Orientation-dependent X-ray Absorption Fine Structure (XAFS) of the Bi-St=Ca-Cu-0 System

C. H. Chou ； W. F. Pang ； J. B. Shi ； H. C. Ku ； I. N. Lin

《Chinese Journal of Physics》 30卷6期 (1992/12) Pp. 861-884

並列摘要

Using the transmission and fluorescence modes at the National Synchrotron Light Source (NSLS) X-23A2 beam line, we have measured polarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for the (Bi1.85Pb0.15)Ca2.2Sr1.8Cu3O10+δ sample and unpolarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for Bi2Ca1.2Sr1.8Cu2O8+δ, Pb0.2Bi1.9Sr1.9Cu1O6=δ and (PbxBi2-x)Sr2Ca2Cu3Oy (x = 0, 0.2, 0.4) samples. All XAFS spectra were measured at room temperature. The incommensurate modulations in the Bi-0 layers are discussed in terms of the XAFS technique. The XAFS spectra of Bi L3-edge do not vary substantially for different sample orientation with respect to the X-ray polarization vector E. There is evidence for the existence within the Bi-Sr-Ca-Cu-0 system of Bi atoms having the oxidation number exceeding 3. Pb ions predominantly substitute for Bi ions in the Bi-Sr-Ca-Cu-0 system because there is no significant difference between the XANES spectra of the Pb and Bi L3-edges, respectively. The oxidation numbers of substituted Pb in Bi-Sr-Ca-Cu-0 system differ much from those of Pb & in PbO2 in which the oxidation number of all Pb atoms is 4. In agreement with other investigators, our results on the polarized XANES spectra of the Cu K-edge indicate that a satellite about 7 eV above the main K-XANES peak in both polarizations is a multielectron excitation to the final-state configuration 1s3d^9εp^1’, and that a shoulder on the rising absorption edge, present for many Cu compounds and generally assigned to a shake-clown multielectron excitation, is a transition to a final state of the Cu compounds. We have also deduced that the structure of the Cu-0 planes in the Bi-Sr-Ca-Cu-0 samples is ordered.