A high-T_c superconducting phase in the Ba-Y-Cu-O system

The crystal structure of high-T_c 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-T_c superconducting transition at about 90 K. The high-T_c superconducting phase is pseudo-tetragonal with triple periods (3a_p) along the (001) direction. The different diamagnetic and resistance behaviour is connected with different proportions of this phase in the samples.