An optical method was used to study oxidation kinetics. A controlled amount of Fe impurity defined the defect chemistry with regard to unknown impurity background effects and also allowed for the possibility of detecting O uptake/removal by means of absorption spectroscopy. The kinetics were shown to be diffusion-limited at high temperatures (and large sample thicknesses) and to be surface reaction-limited at low temperatures (and small sample thicknesses). It was shown that, in the case of trapping effects, the chemical diffusion coefficient could not be analyzed in terms of the conventional theory of chemical diffusion. It was deduced that the chemical diffusion coefficient could, due to trapping effects, be even lower than the diffusivity of mobile ionic and electronic carriers. The analysis also showed that evaluation of the relaxation behavior of various species during stoichiometric relaxation did not necessarily lead to the same effective diffusion coefficients.

T.Bieger, J.Maier, R.Waser: Solid State Ionics, 1992, 53-56, 578-82