X-ray resonant powder diffraction

X-ray resonant diffraction can be applied in structural chemistry studies on powder samples. It enables an important limitation of powder diffraction to be overcome. This limitation is related to the low ability of powder diffraction to differentiate elements with close atomic numbers when they occupy the same or close crystallographic sites (mixed occupancy case) and also to discriminate cations with different valence states in different sites. However the resonant effect usually has a second order influence on the measured intensity. As a consequence, the efficiency of this method directly implies the need for excellent quality data collection and has generally been better assessed on elements present in single phase powder samples. In recent years, instrumental developments have been made in synchrotron radiation facilities which allow easier use of resonant powder diffraction for site-specific contrast and valence i.e. oxidation state analyses. Moreover, resonant contrast diffraction tools also have been proposed for better visualization of the anomalous effect both in direct and reciprocal space by using differences between electron density maps or diffraction patterns. Finally the potentialities of this technique for de novo structure solution on macromolecular systems are mentioned.