OXYGEN DIFFUSION AND PHASE-DECOMPOSITION IN BI2SR2CACU2OY SINGLE-CRYSTALS UNDER BOTH OXIDIZING AND REDUCING THERMAL TREATMENTS

For a number of Bi2Sr2CaCu2Oy (Bi2212) single crystals grown from Bi-r ich melts, there are two kinds of as-grown crystals (A and B) which sh ow different superconductivity variations under oxidizing or reducing thermal treatment at 750 degrees C. After annealing in flowing oxygen and nitrogen, the superconducting transition temperature T-c of sample A increased and decreased, respectively, relative to the as-grown T-c . For sample B, however, after annealing in either flowing oxygen or n itrogen, all the T-c increased, with the T-c of oxygen-annealed crysta l lower than that of nitrogen-annealed crystal. The phase decompositio n in annealed crystals was also studied by x-ray diffraction and found to be quite different for the two types of crystals when annealed in the same atmosphere. The relationship between oxygen diffusion, struct ural changes and superconductivity of Bi2Sr2CaCu2Oy single crystals an nealed in oxidizing or reducing atmosphere is discussed.