Structure and valence from complementary anomalous X-ray and neutron powder diffraction

Abstract

We introduce a new type of crystallography whose major aim is to study the crystal chemistry and the electronic structure of constituent atoms or ions in crystals, rather than the crystal structure itself. These results are obtained from recent experiments that take advantage of the complementary nature of anomalous X-ray and neutron diffraction data, and of the fact that anomalous scattering can provide substantial changes in the scattering factors for atoms or ions with similar atomic number Z or with the same Z but different valence. Examples presented include: the determination of constituent ordering among several sites in Fe/Co/V alloys and in certain high temperature superconductors; the study of ordering that arises via valence fluctuation in metallic cerium; the determination of anomalous X-ray scattering factor variation for copper with oxidation state; and finally, the determination of site-specific valence of the Cu ions in the chain and plane sites in the high temperature superconductor YBa 2Cu 3O 6+ x as a function of oxygen stoichiometry.