ARRANGEMENT OF GOLD CLUSTERS IN YBCO-AU

Au-197 Mossbauer spectra of fully oxygenated YBCO:Au samples show a do minant contribution from Au3+ in, Cu(1) sites with small contributions from an Au+ surface species and precipitated Au-0. A model is suggest ed in which the gold atoms substitute onto the Cu(1) sites, clustering in small square arrays of approximately 4 x 4 atoms. When fully oxyge nated, pairs of Au3+ atoms flank an interstitial oxygen in the O(5) po sition, producing 5-coordination. Deoxygenation causes a valence chang e from Au3+ to Au+ when the oxygen co-ordination drops from 4 to 3, wi th the final state being Au+ linearly co-ordinated to two O(4) atoms.