SYNTHESIS, COMPOSITION, AND STRUCTURE OF SUPERCONDUCTING (Y,CA)(SR,BA)(2)CU2GAO7-DELTA

The superconducting properties of Y1-yCaySr2Cu2GaO7-delta have been ex amined and related to the Ca content, y, and the use of annealing trea tments at 350 bar oxygen. Superconductivity with T-c up to 41 K was fo und only for high-pressure-annealed samples, and the structural effect s of Ca substitution and high-pressure treatment were examined using p owder neutron diffraction. Small but significant changes in Cu-Cu and Cu-O distances were found and suggest that the Cu ions are more highly charged in superconducting samples. Partial substitution of Ba for Sr was found to be possible (up to 20%) to give samples which, after ann ealing in high-pressure oxygen, were superconducting at temperatures u p to 68 K.