A. S. Avilov

X-ray and Electron Diffraction Study of MgO

Precise X-ray and high-energy transmission electron diffraction methods were used for the study of electron density and electrostatic potential in MgO crystals. The structure amplitudes were determined and their accuracy estimated using ab initio Hartree-Fock structure amplitudes as criteria. The electrostatic potential distributions, reconstructed using Fourier series from both X-ray and electron diffraction data, are in satisfactory mutual agreement and are similar to the theory. They, however, suffer from restricted experimental resolution and, therefore, the reconstruction of the electrostatic potential via an analytical structural model is preferable. The model of electron density was adjusted to X-ray experimental structure amplitudes and those calculated by the Hartree-Fock method. The electrostatic potential, deformation electron density and the Laplacian of the electron density were calculated with this model. The critical points in both experimental and theoretical model electron densities were found and compared with those for procrystals from spherical atoms and ions. A disagreement concerning the type of critical point at (,,0) in the area of low, near-uniform electron density is observed. It is noted that topological analysis of the electron density in crystals can be related with a close-packing concept.

Scanning system for high-energy electron diffractometry

A new electron diffractometer with a diffraction-pattern scanning system in front of a fixed counter has been developed. Significant improvement was achieved in the measured diffraction intensities by using fast electronics and additional control of the stability of the electron beam. The measurement of and accounting for the gear-frequency characteristic of the registration system was performed, and the signal accumulation mode for intensity measurements together with advanced statistical data processing were employed. Good agreement between the experimental and Hartree–Fock structure factors for LiF, NaF and MgO was achieved (to avoid strong extinction effects, rather thin polycrystalline films were used as samples).

Mechanochemical synthesis of nonstoichiometric nanocrystals La1 − yCayF3 − y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

The nonstoichiometric phases La1 − yCayF3 − y (y = 0.15, 0.20) with a tysonite (LaF3) structure have been prepared for the first time by the mechanochemical synthesis from CaF2 and LaF3 crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF2-LaF3 system. The “mechanohydrolysis” of the La1 − yCayF3 − y phase has been observed for the first time. Under these conditions, the La1 − yCayF3 − y phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La1 − yCayF3 − y nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 108 Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10−4 S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials.

Crystal structure refinement from electron diffraction data

A procedure of crystal structure refinement from electron diffraction data is described. The electron diffraction data on polycrystalline films are processed taking into account possible overlap of reflections and two-beam interaction. The diffraction from individual single crystals in an electron microscope equipped with a precession attachment is described using the Bloch-wave method, which takes into account multibeam scattering, and a special approach taking into consideration the specific features of the diffraction geometry in the precession technique. Investigations were performed on LiF, NaF, CaF2, and Si crystals. A method for reducing experimental data, which allows joint electron and X-ray diffraction study, is proposed.

Multipole analysis of the electron density and electrostatic potential in germanium by high-resolution electron diffraction

Accurate electron structure factors measured by significantly improved transmission electron diffraction technique were used in a high-resolution quantitative study of the electron density and electrostatic potential in Ge polycrystalline sample. The parameters of the multipole model adapted for electron diffraction were found and topological features of the electron density and electrostatic potential were determined with this model.

Effect of methylene blue modification on the structural, morphological, and photocatalytic properties of nanosized η-TiO2

The structural and morphological features of samples containing titanium dioxide, obtained by the sulfate method in the presence of methylene blue dye MeB (sample with η-TiO2) and in its absence (sample with anatase), have been determined using a complex of analytical methods. For the sample with η-TiO2, the sizes of titanium dioxide crystallites (5.0(2) nm) are smaller than the nanoparticle sizes (5.5–9.0 nm), which does not exclude their defect structure. This sample exhibited a higher tendency of nanoparticles to agglomeration (the presence of strong bonds between nanoparticles) and a low tendency to aggregation (the presence of weak bonds). It is shown that a MeB-modified sample with η-TiO2 has high photocatalytic activity in the visible spectral range in the model reaction of methyl orange dye decomposition.

Methods for studying the coherent 4D structural dynamics of free molecules and condensed state of matter

Studies in the coupled 4D spatial and temporal continuum are necessary for understanding the dynamic features of molecular systems with a complex profile of the potential energy surface. The introduction of time sweep into diffraction methods and the development of principles for studying coherent processes have revealed new approaches to the analysis of the dynamics of wave packets, the intermediate products and the transition state of the reaction center, and short-lived compounds in gaseous and condensed media. The use of picosecond and femtosecond electron probe pulses, synchronized with excitation laser pulses, determined the development of ultrafast electron crystallography, time-resolved X-ray diffraction, and dynamic transmission electron microscopy (DTEM). One of the most promising applications of the developed diffraction methods is the characterization and visualization of the processes occurring upon the photoexcitation of free molecules and biological objects and the analysis of surface and thin films. The whole set of spectral and diffraction methods based on different physical principles, which are complementary and make it possible to perform the photoexcitation of nuclei and electrons and carry out diagnostics of their dynamics at ultrashort time sequences, reveal new possibilities for studies with the necessary integration of the “structure-dynamics-function” triad in chemistry, biology, and materials science.

Precision Electron Diffraction Structure Analysis and Its Use in Physics and Chemistry of Solids

The present level of the precision electron diffraction provides the quantitative analysis of the electrostatic potential and electron density in crystals and allows us to approach the direct study of properties of solids by electron diffraction. This is demonstrated by examples of ionic compounds with an NaCl structure and a covalent Ge crystal. Using the analytical structure models of crystals, one can quantitatively characterize chemical bonding and study topological characteristics of the electrostatic potential by electron diffraction data. It is established that the internal crystal field is well structurized, whereas the topological analysis revealed some important characteristics of its structure. These data considerably enrich our knowledge on atomic interactions in crystals.

Electron diffraction study of ordering in the Er0.715Ca0.285F2.715 tysonite phase

The structure of Er0.715Ca0.285F2.715 crystals as grown from the melt (without heat treatment) has been studied by electron microscopy. It is found that the structure is inhomogeneous, consisting of an ordered tysonite matrix with the hexagonal crystal structure (supercell reflections exhibit the vector 1/8〈203〉 in the units of small tysonite cell), in which crystallites of the other laminar phase with layers ∼10 Å thick are incorporated.

Effect of high-energy electron irradiation in an electron microscope column on fluorides of alkaline earth elements (CaF2, SrF2, and BaF2)

The effect of high-energy (150 eV) electron irradiation in an electron microscope column on crystals of fluorides of alkaline earth elements CaF2, SrF2, and BaF2 is studied. During structural investigations by electron diffraction and electron microscopy, the electron irradiation causes chemical changes in MF2 crystals such as the desorption of fluorine and the accumulation of oxygen in the irradiated area with the formation of oxide MO. The fluorine desorption rate increases significantly when the electron-beam density exceeds the threshold value of ∼2 × 103 pA/cm2). In BaF2 samples, the transformation of BaO into Ba(OH)2 was observed when irradiation stopped. The renewal of irradiation is accompanied by the inverse transformation of Ba(OH)2 into BaO. In the initial stage of irradiation of all MF2 compounds, the oxide phase is in the single-crystal state with a lattice highly matched with the MF2 matrix. When the irradiation dose is increased, the oxide phase passes to the polycrystalline phase. Gaseous products of MF2 destruction (in the form of bubbles several nanometers in diameter) form a rectangular array with a period of ∼20 nm in the sample.