Optimization of Azimuthal Scan Procedure for Absolute Structure Determination

Abstract

The paper describes the application of the azimuthal scan technique for absolute structure determination using X-ray anomalous dispersion. A diffraction pattern with rotation around the scattering vector provides information about various anisotropic effects: absorption, extinction and multi--beam interaction. Recording of azimuthal scans for Friedel pairs is proposed as an alternative to single reflection measurement. Several factors affecting the smoothness of the azimuthal scan curves are discussed and a procedure for eliminating various sources of errors, involving a Fourier filtering, is proposed in order to increase feasibility of chiral discrimination.

Absolute Structure Determination in Absence of Heavy Scatterers

“…[41]. The analysis of azimuthal scan recordings for the Bijvoet pairs of several crystal samples with weak anomalous scatterers clearly shows the reason why usually not all individual signs of the Bijvoet inequalities between intensities measured at one discrete ψ value are consistent with the model predictions.…”

Identification of inversion domains in KTiOPO4viaresonant X-ray diffraction

“…The parameter is then considered as variable during the least-squares refinement of the crystal structure, alongside the atomic coordinates. In those cases in which the basic crystal structure is known and only the inversion domain is to be determined, similar to the case that we present here, other approaches have sought to restrict the measurement to those reflection pairs for which the discrepancy from the Friedel law is more pronounced (Page et al, 1990;Grochowski, 1997;Grochowski & Serda, 1997). Unfortunately, these methods presuppose collecting the intensities of reflections opposite to each other from the same scattering volume, which may not be feasible in crystals whose thickness is several mm, due to beam attenuation; even in the case of small crystals, the refinement required by Flack & Bernardinelli (1999) relies on an often subtle contrast between diffraction intensities in different geometries, that may be influenced by shape effects and be susceptible to systematic errors due to absorption or anisotropic extinction.…”