VALENCE AND COORDINATION OF CU IONS IN HIGH-TC SUPERCONDUCTOR BAXY1-XCUO3-Y

Cu K near-edge and extended X-ray absorption fine structure (EXAFS) have been measured for 90 K superconductor Ba//xY//1// minus //xCuO//3// minus //y with x equals 0. 6. The Fourier analysis of EXAFS shows that Cu ions are coordinated with basal plane oxygens having the average Cu-O distance of 1. 97 Angstrom. Determined average oxygen coordination number of ca. 4. 8 suggests the presence of axial oxygens with a similar Cu-O distance. The near-edge spectra provide an evidence for the mixed valence character of Cu ions in Ba//xY//1// minus //xCuO//3// minus //y between predominalty divalent states and trivalent states. The strong electron-phonon interaction may be further enhanced by a charge-coupled motion of oxygens induced by the Cu valence fluctuation.