High-T-c superconductivity in Bi1.6Pb0.4Sr2Ca2 Cu2+xOy compounds

Superconducting samples with the nominal composition Bi1.6P0.4Sr2Ca2Cu2+xOy (x = 1.6, 1.81, 2.0) have been prepared by a solid state reaction technique. The resistivity data reveal the occurrence of multiphases in all the samples. By a process of ageing over 2 years then reannealing, a dramatic increase in T-c was observed in the above system for x = 2.0. We achieved zero resistance at 130 K, which is the highest in the composition Bi1.6Pb0.4Sr2Ca2Cu2+xOy (x = 1.6, 1.8, 2.0) reported so far but the recipe is often not reproducible. This shows that the system is sensitive to Cu composition and to the conditions of preparation. Furthermore, long storage leads to a detoriation in the superconducting properties. X-ray diffraction studies show that these samples mainly contain the (2223) phase. However, in each sample, a few lines could not be indexed, which may indicate the presence of some new phase.