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

The oxygen content, z, of tetragonal, Ca-doped (Formula presented)(Formula presented)(Formula presented)(Formula presented) (La336) superconductors can be increased to values significantly greater than 7.0 by annealing in high-pressure (Formula presented). For instance, for (Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented), z=7.25 after heating at 400 °C in 350 bars (Formula presented). This contrasts with the behavior of (Formula presented)(Formula presented)(Formula presented), which decomposes on attempting to increase z greater than 7.0. The critical temperature, (Formula presented), 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. (Formula presented) data for a range of Ca-doped compositions fall approximately on a single dome-shaped master plot. The crystal structure of (Formula presented)(Formula presented)(Formula presented)(Formula presented) 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 (Formula presented), nor to influence the value of (Formula presented). This behavior of Ca-doped materials is different from that of (Formula presented)(Formula presented)(Formula presented)(Formula presented), which cannot be made superconducting even though its nominal Cu valence can be increased sufficiently, and of (Formula presented)(Formula presented)(Formula presented), which has a significantly higher maximum (Formula presented) of 94 K and shows an orthorhombic to tetragonal transition with decreasing z.