Bi, PbSrCuO system including a modulation-free superconductor

Abstract

Monophasic compositions of Bi 2− x + y Pb x Sr 2− y Cu 1+ y /4 O z with 0≤ x ≤0.5 and 0.1< y ≤0.5 were pr epared. Three different types of modulation modes were found, which have been interpreted as an indication that the Pb substitution site changes between the Bi site and the Sr site depending upon x and y . The modulation was found to completely disappear in a narrow range around the Pb solubility limit in the Bi-poor side of x ≥0.4 and y =0.125. The modulation free composition with z ≅6.0, which could be prepared in an Ar stream of 1 atm but not in air, behaved as a superconductor with T c ≅14 K . An over-doped normal metal with x =0.35, y =0.125 and z =6.08 prepared in air also became superconducting below the same T c after reduction to z =6.02. These relatively Pb-rich and Bi-poor compositions showed an interesting segregation reaction when annealed at low temperatures in air as found already for the Pb-substituted 2223 phase.