A. P. Oreshko

Numerical simulation of experimental resonance absorption spectra and diffraction of synchrotron radiation in yttrium iron garnet

Extended X-ray absorption fine-structure spectra of yttrium-iron garnet, measured near the absorption edges of iron and yttrium at the Kurchatov synchrotron radiation source are reported. Numerical simulation of the X-ray absorption near-edge structure of these spectra is performed and good agreement with the experimental data is obtained. It is shown theoretically that “forbidden” Bragg reflections can be observed near the absorption edges of iron and yttrium. The indices of these reflections are determined. The energy structure of the “forbidden” reflections 006 and 110 is calculated.

Study of the amorphization of surface silicon layers implanted by low-energy helium ions

The structural changes in surface layers of Si(001) substrates subjected to plasma-immersion implantation by (2–5)-keV helium ions to a dose of D = 6 × 1015–5 × 1017 cm–2 have been studied by highresolution X-ray diffraction, Rutherford backscattering, and spectral ellipsometry. It is found that the joint application of these methods makes it possible to determine the density depth distribution ρ(z) in an implanted layer, its phase state, and elemental composition. Treatment of silicon substrates in helium plasma to doses of 6 × 1016 cm–2 leads to the formation of a 20- to 30-nm-thick amorphized surface layer with a density close to the silicon density. An increase in the helium dose causes the formation of an internal porous layer.

Absolute Intensity and Phase of the Resonant X-ray Scattering from a Germanium Crystal

The 222 and 600 reflections near the germanium absorption K edge were studied on the Kurchatov synchrotron radiation source. The energy spectrum of the 222 reflection is caused by the interference of the weak nonresonant and purely resonant contributions to the tensor atomic factor, whereas the 600 reflection is purely resonant. The energy dependence of the magnitude and phase of the resonant contribution to the scattering amplitude was determined from a change in the interference pattern. The numerical simulation of the energy spectra of reflections with the inclusion of the dipole-quadrupole and thermally induced contributions shows that the latter is dominant at room temperature.

X-Ray Natural Circular Dichroism in Langasite Crystal at the Ga and La Edges

X-ray natural circular dichroism (XNCD) and its structural nature have been investigated in a langasite (La3Ga5SiO14) crystal at an incident radiation energy close to the Ga K-absorption edge and the La L2,3-absorption edges. An XNCD signal was observed mainly beyond the absorption edge, which confirmed the existence of delocalized mixed p–d electronic states in Ga and d–f and d–p electronic states in La. Calculations with application of the multiple scattering method have made it possible to separate the contributions from three crystallographically nonequivalent Ga sites to the absorption spectrum and the XNCD signal and explain adequately the largest contribution of the Ga atom occupying the 1a site to the XNCD signal.

X-ray natural circular dichroism in copper metaborate

The local electronic structure of copper ions in a copper metaborate CuB2O4 crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB2O4 to the absorption and X-ray circular dichroism spectra of the crystal.

Proton configurations in the hydrogen bonds of KH2PO4 as seen by resonant x-ray diffraction

KH2PO4 (KDP) belongs to the class of hydrogen-bonded ferroelectrics, whose paraelectric to ferroelectric phase transition is driven by the ordering of the protons in the hydrogen bonds. We demonstrate that forbidden reflections of KDP, when measured at an x-ray absorption edge, are highly sensitive to the asymmetry of proton configurations. The change of average symmetry caused by the “freezing” of the protons during the phase transition is clearly evidenced. In the paraelectric phase, we identify in the resonant spectra of the forbidden reflections a contribution related to the transient proton configurations in the hydrogen bonds, which violates the high average symmetry of the sites of the resonant atoms. The analysis of the temperature dependence reveals a change of relative probabilities of the different proton configurations. They follow the Arrhenius law, and the activation energies of polar and Slater configurations are 18.6 and 7.3 meV, respectively.

Ab initio simulation of resonant forbidden reflections in Ge crystals

Forbidden reflections are observed in the case of diffraction of synchrotron radiation with wave-lengths close to the absorption edges in crystals. A new method for calculating the intensity of thermal-motion-induced (TMI) forbidden reflections is proposed in this paper. It includes two stages: simulation of instantaneous thermal atomic displacements using ab initio molecular dynamics and subsequent quantum-mechanical calculations of the resonance scattering amplitude for various configurations. This procedure is used to calculate the temperature dependence of the 600 reflection intensity for Ge. The proposed method for simulating forbidden TMI reflections is suitable for any crystal structures and can explain many results so far obtained using synchrotron.

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 o...

Dynamic theory of resonant X-ray Bragg diffraction in perfect crystals

An exact solution is found to the problem of mirror reflection and resonant Bragg diffraction of X rays in a perfect crystal. An anomalous transmission of radiation with the energy close to the K-edge absorption of the atoms is shown to be feasible.

X-ray specular reflection under conditions of extremely asymmetric noncoplanar diffraction from a bicrystal

The angular dependence of the intensities of X-ray specular reflection has been rigorously analyzed under conditions of noncoplanar grazing Bragg diffraction in a crystal coated with a crystalline film (bicrystal). It is shown that the anomalous angular dependence of the specular-reflection intensity is extremely sensitive to the thickness (from fractions of a nanometer up to several nanometers), deformation, and the amorphization degree of the crystalline films. The optimum conditions for recording intensities are attained at grazing angles equal to 1.5–4.0 of the critical angle of the total external reflection.