Processing related microstructure and superconductivity in YBCO-247

Abstract

Two differently processed Y₂Ba₄Cu₇O_15-x (247) specimens were investigated by transmission electron microscopy. From X-ray diffraction both samples were found to be 247 single phase with slightly different lattice parameters. They showed different superconducting transition. The first specimen with T _c = 95K was found to contain a number of dislocations and lattice distortion due to cation disordering from standard and high-resolution electron microscopy. The dislocations investigated have Burgers vectors of the type [100] and [110] with a (001) glide plane, they were determined by g·b = 0 rule according to the two-beam case analysis. The second specimen showed two T _c, values, 80 and 56 K. This specimen was found to contain a large number of almost periodically separated stacking faults on the (001) plane. The faults were due to CuO layers mostly added to a single CuO layer and a few removed from a double (CUO)₂ layer giving locally unit-cell size thick layers with 124 and 123 structure, respectively. The stacking faults showed locally superstructure-like ordering with a repeat distance that varied with position in the specimen. Typically the period was found to be from 20 to 40 nanometres. The ordered extra CuO layers seem to be related to the double superconductingtransitions of this specimen.