A high-Tcsuperconducting phase in the Ba-Y-Cu-O system

The crystal structure of high-Tc superconductors in the Ba-Y-Cu-O system has been investigated by means of X-ray powder diffraction and electron microscopy. The results indicate that the superstructure based on the distorted cubic perovskite subcell is responsible for the high-Tc superconducting transition at about 90 K. The high-Tc superconducting phase is pseudo-tetragonal with triple periods (3ap) along the (001) direction. The different diamagnetic and resistance behaviour is connected with different proportions of this phase in the samples.