Discussion of x-ray-absorption near-edge structure: Application to Cu in the high-Tc superconductors La1.8Sr0.2CuO4 and YBa2Cu3O7

Abstract

A model for interpretation of x-ray-absorption near-edge structure (XANES) in transition-metal compounds is presented based on λ=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 ΔE vs R from 1.5 to 3.5 Å with an accuracy of ±0.1 Å. 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 ∼2.6 Å 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.