Superconductivity in the ‘triple-fluorite-layer’ copper oxides (Cu,M)-1232 (M=Mo, W, Re, Pb)

Here two new high-Tc superconductors are reported of the (Cu,M)-1232 structure containing a fluorite-structured (Ce,Y)–O2–(Ce,Y)–O2–(Ce,Y) triple-layer block between two adjacent superconductive CuO2 planes. The new superconductors were found through substitution studies at the Mo site in the recently synthesized superconductor, (Cu0.75Mo0.25)Sr2(Ce0.67Y0.33)3Cu2O11+δ or (Cu0.75Mo0.25)-1232 [Y. Morita, T. Nagai, Y. Matsui, H. Yamauchi, M. Karppinen, Phys. Rev. B 70 (2004) 174515. [10a]]. Essentially single-phase samples of the (Cu0.75M0.25)-1232 phase with M=W, Re and Pb were obtained by an ambient-pressure solid-state synthesis method. Through a subsequent high-pressure oxygenation (HPO) treatment carried out at 5 GPa and 500 °C in the presence of Ag2O2 as an oxygen source the M=W and Re samples were successfully ‘superconductorized’. Introduction of Pb was found to markedly enhance the speed of the formation of the 1232 phase, but the (Cu,Pb)-1232 samples did not exhibit superconductivity. Among the superconductive (Cu,M)-1232 samples, the highest Tc value of 56 K was achieved for (Cu0.75Re0.25)-1232. It is likely that the higher the average valence of cations in the (Cu,M)O1+δ charge-reservoir is, the more can the phase accommodate excess oxygen upon HPO treatment and the higher is the resulting Tc value.