INFLUENCE OF INTERNAL REACTIONS ON CHEMICAL DIFFUSION - APPLICATION TO FE-DOPED SRTIO3

Valence changes of ionic defects (trapping effects) play an essential role with respect to chemical diffusion in many electronically or mixe d conducting materials e.g. SnO2, PbO, YBa2Cu3O6+x, SrTiO3, as well as in ionic conductors (e.g. ZrO2). Following the concept of ''Conservat ive Ensembles'', it is shown haw short-range interactions can be expli citly handled in the chemical diffusion formalism if they are in local equilibrium with the minority carriers. It is also outlined how they Influence electrochemical polarisation or tracer experiments. Emphasis is put on our recent investigation of oxygen transport in SrTiO3 in w hich the chemical diffusion coefficient has been measured, in-situ and spatially resolved, by an optical technique using colour centers as l ocal probe. In SrTiO3 there has been a disagreement for the chemical d iffusion coefficient measured and computed by the conventional ambipol ar diffusion model by several orders of magnitude at low temperatures. On the other hand, the defect chemistry in SrTiO3 is sufficiently wel l understood. It is shown that the internal redox changes due to trans ition metal dopants (impurities) are of basic influence (not only due to the trivial modification of the carrier concentration but more basi cally) due to the internal buffer effect of the transition metal ions. Using the generalised approach in SrTiO3 for which all the necessary materials constants are available the agreement is quantitative ((D) o ver tilde = f(T, P-O2, impurity content)). The results are confirmed b y permeation, electrocoloration, and polarisation experiments. Finally , novel results with respect to interfacial kinetics (chemical kinetic s through grain boundaries, surface exchange kinetics) are presented.