Abstract

We use x-ray absorption near-edge structure (XANES) spectroscopy to investigate metal valence in high-temperature superconducting oxides. The experimental results presented here were obtained at the Cu K-absorption edge of La2−xSrxCuO4(x = 0, 0.15) and YBa2Cu3Ox(x = 6.3, 6.5, 6.9) and at the Bi L3-absorption edge of Ba1−xKxBiO3(x = 0 → 0.5). We also have studied some standard oxides for comparison. In all three superconducting systems, transition temperatures are a function of x and reach a maximum for those values of x suggestive of nonintegral valences of the metal ions. Our results show that changes in the O content of YBa2Cu3Ox produce significant changes in the average charge environment of these Cu atoms, as reflected in K-edge XANES. In contrast, we find no significant changes in the charge environment of Cu in La2−xSrxCuO4 or of Bi in Ba1−xKxBiO3, as indicated by our XANES results for various values of x. We discuss the question of the metal-ion valence and its relation to superconductivity.

© 1989 Optical Society of America

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