Copper valency of superconducting phases in the Pb-doped Bi-Sr-Ca-Cu-O system

Abstract

Pb-doped Bi-Sr-Ca-Cu high- T c superconducting oxides have been prepared by a modified sol-gel process via the EDTA route. Upon decomposing the X-ray photoemission spectrum of the Cu 2p 3 2 core level of the superconducting oxide into three components Cu 1+, Cu 2+ and Cu 3+, we have found an obvious trend: the corresponding [Cu 1+]/[Cu 2+] ratio increased as more 110 K phase was formed in the sample. According to this trend, a defect model was proposed; it can reasonably explain some previous experimental results, such as a reduced oxygen atmosphere sintering, which effectively facilitates the formation of the 110 K phase, and the coexistence of copper and oxygen vacancies in Bi-system superconductors, observed by high-resolution transmission electron microscopy. On the other hand, the dimerization of holes in oxygen sites was also suggested to be a possible mechanism responsible for the increase in the ratio of [Cu 1+]/[Cu 2+].