Modeling of the Thermal-Motion-Induced Effects in Resonant X-ray Diffraction Observed for Ge and ZnO

Diffraction of X-rays near absorption edges of atoms (resonant diffraction) is considered theoretically, modeled with modern computer programs, and compared with available experimental data. The process of resonant diffraction includes virtual excitation of an electron from an inner shell to empty electronic states. Therefore this process is very sensitive to atomic environment and to electronic and phonon properties of crystals. In particular, the X-ray scattering amplitude becomes an anisotropic tensor with the symmetry corresponding to the temporal local symmetry of atomic position. The most spectacular result of this anisotropy is the excitation of additional X-ray reflections otherwise forbidden by screw-axis or glide-plane symmetry of crystals. In comparison with EXAFS, the resonant diffraction is more sensitive to the symmetry of atomic environment. For example, the forbidden reflections can be caused by opposite chirality of atomic positions in centrosymmetric crystals or by phonon displacements of atoms. The presented here modeling of the thermal-motion-induced (TMI) reflections observed in Ge and ZnO demonstrates remarkable agreement between simulations and experimental data.