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 La_2−xSr_xCuO₄(x = 0, 0.15) and YBa₂Cu₃O_x(x = 6.3, 6.5, 6.9) and at the Bi L₃-absorption edge of Ba_1−xK_xBiO₃(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 YBa₂Cu₃O_x 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 La_2−xSr_xCuO₄ or of Bi in Ba_1−xK_xBiO₃, 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

Extensive study of the optical spectra for high-temperature superconducting oxides and their related materials from the infrared to the vacuum-ultraviolet energy region

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