EFFECT OF OXYGEN-DIFFUSION POSSIBLY INVOLVED IN PEROVSKITE UNIT ON SUPERCONDUCTIVITY OF VARIOUS OXYGEN-DOPED BI2SR2CACU2O8-CRYSTALS(Y SINGLE)

For a highly oriented Bi2Sr2CaCu2O8+y single crystal with a lower as-g rown T-c of 72K, on air-annealing at 250 degrees C for 2h, its T-c rem ained unchanged, while increased to 83K after annealing for 25h. For a nother crystal with a higher as-grown T-c of 86.7K and annealed under the same condition, its T-c was unchanged except for a slight improvem ent of Delta T-c and was independent of the annealing time. X-ray diff raction showed that there were no changes of c-axis lattice parameter for the annealed crystals. If the above two crystals, after annealing at 250 degrees C for 25h, were further annealed at 600 degrees C for 2 0h, then T-c was increased to the same value of almost 88K and the c-a xis lattice parameter showed a dramatic contraction. We suggested that the improvement of superconductivity of the sample annealed at much l ower annealing temperature (250 degrees C) are associated with the oxy gen diffusion involved in the perovskite unit and this diffusion from the perovskite unit occurs earlier than that from Bi-O layers.