Anisotropic X-ray anomalous scattering by incommensurately modulated crystals

The possibility of the ATS (anisotropy of the tensor of susceptibility) ref lections appearance in X-ray anomalous anisotropic diffraction near the abs orption edges is considered for the crystals with incommensurate modulation . The superspace groups are used to describe symmetry properties of the sus ceptibility tensor for incommensurately modulated crystals. It is shown tha t the modulation can change both the resonant spectra and the tensorial pro perties of susceptibility. No ATS reflections may appear in the incommensur ately modulated crystals owing to the modulation of the resonant spectra, b ut the modulation of the angular part of the susceptibility tensor can lead to the appearance of the ATS main reflections and ATS satellites. The inco mmensurate modulation can violate the reflection extinctions which correspo nd to the glide planes, the screw axes and to the centering translations. T he possibility of the ATS reflections near the K-edge of Ni in the diffract ion pattern of La2NiO4+x is considered. (C) 1999 Elsevier Science S.A. All rights reserved.