High-pressure oxidation of the tetragonalLa1.5−xBa1.5+x−yCayCu3Ozsuperconductors

Abstract

The oxygen content, z, of tetragonal, Ca-doped La{sub 1.5}Ba{sub 1.5}Cu{sub 3}O{sub z} (La336) superconductors can be increased to values significantly greater than 7.0 by annealing in high-pressure O{sub 2}. For instance, for La{sub 1.1}Ba{sub 1.5}Ca{sub 0.4}Cu{sub 3}O{sub z}, z=7.25 after heating at 400{degree}C in 350 bars O{sub 2}. This contrasts with the behavior of YBa{sub 2}Cu{sub 3}O{sub z}, which decomposes on attempting to increase z greater than 7.0. The critical temperature, T{sub c}, passes through a maximum of 79 K at an average Cu valence of 2.3{endash}2.4 and decreases to as low as 35 K in the overdoped region of higher Cu valence. T{sub c} data for a range of Ca-doped compositions fall approximately on a single dome-shaped master plot. The crystal structure of LaBa{sub 1.5}Ca{sub 0.5}Cu{sub 3}O{sub z} shows a complex {ital A}-site distribution; in the as-prepared sample, Ca substitutes onto La sites with displacement of La onto Ba sites. After high-pressure treatment, however, Ca substitutes onto Ba sites and La sites are fully occupied by La. The change in A-site distribution appears not to correlate with the maximum in T{sub c}, nor to influence the value of T{sub c}. This behavior of Ca-doped materials is different from thatmore » of La{sub 1.5}Ba{sub 1.5}Cu{sub 3}O{sub z}, which cannot be made superconducting even though its nominal Cu valence can be increased sufficiently, and of LaBa{sub 2}Cu{sub 3}O{sub z}, which has a significantly higher maximum T{sub c} of 94 K and shows an orthorhombic to tetragonal transition with decreasing z. {copyright} {ital 1997} {ital The American Physical Society}« less