Oxygen diffusion and surface exchange kinetics in YBa2Cu3O6+x

In the present work oxygen chemical diffusion and 18O tracer diffusion has been studied in the high temperature superconductor YBa2Cu3O6+x as a function of temperature (400 C ≤ T ≤ 700 C) and oxygen partial pressure (103 Pa ≤ PO2 ≤ 105 Pa). In the former case conductance relaxation experiments have been performed, while in the latter case 18O isotope depth profiles were analyzed by secondary ion mass spectrometry (SIMS). The combined investigation of single crystal and polycrystalline YBa2Cu3O6+x samples allowed us to determine the strong anisotropy of oxygen transport as well as to separate lattice and grain boundary contributions. The obtained oxygen diffusion results are in good agreement with existing diffusion data in the literature. The grain boundary diffusivity in polycrystalline YBa2Cu3O6+x is about 3-4 orders of magnitude higher than the lattice diffusivity (grain boundary width assumed to be δ ∼ 1 nm) and is discussed in terms of grain boundary chemistry. At lower temperatures the oxygen exchange is kinetically determined by a surface reaction. Besides oxygen diffusion coefficients (D, DTr), rate constants (k, kTr) of the oxygen exchange reaction have been obtained from the diffusion profiles. The diffusion results (chemical and tracer diffusion) along with the behaviour of the electronic conductivity are interpreted in terms of existing defect models. (orig.)