Oxygen transport through the best mixed ionic−electronic conducting oxides is often governed by the transfer of oxygen across the material’s surfaces rather than oxygen diffusion through the bulk phase. This transfer process is rather complex, involving oxygen containing molecules becoming oxide ions (or vice versa) in a multistep reaction with various reaction intermediates. Generally, the surface is simply taken as the planar interface between the gas phase and the solid. This simplification is problematic because it ignores that the surface has its own structure, which depends on the surface orientation, and that the surface may be charged, with an attendant space-charge zone within which charge-carrier concentrations are drastically altered.

Taking the prototypical perovskite SrTiO₃ as a model system, we examined both the influence of the surface orientation and of the presence of water in the gas phase on the surface exchange coefficient and the surface space-charge potential was investigated [1, 2]. We employed two different isotope exchange experiments to investigate these parameters: either a dry (¹⁸O₂ / ¹⁶O₂) or a wet (H₂¹⁸O / H₂¹⁶O) isotope exchange followed by determination of the isotope diffusion profile with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). We obtained data for single-crystal samples of SrTiO₃ with three different surface orientations, (100), (110) and (111), and for dry and wet atmospheres for the (100) surface orientation. Surprisingly, the difference in isothermal surface exchange coefficients measured for the different surface orientations was less than an order of magnitude. A further surprise was that the (110) and (111) surfaces exhibited a clear change in activation enthalpy, whereas the (100) surface did not. A third surprise was that the behaviour of the surface exchange coefficients was similar to that for the surface space-charge.

For two of these surprising results (the latter two), we found a single explanation. We started with a careful analysis of the surface exchange data, attributing the high activation enthalpy at high temperatures to a mechanism limited by charge transfer and the low activation enthalpy at lower temperatures to a mechanism involving adsorbed water species (due to trace amounts of water present in the oxygen exchange gas), which was further explained by the wet exchange experiments. The common behaviour of the two parameters is then linked to the presence of charged OH surface species at lower temperatures.