Grain boundary effects in ionic and mixed conductors

Ionic and electronic transport phenomena rely on the presence of excess and missing particles, i. e. interstitial ions and vacancies in the case of ions, excess electrons and electron holes in the case of electrons. In the first and introductory part it is shown how the equilibrium defect concentrations are calculated as a function of the decisive parameters, in particular temperature (T), doping content (C), and component potential (mu) The latter determines the position in the phase diagram and relates the electronic entities to the ionic ones. In the second part, these considerations are extended to interfaces at which redistribution of defects occurs as a consequence of the interfacial chemistry. The implications in particular for the ionic conductivity but also with respect to electronic transport properties are discussed. Large conductivity anomalies occur at phase boundaries or grain boundaries giving rise to conductive pathways or to blocking effects. The effects can be tuned by modifying the interfacial chemistry. In turn, they can be used for the purpose of sensing chemicals. In the third part nano-crystalline particles are considered and the effects discussed which occur if the sample dimension becomes comparable with or lower than the Debye-length. In general the paper stresses as much as possible the general case of a mixed conductor in which electronic and ionic effects are combined.