SrTiO3 as a prototype of a mixed conductor. Conductivities, oxygen diffusion and boundary effects

Partial conductivities and profiles of oxygen stoichiometry have been investigated in acceptor doped SrTiO3 in the temperature range 450 to 1000 K and oxygen partial pressures between 10(0) and 10(5) Pa. For the first time, in-situ profiles could be observed as a function of time and evaluated according to the transport equations. Experimentally, the optical absorbance due to the corresponding valence states of internal redox-couples was recorded by a CCD-camera. If the effect of internal reaction sinks or sources being in local equilibrium is taken into account, the chemical diffusion coefficient is in remarkable agreement with the defect chemical analysis. Within the same model, partial conductivities can be understood without adjustable parameter. To achieve this, and also to apply appropriate techniques of measurement, it has to be taken into account that the surface exchange reaction becomes rate-limiting and very sluggish at low temperatures. This kinetic constraint has to be introduced in the low temperature analysis. The use of YBa2Cu3O6+x electrodes extends the diffusion controlled oxygen exchange to far lower temperatures (T approximate to 450 K). Further investigations on grain boundaries by impedance, TEM, EDX and EELS measurements yield detailed information on electrical behaviour and the chemical composition of the different types of interfaces (free surfaces, crack surfaces, tilt and twist grain boundaries). Depletion layers along the grain boundaries and adjacent to the metallic electrode contact region can be identified.