ИСТИНА

Resonant X-ray diffraction (RXS) has been proved to be a method sensitive to the various kinds of ordering in crystals: magnetic, orbital etc. The enhancement of the resonant X-ray scattering amplitude near absorption edges occurs when the energy of the incident radiation approaches the value required to excite an inner-shell electron into an empty states, which are strongly affected by various anisotropic factors. If the degeneration of the valence-electron states is broken down owing to these factors, it provides the environment-induced anisotropy of resonant scattering. In most cases, the contribution of the resonant effects are rather small in comparison with conventional X-ray scattering. However, this contribution is anisotropic and strongly enhanced in the resonant regions at absorption edges of atoms, where it depends on the frequency and polarization of diffracting beam. Owing to this, it is very helpful to observe the small effects, the so-called ‘‘forbidden’’ reflections, which are absent in conventional X-ray diffraction due to symmetry elements with translational components. The interference between various resonant scattering channels makes the energy spectra of the forbidden Bragg reflections especially sensitive to the details of local properties of a crystal. This lecture presents a survey of recent RXS studies of charge and orbital ordering, phase transitions, thermal vibrations, magnetic moments orientation, Dzyaloshinskii-Moriya interaction and other local properties of a substance.