Determination of valence and cation distributions by resonant powder X-ray diffraction

Abstract

X-RAY crystallographic methods are important in determining the ordered atomic structures of matter from single-crystal or powder¹diffraction data. But because the scattering factor depends on atomic number, it is not always possible to distinguish between atoms with very similar numbers of electrons, such as neighbouring elements or different valence states of the same element. By selecting an X-ray wavelength at an elemental absorption edge, the scattering power of that element is drastically altered by anomalous dispersion. Here I show that the resulting resonant X-ray diffraction data can provide information about valence states and cation distributions. I use powder data collected at the europium Lm edge using the SERC Daresbury Synchrotron Radiation Source to study the valence distribution in Eu₃O₄ and to determine the composition and cation and valence ordering in a new compound, EuSm₂O₄. Such resonant X-ray diffraction methods should be applicable to almost all elements heavier than calcium for contrasting different electronic states of the resonant element and for differentiating it from neighbouring elements.