The Relation between Superconductivity and Crystal Structure on YBa_2-xSr_xCu₃O_7-δ (x=0-1.2)

Abstract

A systematic study on superconductivity (T_c) and structural parameters of YBa_2-xSr_xCu₃O_7-δ (0≤x≤1.2) has been carried out by the Rietveld analysis of X-ray powder diffraction patterns and by measurement of d. c. electrical resistivity. Single phase samples were obtained in the whole range of x=0-1.0, and Sr atoms were substituted exclusively for the Ba sites in YBa₂Cu₃O_7-δ. T_c and lattice constants decreased linearly with the increase of x from x=0 to 1.2 except for x=1.0. However, a discontinuous raise of T_c and a change in the axial ratio (a/b) was observed at x=1.0. In this structure the T_c's have obvious relations with, the oxygen occupancy in a Cu-O one dimensional chain as well as Ba(Sr)-O and Y-O length. The oxygen atoms in this system play an important role in superconductivity. The orthorhombic-tetragonal phase transition in x=1.0 compound took place at 550°C, where the oxygen content was 6.7. The structure of the high temperature phase was refined as tetragonal (P4/mmm) by Rietveld analysis using X-ray powder diffraction patterns at room temperature. The low temperature phase which showed superconductivity was analysed using an orthorhombic structural model. However, we refined the structure using a micro-twin model, which fitted the observed diffraction patterns much better than the single phase model.