced by the electronic structure of the material, which can be changed through acceptor or donor doping and is the oxygen surface exchange coefficient dependent on the surface orientation of the system. To answer these questions a combination of experimental and theoretical methods was employed. The experimental methods included conductivity relaxations measurements and oxygen isotope exchange experiments with subsequently analysis by secondary ion mass spectrometry. The theoretical methods used were molecular-dynamics simulations and finite-element modelling. A fundamental difference in the diffusion properties of the two oxygen point defects was found; at a given temperature oxygen vacancies were more mobile than oxygen interstitials. Additionally, a lower activation enthalpy of migration was obtained for the vacancy migration than for the interstitial migration. Regarding the surface properties the initial question could not be fully answered. For ceria thin films with different surface orientations different oxygen surface exchange coefficients were obtained. The highest surface exchange coefficient was obtained for the (111) oriented surface. Regarding the dependence on the electronic structure of the system no concluding answer was found due to the following reason: For all considered ceramic samples a strong enrichment of silica was observed at the surface. This unspecified silica phase prevented quantification of the surface exchange coefficient.