Crystallography Reports最新文献

Millimeter-sized crystals of sodium hexafluorosilicate Na₂SiF₆ have been grown by the hydrothermal method. X-ray diffraction analysis revealed that Na₂SiF₆ samples are twinned according to the merohedral law and crystallize in the sp. gr. Р321 with the following unit-cell parameters at 295 K: 〈a〉 = 8.8582(12) Å, 〈c〉 = 5.0396(11) Å, 〈V〉 = 342.47(17) ų (averaged results of repeated measurements). A multitemperature diffraction study of Na₂SiF₆ was performed; the results obtained were used to calculate the temperature dynamics of the optical properties of crystals. Structural similarity was revealed between Na₂SiF₆ crystals and crystals of the langasite family La₃Ga₅SiO₁₄. This made it possible to explain the optical activity of Na₂SiF₆ by considering the electron density helices similar to those in langasite, twisted around the threefold symmetry axis passing through the origin of the Na₂SiF₆ cell. The kinks in the temperature dependences of the refractive index and rotation of the plane of polarization of light are explained taking into account the anomalous features of interatomic interactions along the threefold axis of the unit cell, passing through the Si2(2d) site with the coordinates (1/3, 2/3, z). It was found that main factor affecting the temperature dynamics of optical parameters is the Si2(2d)–F2(6g) distance, which increases abnormally upon cooling.

The molecular dynamics of two types of lysozyme octamers was simulated under crystallization conditions with the MARTINI coarse-grained force field. A comparative analysis of the obtained results and the simulation data for the same octamers modelled with the Amber99sb-ildn all-atom force field showed that octamer A is more stable compared to octamer B in both force fields. Therefore, the results of molecular dynamics simulations of octamers using both force fields are consistent with each other. Despite some differences in the behavior of the protein in different force fields, they do not affect the validity of the data obtained using MARTINI. This confirms the applicability of the MARTINI force field for studying crystallization solutions of proteins.

The results of computer simulation of images of gas pores in a silicon carbide crystal in section topograms, i.e., during diffraction of a narrow X-ray beam in the crystal, are reported for the first time. The simulation was performed using a special module of the universal computer program XRWP. This program is being developed by the author to calculate the effects of coherent X-ray optics. The calculation method combines two previously known methods, specifically, the Fourier transform method (Kato method) and the method of solving the Takagi–Taupin equations. It is shown that gas pores can produce a wide variety of images, depending on the experimental conditions and the pore position inside the crystal.

Triethylenediaminium ethylenediaminetetraacetatozincate trihydrate ([HN(CH₂CH₂)₃NH]ZnL⋅ 3H₂О) and tetramethylethylenediaminium ethylenediaminetetraacetatozincate dihydrate ([H(CH₃)₂NCH₂CH₂N(CH₃)₂H]ZnL⋅2H₂О) were synthesized by the reaction of zinc oxides with ethylenediaminetetraacetic acid and then with its triethylenediaminum or tetramethylethylenediaminium salts. The synthesized compounds were isolated from aqueous solutions or organic solvents as fiber-like, cubic, and pyramidal crystals, which were characterized by elemental analysis, scanning electron microscopy, IR spectroscopy, thermogravimetry, and differential scanning calorimetry.

The results of investigation of DNA-protein complexes by two independent structural methods, namely, molecular dynamics (MD) and small-angle X-ray scattering (SAXS), were compared. Molecular dynamics is a computational method enabling the visualization of the behavior of macromolecules in real media, which is based on the laws of physics, but it is limited by numerous simplifications. Small-angle X-ray scattering is an X-ray method, which allows the reconstruction of the three-dimensional structure of a system in solution from a one-dimensional small-angle scattering profile, but it faces the problem of ambiguity in solving inverse problems. The use of structural characteristics of the complexes determined by SAXS for the validation of 3D structural models generated by MD simulations made it possible to significantly reduce the ambivalence of theoretical predictions and demonstrated the efficiency of a combination of MD simulations and SAXS for solving problems of structural biology.

A new concept into crystallography: the polarity of the reticular faces of a crystal. Reticular faces are divided into basal and additional. Basal faces, being systematic in a face system, increase only as the crystal grows. The shape of a crystal is invariant, stable within the limits of its stability, and does not affect its equilibrium state. Additional faces, on the contrary, shrink and disappear, and therefore are variable extensive parameters of the face system. They do not form their own system and associate with the basal faces, determining the variation of the face system. The polar system is nonequilibrium; it spontaneously and irreversibly transforms into the final basic face system, and is in quasi-static equilibrium. The habit of a basic crystal of an invariant shape changes during its growth according to the translational symmetry mechanism.

The nature of increasing the strength of disordered two-component solid solutions in comparison with materials consisting of atoms of one component is studied. For this purpose, the contribution of extreme fluctuations in the distribution of solution atoms, which create obstacles for the movement of dislocation kinks, is calculated. It is shown that the slow (power-law) decrease in the probability of large delays on such obstacles leads to anomalous kinetics of kinks. It is accompanied by a slowdown of dislocation motion. This may be the reason for the hardening of the material.

The temperature properties of the incredible high performance (I.H.P.) structures on a multilayer lithium tantalate/silicon dioxide film/silicon substrate have been studied; these structures are used to improve the characteristics of devices on surface acoustic waves. Test structures have been simulated by the finite-element method in the COMSOL software and the temperature coefficient of frequency has been calculated. The calculated transfer coefficients of a resonator filter on a conventional single-crystal 36°YX-cut lithium tantalate substrate and an I.H.P. filter are compared at different temperatures. It is shown that the temperature coefficient of frequency can be minimized by fitting the thicknesses of the substrate layers. A comparison shows that the results obtained are in good agreement with the known data. The practical importance is that the simulation results and calculated parameters can be used for designing devices of different classes based on multilayer substrates (including those with I.H.P. structures).

IR light-emitting diodes (LEDs) based on InGaAs/AlGaAs multiple quantum wells (MQWs) and Al_xGa(_{{1-x}})As_yP(_{{1-y}}) layers, compensating stress in the active area, have been developed. The optical losses caused by absorption of the radiation generated by the active area (λ = 940 nm) have been investigated at different doping levels of n-GaAs substrates. It is shown that reduction of the donor doping level from 4 × 10¹⁸ to 5 × 10¹⁷ cm^–3 gives an increase in the LED quantum efficiency of ~30%. A technology making it possible type to eliminate completely the optical losses caused by absorption during radiation output has been developed. Removal of the substrate and transfer of the device structure to a carrier substrate with formation of a rear metal reflector made it possible to create LEDs demonstrating a twofold increase in the external quantum efficiency (EQE) and efficiency (~40%) as compared to the technology of radiation output through an n-GaAs substrate.