Resonant X-ray scattering and “forbidden” reflections in crystals with local anisotropy

Near the absorption edges there are different possibilities for appearance of additional X-ray reflections. These reflections are forbidden in the diffraction pattern far from the absorption edges and practically can be observed only with the synchrotron radiation. Several reasons for the appearance of those “forbidden” reflections are considered, among them: the anisotropy of the tensor of the susceptibility (ATS) in regular crystals, a modulation of the susceptibility tensor in the modulated crystals and the combined effect of magnetic ordering together with the local crystal anisotropy or the orbital ordering. The examples of crystal structures in which the listed effects can be observed are presented.