TEM IMAGING OF CLUSTERED AU SUBSTITUTIONAL IMPURITIES IN YBA2CU3O6.9

Evidence for clustering of Au impurity atoms into randomly distributed 1-2 nm rods is presented for YBa2Cu3O7-delta (delta < 0.1) grown by t he self-flux technique in Au crucibles. Electron-diffraction patterns show very weak diffuse streaks along [100] and [010], and these streak s persist over several degrees of tilt indicating rod-like defects. Mu ltislice 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 Au-O vacancy rods along [010] and Au-O interstitial rods along [100]. In light of flux-pinning data comparing YBCO crystal s with and without Au doping, these defects should be considered as fl ux-pinning sites in as-grown Au doped crystals.