Comparative Study of the Polarized XANES at the Cu K Edge of Bi₂Sr₂Ca₁Cu₂O₈, Bi₂Sr₂Ca₂Cu₃O₁₀, and Y₁Ba₂Cu₃O₇ Thin Films

Abstract

The angular dependence of the Cu K edge in films of BiSrCaCuO (2212, 2223) and YBa2Cu3O7 are measured for both orientations (e ⟂ c, e ∥ c). A similar XANES is observed (but magnitudes and energy positions change slightly) although the compounds are quite different except the first shell. Specifically, anisotropy spectra are very close. After connecting the same XANES features to the local arrangement, it is found that the pyramidal site in 2223 is flattened compared to the one in 2212 by only (0.003 ± 0.001) nm. In addition, it is deduced that the electron density in the copper sphere is similar in all compounds whatever the site (CuII). Then, taken into consideration that pyramidal sites in 2212 and in YBaCuO are rather close to each other, an estimate is deduced of the absorption edge arising from the chain site following the two polarization directions. The major resonance on these spectra yields 0.009 nm of distance anisotropy, a result in very good agreement with the XRD data. In the course of the discussion, it is explained why the anisotropy spectrum is smaller in YBaCuO than in 2212. Lastly, the difference spectrum published by Tranquada et al. on oxygen reduction of YBa2Cu3Ox is revisited. It is deduced that the electron density increases in the copper sphere as soon as x = 7 when the copper atoms in the chains change from four to three oxygen coordination. Then, when the local structure evolves from three to two oxygen coordination, the local valence on the copper sphere does not change anymore but on the bridging oxygen.