High-temperature superconducting phases in oxides Bi2Sr2Ca4Cu8O17+δ

We have investigated the superconducting behaviour of the oxide Bi2Sr2Ca4Cu8Ox (2248) and compared it with that for Bi2Sr2CaCu2Ox. The samples were prepared in two stages: at first, the corresponding oxide in the Sr-Ca-Cu-O subsystem was synthesized at elevated temperatures (950-970 deg C) and afterwards this oxide was sintered with Bi2O3 at moderate temperatures (820-860 deg C). Samples with the starting composition 2248 were prepared under a variety of sintering, annealing and quenching regimes. The temperature dependence of the electric resistivity of all the 2248 samples exhibit more or less pronounced step at about 110 K which indicates the formation of a phase with critical temperature (Tc ∼ 107 K) higher than that characteristic for the 2212 phase. No clear evidence for the presence of phases with still higher Tc has been obtained. Considerations are put forward about a possible mechanism for the high-Tc superconduction in the system of bismuth superconductors. In this mechanism, pairing of free charge carriersin the adjacent CuO2 planes is supposed, mediated by the exchange-correlation hole produced by these carriers in the quasimetallic Ca layer, which separates the CuO2 planes