Fast oxygen transport in acceptor doped oxides

The basic expressions governing the ionic conductivity of fast ion conducto rs are examined. Acceptor doped oxides displaying fast ion conduction are t aken as an example. For selected materials, the role of dopant-vacancy inte ractions in influencing the concentration of mobile vacancies is assessed. Examination of experimental data and the results from atomistic lattice sim ulations lead to the following conclusions. For acceptor dopants where the effective charge of the substitutional ion is - 1, a minimum is seen in the concentration dependence of the activation energy for oxygen ion conductio n. This minimum is a characteristic feature and can be used as an indicator of dopant-vacancy interactions. The activation energy for conduction is al so dependent upon the size of the dopant, through a size dependence of the association enthalpy of the dopant-vacancy pairs. The activation energy is minimised when the dopant is close to the size of the host cation. A partic ular example of materials where this is optimised is the Sr'(La) substituti on found in many of the mixed conducting perovskites, which may go part way in explaining the very high diffusivities of oxygen found for these materi als. (C) 2000 Elsevier Science B.V. All rights reserved.