Structural analysis of superconducting NdBa 1.5Sr 0.5Cu 3o 7−δ

Abstract

Two samples of NdBa 1.5Sr 0.5Cu 3O 7−δ with δ<0.1, prepared by the solid state reaction, were subjected to two different annealing conditions. Based on Rietveld refinement of powder neutron diffraction (PND) patterns, the occurrence of a single phase with tetragonal symmetry was found for the sample, which was annealed at high temperature in oxygen atmosphere (T-sample). The other sample, which was heated at high temperature in argon atmosphere followed by low temperature annealing in oxygen atmosphere, however, showed coexistence of both orthorhombic and tetragonal symmetries (O/T-sample). Refinement of powder X-ray diffraction data of the T-sample indicated cation disorder between Sr and Nd ions at the ( 1 2 , 1 2 , z ) and the rare earth sites, respectively. The relation to oxygen disorder is discussed. Superconducting transition temperatures, obtained by electrical resistance measurements, were T c ( R = 0) = 83 K and T c ( R = 0) = 71 K for the O/T-and T-sample, respectively. The lower T c for the T-sample is correlated with an elongated bond distance between the Cu(2) ion and apical oxygen.