Resonant elastic X-ray scattering in chemistry and materials science

Abstract

Abstract. The applications of anomalous scattering for locating metal atoms and discriminating between different elements has increased when optimised with synchrotron X-radiation.The on-resonance effect enhances the targeted elemental signal and allows small occupancies to be determined, including in situations of a mixed metal population at a single atomic site. Thus the applications of resonant elastic X-ray scattering in biological, inorganic and materials chemistry is being widely applied to single crystals, which is our emphasis, but also powders, fibres, solutions, amorphous and thin film states of matter. Recent developments have included the use of high photon energies (upto 100 keV) as well as softer X-rays (2 keV). The various instrument and technical capabilities have improved in the last 15 years. This ease of measurement of the resonant scattering signals along with absorption edge shifts indicates an expansion to the measurement of multiple data sets, to allow monitoring of redox changes. Whilst crystal structure determination in biological crystallography has been revolutionised by the MAD method, it is not a requirement for chemical or materials crystallography, as other phasing techniques are routine. Synchrotron source upgrades will allow nano-sized X-ray beams to be more widely available. The new X-ray lasers suggest new capabilities too.

Resonant X-ray scattering of biological assemblies

“…For example, resonant powder diffraction, diffraction anomalous fine structure (DAFS), anomalous small-angle X-ray scattering (ASAXS), grazing incidence diffraction with multiwavelength diffraction methods, and grazing incidence small angle scattering near resonances (A+GISAXS) have all benefited from advances in X-ray instrumentation 27,34,35 . Recent developments in synchrotron radiation have also enabled resonant X-ray scattering and reflectivity experiments in the soft and tender X-ray regimes.…”

Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques

“…Indeed, synchrotron sources uniquely allow the energy adjustment of the incident X-ray beam, enabling detailed analysis of the scattering process in proximity of selected absorption edges. Hence, exploiting the resonant (or anomalous) diffraction process, element-specific structural sensitivity can be obtained [138,[227][228][229][230][231]. The relevance of these methods in chemical crystallography will be discussed in more detail in Section 2.7.…”

“…Tuning the incident wavelength in the closest proximities of the edge (resonant conditions), the incident X-ray photons can interact with the bound inner electrons of the correspondent atom, be absorbed, and give photoelectric effect. Importantly , the anomalous effects near an absorption edge for an atom contained in the sample, selectively enhance the contribution of that atom to the elastically scattered intensity [138,228,230,[544][545][546]. This phenomenon can be exploited in a variety of cases.…”

“…crystal analysers with an enlarged mosaicity [229]. Applications span a variety of research areas in solid state chemistry [138,[228][229][230]564], including the discrimination of Zn and Ga in microporous materials [565] or of Pb and Bi in minerals [566], the investigation of the distribution of cations in multi-cationic zeolites [229,567,568], substitution of Cu by Fe, Co, Ni, and Zn in high-critical-temperature superconductor cuprates [556]. A few recent applications of contrast-mode anomalous X-ray diffraction specifically deal with the structural determination of coordination compounds, including multinuclear metal complexes [569][570][571] and MOFs [572].…”