Xray Absorption Fine Structure (XAFS) Study of the High-Tc Superconductor Pb-Bi-Sr-Ca-Cu-0 System

周鑑恆(C. H. Chou) ； 彭維鋒(W. F. Pong) ； 林諭男(I. N. Lin) ； 蔡尚芳(S. F. Tsai)

《Chinese Journal of Physics》 29卷3期 (1991/06) Pp. 263-282

並列摘要

Samples of (PbxBi2-x)Sr2Ca2Cu3Oy were prepared with appropriate proportions of PbO, Bi2O3, SrCO3, CaCO3 and CuO powders. For the high-Tc superconductive samples in which x = 0, 0.2, 0.4, 0.6, we have measured the XAFS spectra of the Cu K-edge, the Pb L3-edge and the Bi L3-edge at room temperature in the transmission mode at NSLS using the X-23 beamline. The results of our study indicate that: (1) Pb atoms within PbO partially substitute for Bi atoms in Bi-Sr-Ca-Cu-0 superconductors; (2) there is evidence for the existence of unreacted PbO, particularly in the sample in which x = 0.6; (3) the oxidation number of most Pb ions in the Bi site within the Bi-Sr-Ca-Cu-0 system remains 2; (4) a sufficient proportion of PbO is essential for the formation of the high-Tc phase, (5) and the addition of PbO is responsible only for the phase transformation of the low-Tc phase into the high-Tc phase. Moreover, the electronic structure of the Cu-0 layers is essentially independent of the Pb substitution. The oxidation number of Cu ions in the high-Tc phase is slightly larger than that of Cu ions in the low-T, phase, and the average oxidation number of Bi remains nearly constant independent of either the phase transformation or Pb substitution. We have also deduced that the Cu-0 layers have an ordered structure, and comment qualitatively about the incommensurate modulations in the Bi-0 layers.