Chemical diffusion and defect chemistry of grain boundaries in n-type barium titanate ceramics

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Conductivity relaxation experiments have been performed on n-type barium titanate ceramics, co-doped with Y (donor) and Mn (acceptor), as a function of temperature (700–900°C) and oxygen partial pressure (0.001–1.0bar) by employing the van der Pauw technique. The chemical diffusion process owing to a change of the oxygen partial pressure in the surrounding atmosphere can be described by fast transport of oxygen vacancies along the grain boundaries (mainly core regions) and slow diffusion of cation vacancies into adjacent bulk regions (including space charge layers). A careful analysis of the experimental conductivity relaxation experiments allows the determination of the chemical diffusion coefficient for oxygen along grain boundary core regions with typical values in the range of 10−8 to 10−9cm2s−1 and an activation energy of 0.9eV (900>T/°C>700). Furthermore, the partial pressure dependence of the electronic conductivity can be interpreted by means of a defect chemical model involving grain boundary core regions where Mn is segregated and oxygen vacancies are the decisive mobile ionic charge carriers.