Crystallography Reports最新文献

The paper reports an analysis of surface morphology variation and cavity pattern formation in silicon single crystal induced by ion implantation and post-implantation annealing in different regimes. Critical implantation doses required to promote surface erosion are determined for samples subjected to post-implantation annealing and in absence of post-implantation treatment. For instance, implantation with helium ions to fluences below 3 × 10¹⁷ He⁺/cm² without post-implantation annealing does not affect the surface morphology; while annealing of samples implanted with fluences of 2 × 10¹⁷ He⁺/cm² and higher promotes flaking.

Triblock fluorocopolymer (terpolymer) is characterized by two levels of nano-organization. According to the X-ray diffraction and IR spectroscopy data, these levels of organization undergo changes induced by phase transitions. The first level of organization is formed mainly by nanostructures with sizes of 1.5–2 nm, whereas the predominant size of nanostructures at the second level of organization is ~40 nm according to small-angle X-ray scattering data. In particular, three types of nanostructures with sizes of 1.5, 56, and 130 nm and average interplanar distances of  5.11, 3.55, and 7.14 Å, respectively, formed in the terpolymer within 20 min at 170°С, which is ~120°С higher than the viscous flow temperature of the terpolymer. An increase in the time of thermal treatment to 90 min leads to a change in the structural parameters of nanostructures mainly of the last two types. The temperature-induced changes of the thermal expansion coefficient, the rigidity, and the mechanical loss tangent of the terpolymer are determined by the specific features of the complex nano-organization of the polymer.

The number of types of nanostructures 3–80 nm in size, found in ternary copolymers—fluorocopolymers Viton GFLT 600S and Viton GFLT 200S with a change in the their history, exceeds (according to the large-angle X-ray diffraction data) that found in double copolymers—SKF-26 and SKF-32. This process is affected more strongly by the chemical structure of spacings in macromolecules rather than the molecular weight. The complex and different character of change in the dynamic viscosity of fluorocopolymers with increasing temperature is caused by multiple phase transitions. It is shown that the fluoroelastomer SKF-32, in contrast to the three fluorocopolymers, does not pass to the viscous flow state up to ~190°С due to the presence of nanostructures of intermolecular type, 5 nm in size, whose strength is much higher than that of (3–4) nm nanostructures in the fluorocopolymers. It is found that the rotational mobility of the TEMPO radical is determined to a greater extent by the flexibility of passage chains, connecting ordered formations into a unified system, rather than the intermolecular distances in the disordered part of copolymers.

The features of the Bragg diffraction of coherent synchrotron radiation from the atomic lattice of a single crystal in the Laue geometry have been studied theoretically, provided that the radiation beam is limited by a relatively large slit placed in front of the crystal. The method of numerical simulation is used, and dependences of the intensity distribution are obtained for different crystal thicknesses. It is shown that the slit edges introduce inhomogeneous intensity distortions inside the Borrmann fan with an angle of 2θ_B, where θ_B is the Bragg angle. In the area where the triangles intersect the intensity distribution is similar to that for the diffraction from a slit in air at a certain (large) distance. An equation for the correspondence between the distance and crystal thickness is derived, which describes well the numerical calculation results.

The crystal structure of metallocarboxypeptidase T (CPT) from Thermoactinomyces vulgaris in complex with L-phenyl lactate was determined at 1.73 Å resolution. As opposed to pancreatic carboxypeptidase A, which binds one L-phenyl lactate molecule, the ligand in the complex with CPT occupies simultaneously the S1 and S' subsites of the active site. This leads to conformational changes, which differ from those caused by the alternating occupation of the S1 and S1' subsites by tert-butyloxycarbonyl-L-leucine (BOC-leucine) and benzylsuccinic acid. These changes concern the residues E277, E59, L254, G192, S127, and Y218 and are up to 0.77 Å. A conclusion was drawn about the possible role of the residue E59 in the substrate recognition and catalysis by carboxypeptidase Т.

