Discussion of x-ray-absorption near-edge structure: Application to Cu in the high-TcsuperconductorsLa1.8Sr0.2CuO4andYBa2Cu3O7

Abstract

A model for interpretation of x-ray-absorption near-edge structure (XANES) in transition-metal compounds is presented based on \ensuremath{\lambda}=R backscattering from nearby atoms. The model allows identification of XANES features with specific lattice distances. Reference to spectra from model compounds of simple structure allows construction of a calibration curve of \ensuremath{\Delta}E vs R from 1.5 to 3.5 A\r{} with an accuracy of \ifmmode\pm\else\textpm\fi{}0.1 A\r{}. Specific multiscattering paths were also identified in M${\mathrm{O}}_{4}$ molecules. The Cu K spectra of ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ were examined using the model. A unique feature in the spectra was identified with a (Cu---O) bond distance of \ensuremath{\sim}2.6 A\r{} and confirmed with analysis of the extended x-ray-absorption fine structure. This distance is evidence of antisite disorder in these compounds with 0.2--0.3 mole fraction of Cu substituting into the Y or La site. We also find evidence of simultaneous ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cu}}^{3+}$ valences. The structural implications of these findings are discussed.