COPPER SUBSTITUTION EFFECTS ON YBCO MICROSTRUCTURE - OXYGEN ORDERING,STRUCTURAL COHERENCE AND SUPERCONDUCTIVITY

Zn- and Fe-doped YBCO systems were investigated as a function of the d opant concentration and the oxygen stoichiometry and with respect to t he resistive characteristics, bulk simmetry and oxygen microdomain ord ering. It was verified that Zn, while strongly abating T-c, is not ind ucing any appreciable modification in the bulk simmetry and the oxygen ordering with respect to the undoped YBCO. On the contrary, Fe substi tution, which is less effective on T-c at comparable concentration lev el, heavily modifies the structural simmetry and hinders the formation of oxygen ordered superstructures on a scale range appreciable by the electron diffraction technique.