High-T_c superconductivity in three-fluorite-layer copper oxides. Ⅱ. (Cu,Mo)Sr_2(Ce,Y)_3Cu_2O_(11+δ)

摘要

We have synthesized high-quality samples of the s = 3 member of the (Cu, Mo)-12s2 homologous series of layered copper oxides. The (Cu, Mo)-1232 phase forms as a single phase for the cation composition (Cu_(0.75)Mo_(0.25))Sr_2(Ce_(0.67)Y_(0.33))_3Cu_2O_(11+δ) through solid-state synthesis in air, but the samples as synthesized are not superconductive. Superconductivity is induced in the air-synthesized samples by means of high-pressure oxygenation carried out at 5 GPa and 500℃ in the presence of Ag_2O_2 as an excess oxygen source. With increasing ratio of Ag_2O_2 to the (Cu, Mo)-1232 phase the c-axis lattice parameter gradually decreases and T_c increases. The highest T_c value of 53 K reached for samples oxygenated in the presence of 100 mol% Ag_2O_2 exceeds the values reported for Cu-based two-fluorite-layer compounds.