High-pressure oxidation of the tetragonal La1.5−xBa1.5+x−yCayCu3Oz superconductors

Abstract

The oxygen content, z, of tetragonal, Ca-doped La1.5Ba1.5Cu3Oz (La336) superconductors can be increased to values significantly greater than 7.0 by annealing in high-pressure O2. For instance, for La1.1Ba1.5Ca0.4Cu3Oz, z=7.25 after heating at 400 °C in 350 bars O2. This contrasts with the behavior of YBa2Cu3Oz, which decomposes on attempting to increase z greater than 7.0. The critical temperature, Tc, passes through a maximum of 79 K at an average Cu valence of 2.3–2.4 and decreases to as low as 35 K in the overdoped region of higher Cu valence. Tc data for a range of Ca-doped compositions fall approximately on a single dome-shaped master plot. The crystal structure of LaBa1.5Ca0.5Cu3Oz shows a complex 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 Tc, nor to influence the value of Tc. This behavior of Ca-doped materials is different from that of La1.5Ba1.5Cu3Oz, which cannot be made superconducting even though its nominal Cu valence can be increased sufficiently, and of LaBa2Cu3Oz, which has a significantly higher maximum Tc of 94 K and shows an orthorhombic to tetragonal transition with decreasing z.

Source-reported materials — not catalogue approval

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