The temperature dependence of the ionic conductivity of a Bi_0.94Ba_0.06F_2.94 solid electrolyte ceramic sample has been studied by impedance spectroscopy in the temperature range of 293–473 K. The ceramics obtained by the solid-state synthesis (873 K, 3 h) in a closed Cu ampoule is a heterovalent solid solution of tysonite structure (sp. gr. (Pbar {3}c1)) with the lattice parameters a = 7.1482(8) Å and c = 7.3279(5) Å. The room-temperature conductivity and its activation enthalpy are, respectively, σ_cer = 3 × 10⁻⁵ S/cm and ΔH_σ = 0.49 ± 0.05 eV. The ion-conducting properties of the isostructural Bi(_{{1 - y}})Ba_yF(_{{3 - y}}) and La(_{{1 - y}})Ba_yF(_{{3 - y}}) solid electrolytes with similar ionic radii of the matrix cations (1.17 and 1.16 Å for Bi³⁺ and La³⁺, respectively) have been compared. The conductivity of Bi_0.94Ba_0.06F_2.94 ceramics at 473 K exceeds the electrical conductivity of La_0.95Ba_0.05F_2.95 ceramics and single crystal by factors of 6 and 3.3, respectively.

A thin amorphous fluoroplast (polytetrafluoroethylene) film has been obtained and studied by methods of electron diffraction structure analysis. The previously developed method of constructing atomic radial distribution functions from electron diffraction patterns of amorphous structures, which is based on determining the normalization factor by varying the thermal parameter, was applied. The possibility of applying this technique is shown, and its necessary adaptation for the investigated polytetrafluoroethylene sample is performed with allowance for its not quite usual amorphous structure due to the rigid helical conformation of polymer chains …–CF₂–…, which does not allow us to speak about complete absence of the long-range order in the arrangement of atoms along the helix axis. Measurements were performed using the developed registration system on an EMR-102 electron diffractometer.

Polycrystalline films of organic–inorganic perovskite semiconductors are promising as a base for designing functional optical metasurfaces. The requirements to their structural quality, thickness uniformity, homogeneity, and absence of defects are much more stringent than those for perovskite films used in photovoltaics. The results of searching for the optimal conditions of one-stage synthesis of methylammonium-lead bromide films by spin coating are reported, and successful preparation of subwavelength optical gratings based on them using focused ion beam (FIB) processing is described. The measured light transmission spectra demonstrated excellent optical quality of the gratings and confirmed the possibility of forming semiconductor photonic metasurfaces with submicron periodicity and high-Q dielectric resonances.

This work presents the structural and morphological characterization and the results of studies of the luminescence and photocatalytic properties of  ZnO tetrapods synthesized by high-temperature pyrolysis. It was demonstrated that the morphology and structural parameters of  ZnO tetrapods are defined by the location in the synthesis zone. All samples are characterized by pseudo-three-dimensional morphology of tetrapods. The correlation was found between the luminescence properties and photocatalytic activity of tetrapods. The highest photodegradation rates of methylene blue under UV radiation were demonstrated by ZnO tetrapods grown in the zones closest to and farthest from the air-inlet window (the rate constants were 54 × 10^–3 and 50 × 10^–3 min^–1, respectively).

Superproton crystals obtained in the CsHSO₄–CsH₂PO₄–H₂O water‒salt system have been studied by electrical atomic force microscopy. Local I‒V characteristics for the Cs₃(HSO₄)₂(H₂PO₄) and Cs₄(HSO₄)₃(H₂PO₄) samples with different crystallographic orientations have been measured at 296 K, the conductivity anisotropy has been established, and the degree of dependence of the conductive properties on the sample composition has been demonstrated. Twin structures on cleavages have been considered, and their correlation with the atomic structure of monoclinic crystals have been investigated. Common features and differences between the atomic and real structures of the mixed crystalline phases and the effect of the hydrogen subsystems on their properties are discussed.