The structure of gold clusters in YBCO:Au

The addition of a small amount of gold to superconducting YBa2Cu3O7-δ increases its critical temperature. The Au is incorporated into the Cu(1) sites and can be precipitated as Au0. At an oxygen concentration of 7.02 the substitutional Au is Au3+, and reducing the concentration to 6.40 produces complete reduction of the Au3+ to principally Au+, but with also some conversion to Au0. The nature of the gold clusters is important because they presumably act as weak pinning centres in the superconductor. Based on the author's 197Au Moessbauer measurements and previous TEM studies carried by Storey et al., the following model is proposed. The gold atoms form an approximately square array in the Cu(1) sites on the (001) plane, with approximate dimensions 4 x 4 atoms. In the fully oxygenated state, interstitial oxygen atoms join pairs of gold atoms along [100] so that there are half as many interstitial oxygen atoms as gold atoms. Thus the majority of gold atoms are five-coordinated to oxygens, although some end atoms may be only four-coordinated. The first stage of de-oxygenation removes these interstitial atoms to give square-planar coordination. Removal of the next oxygen is accompanied by a change in the gold valence to Au+. Removal of the second O(1) atom allows the Au+ to achieve its preferred linear coordination with oxygen vacancies along [010]. Our model explains the observed TEM rod-like images while, at the same time, providing a simple mechanism of getting from one state to the other