ROLES OF OXYGEN DEFECTS IN COPPER-OXIDE SUPERCONDUCTORS

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La2CuO4+x and HgBa2CuO4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)3Cu2O6+x with various metal compositions and metal-site ordering show that interstitial oxygen defects that form between the CuO2 layers in this structure systematically lower T(c) and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO2 planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO2, where A = La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO2 planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr0.9La0.1CuO2 (T(c) = 42 K) have not found any interstitial oxygen.