Effects of Lanthanum Substitution for Yttrium on the Superconductivity of YBa₂Cu₃O_y Ceramics

Abstract

Effects of La addition substituting for Y in YBa₂Cu₃O_y on the supercoductivity were investigated. Powder mixtures of Y₂O₃, BaCO₃, La₂(CO₃)₃ and CuO were prepared by mechanical method, cold pressed at the pressure of 98 MPa, sintered at 1253K for 86.4 ks followed by 523K for 86.4ks and then furnace cooled in an oxygen atmosphere.
For estimating the superconductivity, electrical resistivity versus temperature was measured. X-ray diffraction analysis was performed to obtain diffraction patterns and determine lattice constants by using graphite monocromatized CuKα radiation and diffractometer.
Substitution of La for Y in YBa₂Cu₃O_y did not influence on Tc up to approximately 0.1mol La of Y in YBa₂Cu₃O_y. However, ΔTc was minimal at approximately 0.1mol La of Y, that is, superconductivity was stabilized by the substitution of La for Y. Over this, Tc inclined to lower with increasing La content. These experimental results may be related to the fact that sintered density of Y_0.9La_0.1Ba₂Cu₃O_y became larger than that of YBa₂Cu₃O_y at the same sintering conditions.
Tc^offset was achieved at 84 K for La_0.64Ba₂Cu₃O_y sample; this is the highest Tc obtained among the LaBaCuO ceramics reported so far.
Lattice parameters of orthorhombic Y_1-xLa_xBa₂Cu₃O_y increased monotonously with the increase of La content up to 1 mol La of Y. Orthorhombic single phase was kept to exist up to approximately 0.2 mol of La for Y. Over this, mixture of orthorhombic phase and other phases were confirmed to be coexistent in the specimens.