Defect Chemistry of Perovskite Titanates: From Bulk Materials to Nanostructured Materials

To describe the electrical conductivities of perovskite titanates (e.g. SrTiO₃ and BaTiO₃) and the relation to parameters like temperature, oxygen partial pressure or acceptor- or donor-type dopants, the defect chemistry has been extensively studied since the late 1960s. The studies finally lead to the formulation of the point defect chemistry on the basis of defect chemical reactions, exchange processes with the atmosphere and changes of the valence state regarding their individual thermal activation and thermo­dynamic equilibriums. Comparing with microstructured counterparts, nanostructured materials have significantly higher interface (surface and/or grain boundary) to bulk fraction, which leads to some unique effects in nanostructured perovskite titanates. One effect is that the space charge layer becomes increasingly important. Owing to the high surface-to-volume ratio, the surface space charge layer plays an important role in thin films. For example, the p-type conductivity of the epitaxial SrTiO₃ thin films decreases, and the n-type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the space charge layer. Moreover, the defect thermodynamics at nanometer-scale is different. E.g. the oxidation enthalpy of nanocrystalline BaTiO₃ ceramics is found to be about one-third of that of microcrystalline BaTiO₃. As a result, the p-type electrical conductivity of nanocrystalline BaTiO₃ is about 2 orders of magnitude higher than that of microcrystalline BaTiO₃.