TEM imaging of clustered Au substitutional impurities in YBa₂Cu₃O_6.9

Evidence for clustering of Au impurity atoms into randomly distributed 1-2 nm rods is presented for YBa₂Cu₃O_7-δ (δ < 0.1) grown by the self-flux technique in Au crucibles. Electron-diffraction patterns show very weak diffuse streaks along «100å and «010å, and these streaks persist over several degrees of tilt indicating rod-like defects. Multislice image simulations, including the effect of amorphous carbon, for a variety of defect types are compared to high-resolution images. The model that best fits the experimental data is defects made up of 3-4 unit cell long AuO vacancy rods along «010å and AuO interstitial rods along «100å. In light of flux-pinning data comparing YBCO crystals with and without Au doping, these defects should be considered as flux-pinning sites in as-grown Au doped crystals.