The Effect of Oxygen Nonstoichiometry on the Structure of the Low Tc Phase in Bi-Sr-Ca-Cu-O Superconductors

Abstract

The oxygen content of the low Tc phase was varied by annealing in N2 and O2 at 500–750 °C. The oxygen unloading increased the Tc up to 100 K and the length of c-axis by 0.01nm. The associated structural change was examined by X-ray diffraction, XPS, and HRTEM. XPS measurements were performed on the clean fracture surface. No major difference was detected on Bi and Cu spectra, indicating that the change of oxygen content was too small to cause detectable change in the valence of Bi and Cu ions. High resolution TEM showed that the average period of modulation in Bi-O layer was unvaried but the modulation was locally disordered by oxygen unloading. These results strongly indicate that oxygen unloading takes place from the Bi-O layer and the Tc of low Tc phase is mainly controlled by the oxygen in Bi-O layer. Preliminary EXAFS data were also consistent with this idea.