SUPERCONDUCTIVITY IN THE BA-Y-CU-W-O SYSTEM

The Ba2YCu2WO(y) system becomes superconductive at 77 K, the electrical resistivity of the samples goes to zero, but the Meissner effect at 77 K is so small that it cannot be measured. This behaviour is similar to that observed for the La2CuO4 system in lanthanum deficient samples prepared at one atmosphere, interpreted as a filamentary type of superconductivity. The behaviour of the superconductor in the Ba2YCu2WO(y) system is observed only when the samples are quenched from a temperature above 1130-degrees-C, but not when they are cooled slowly. From X-ray diffraction analyses we identify in the samples two different phases: 1) Ba8Y3CuW4O24, which is cubic with the space group Pm3m and a lattice parameter of 0.8350(1) nm, and 2) CuO, which acts as a flux medium. Experiments performed on the Ba2YCu2WO(y) and CuO systems with small amounts of YBa2Cu3O7 show that after the heat treatment the YBa2Cu3O7 phase is destroyed. This result together with the fact that the slowly cooled samples are not superconductors, discard the possibility that the observed superconductivity in samples of the Ba2YCu2WO(y) system is originated by small impurities of the superconductor YBa2Cu3O7 oxide.