RESONANT POWDER X-RAY-DIFFRACTION APPLIED TO MIXED-VALENCE COMPOUNDS AND THE POSSIBILITY OF SITE-RESOLVED X-RAY ABSORPTION-SPECTROSCOPY ILLUSTRATED FOR YBA2CU3O6.27

Synchrotron X-ray sources enable resonant diffraction experiments to be performed at energies very close to elemental absorption edges. This gives rise to very large anomalous scattering effects, which may differ significantly between different valence states of the resonant element. The relationships between the X-ray absorption spectrum and the anomalous scattering coefficients are described, and recent resonant powder diffraction studies of the mixed valence compounds EU3O4, Ga2Cl4, and YBa2Cu3O(x), and the valence fluctuations in alpha-Ce are reviewed. These results establish powder X-ray diffraction as a tool for studying electronic structure as well as the spatial electron distribution. Such experiments also give rise to the possibility of using resonant diffraction to determine site-resolved X-ray absorption spectra, and a simple method is outlined. This has been applied to polycrystalline YBa2Cu3O6.27 and the resolved K edge spectra for the two Cu sites are in qualitative agreement with other results for the valence distribution in this system.