VARIATION OF YBA2(CU1-YCOY)3O6+X SYMMETRY AT CONSTANT STOICHIOMETRY - RELATION WITH SUPERCONDUCTING PROPERTIES

We performed heat treatments (800-830 °C) in inert atmosphere followed by oxidation at low temperature (400 °C) on Co-substituted YBa2(Cu1-yCoy)3O6+x, leading to the stabilization of the orthorhombic phase beyond y = .025. For example, orthorhombic symmetry is obtained with y = .06 with twin boundaries spacings as large 1000Å. Our STEM data on and between twin walls, and the presence of diffuse crosses around spots of each orthorhombic individual indicate that Co clusters exist also inside the orthorhombic domains. The orthorhombic samples recover the tetragonal symmetry by heating in flowing O2 at 600 °C for 30mn. They present the tweed-like structure and the same type of diffusion as for the native compounds. In order to have a better understanding of the correlations between the structural details and the superconducting properties, ac-suceptibility and dc-magnetization mesurements are being carried out. Our first results show that the middle of the transition of the 6% Co-substituted orthorhombic sample is at lower temperature than that of same sample before heat treatment. © 1990.