Synthesis and structure of copper- and yttrium-rich YBa₂Cu₃O_7-x superconductors

Copper- and yttrium-rich modifications of the YBa₂Cu₃O_7-x superconductors have been synthesized using a liquid nitrogen quenched followed by annealing. In both types of cation-rich materials the starting materials is highly disordered but upon annealing the extra copper or yttrium precipitates out in the form of planar defects normal to the c axis which on annealing at higher temperatures start to coarsen from single plane defects to multiple plane defects and become better ordered normal to the c axis. In all cases X-ray diffraction gives an average orthorhombic structure. The existence of these defects appears to have no effect upon the sharpness of the superconducting transition for specimens annealed to 800°C, leads to a small broadening of the transitions in specimens annealed at 700°C, while the specimens annealed at 300°C show only a hint of superconducting behavior. This supports the view that these materials are one- or two-dimensional superconductors normal to the c axis with only a small contribution to the bulk superconductivity along the c axis.