Superconductivity in CoSr2(Y1-xCax)Cu2O7+δ

The roles of aliovalent Ca-II-for-Y-III substitution and high-pressure-oxygen annealing in the process of 'superconducterizing' the Co-based layered copper oxide, CoSr2(Y1-xCax)Cu2O7+delta (Co-1212), were investigated. The as-air-synthesized samples up to x = 0.4 were found essentially oxygen stoichiometric (-0.03 less than or equal to delta less than or equal to 0.00). These samples, however, were not superconducting, suggesting that the holes created by the divalent-for-trivalent cation substitution are trapped on Co in the charge reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500 degreesC for 30 min in the presence of Ag2O2 as an excess oxygen source induced bulk superconductivity in these samples. The highest T-c was obtained for the high-oxygen-pressure treated x = 0.3 sample at similar to40 K. (C) 2003 Elsevier Ltd. All rights reserved.