High-Pressure Oxidation Of The Tetragonal La1.5-Xba1.5+X-Ycaycu3o2 Superconductors

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 O-2. For instance, for La1.1Ba1.5Ca0.4Cu3Oz, z = 7.25 after heating at 400 degrees C in 350 bars O-2. This contrasts with the behavior of YBa2Cu3Oz, which decomposes on attempting to increase z greater than 7.0. The critical temperature. T-c, 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. T-c 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. Co substitutes onto La sites with displacement of La onto Pa site. 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-c, nor to influence the value of T-c. This behavior of Ca-doped materials is different from that of La1.5Ba1.5Cu3O2, which cannot be made superconducting even though its nominal Cu valence can be increased sufficiently, and of LaBa2Cu3Oz. which has a significantly higher maximum T-c of 94 K and shows an orthorhombic to tetragonal transition with decreasing z.