Effects of composition and processing on the superconductivity of La_1+zBa_2-zCu₃O_y

The effects of composition, processing time (t), temperature (T) and O₂ partial pressure (p_O2) on the superconducting transition temperature, T_c, were investigated for La_1+zBa_2-zCu₃O_y. Tallon and Flower's prediction of T _c ≈ 100 K [Physica C 204 (1993) 237] at z = 0 could not be attained. Instead, the T_c maximum of 93-94K is similar to that of the yttria analog. As z was increased, the T_c values increased gradually by a few kelvin until z = 0.2, which is at the orthorhombic to tetragonal transition, and then decreased more abruptly, with no superconductivity for z≥0.5. Crucial steps in the process at all z values include the highest temperature anneal (1065°C) in 0.1 MPa O₂, reduction of the oxygen content at 950°C and cooling to 300-400°C, both at 100-1000 Pa O₂, and reoxygenation at 0.1 MPa O₂ during slow cooling (5-20°C/min) to < 150°C. Additional experiments demonstrated that the intragranular critical current was less than that for the yttria analog. Values of T_c decreased fairly monotonically with decreasing y at z = 0.08, and a La excess of 0.2 and Cu nonstoichiometry on both sides of Cu/(La+Ba) = 1 generally depressed T_c.