Vol 61, No 3 (2016)

Three tetragonal, partially occupied oxygen sublattices with a translational symmetry higher than the lattice translational symmetry can be introduced into the cubic perovskite structure. This higher symmetry is determined in the 9D space constructed on three orthogonal 3D subspaces corresponding to the sublattices. The generating matrices, metric tensors, and point symmetry matrices of this multidimensional space are determined. Irreducible stars of the representations of multidimensional space group are projected onto the irreducible stars of the space crystal group.

Nonlinear effects accompanying the propagation of high-intensity beams of X-ray free-electron lasers are considered. It is shown that the X-ray wave field in the crystal significantly changes due to the formation of “hollow” atomic shells as a result of the photoelectric effect.

A new approach to the study of the structural quality of crystals is proposed. It is based on the use of X-ray standing-wave method without measuring secondary processes and considers the multiwave interaction of diffraction reflections corresponding to different harmonics of the same crystallographic reflection. A theory of multiwave X-ray diffraction is developed to calculate the rocking curves in the X-ray diffraction scheme under consideration for a long-period quasi-one-dimensional crystal. This phase-sensitive method is used to study the structure of a multilayer lead stearate film on a silicon substrate. Some specific structural features are revealed for the surface layer of the thin film, which are most likely due to the tilt of the upper layer molecules with respect to the external normal to the film surface.

A sample of holotypic calcium catapleiite from the Burpala alkaline massif (Northern Baikal, Russia) is studied by single crystal X-ray analysis at 120 K and IR spectroscopy. The empirical formula of calcium catapleiite is Ca_0.97Na_0.02Zr_1.01Si₃O₉ • 2H₂O (Z = 4). The X-ray diffraction study confirms the orthorhombic unit cell with the following parameters: a = 7.406(1), b = 12.687(1), and c = 10.112(1) Å; V = 950.1(2) ų; space group Pbnn. The crystal structure is refined in the anisotropic approximation of atomic displacement parameters using 1177 reflections with I > 2σ(I) to the final R = 2.91%. The structure of calcium catapleiite under study is based on the microporous heteropolyhedral framework formed by ZrO₆ octahedra and threemembered silicon–oxygen rings [Si₃O₉]. It is on the whole analogous to the structures of the samples studied earlier, but differs from them by a high degree of ordering of calcium and vacancies at extraframework positions. The distribution of calcium over Ca1 and Ca2 positions in the calcium catapleiite structure leads to the formation of zigzag chains of the …Сa1–Zr–Ca1–Zr… and …Сa2–h–Ca2–□… types. Low occupancy of the Ca2 position and its alternation with the vacancy are prerequisites for potential Ca²⁺ cationic conduction.

Two new malonate-containing uranyl complexes with carbamide of the formulas [UO₂(C₃H₂O₄)(Urea)₂] (I) and [UO₂(C₃H₂O₄)(Urea)₃] (II), where Urea is carbamide, and one uranyl oxalate complex of the formula [UO₂(C₂O₄)(Urea)₃] (III) were synthesized, and their crystals were studied by X-ray diffraction. The main structural units in crystals I are the electroneutral chains [UO₂(C₃H₂O₄)(Urea)₂]_∞ belonging to the crystal-chemical group AT¹¹M₂¹ (A = UO₂²⁺, T¹¹ = C₃H₂O₄^2-, M¹ = Urea) of uranyl complexes. Crystals II and III are composed of the molecular complexes [UO₂(L)(Urea)₃], where L = C₃H₂O₄^2- or C₂O₄^2-, belonging to the crystal-chemical group AB⁰¹M₃¹ (A = UO₂²⁺, B⁰¹ = C₃H₂O₄^2- or C₂O₄^2-, M¹ = Urea). The characteristic features of the packing of the uranium-containing complexes are discussed in terms of molecular Voronoi–Dirichlet polyhedra. The effect of the Urea: U ratio on the structure of uranium-containing structural units is considered.

Aqua{pentahydrogennitrilotris(methylenephosphonato)}lithium hydrate is a linear coordination polymer. Its crystal structure is described in space group P^–1, Z = 2; a = 5.5732(2), b = 7.0106(2), and c = 16.9010(5) Å; α = 97.515(2)°, β = 94.551(2)°, and γ = 95.123(2)°. The tetrahedral coordination of the Li atom includes two oxygen atoms of a phosphonate ligand, one oxygen atom of another phosphonate ligand, and a water molecule. Complex formation is accompanied by closing of the eight-membered Li–O–P–C–N–C–P–O chelate ring. Polymeric chains run along the [100] direction. The chains are connected by hydrogen bonds.

The structure of the title compound C₁₁H₇FN₂O₂S was characterized by single crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P2₁/n with Z = 12, i.e. with three molecules in asymmetric unit. The molecules are not planar: the dihedral angles between the planes of thiophene and the benzene rings are 42.3(3)°, 42.0(3)°, and 48.9(2)°. In the crystal, intermolecular C–H···F interactions link the molecules through R₂² (14) ring motif. The crystal packing is also stabilized by π···π interactions.

Dihydrolipoamide dehydrogenase from Escherichia coli (LpD) is a bacterial enzyme that is involved in the central metabolism and shared in common between the pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase complexes. In the crystal structure, E. coli LpD is known to exist as a dimer. The present work is focused on analyzing the solution structure of LpD by small-angle X-ray scattering, molecular docking, and analytical ultracentrifugation. It was shown that in solution LpD exists as an equilibrium mixture of a dimer and a tetramer. The presence of oligomeric forms is determined by the multifunctionality of LpD in the cell, in particular, the required stoichiometry in the complexes.

The composition nonstoichiometry and structural quality of undoped gallium nitride layers grown by the hydride vapor phase epitaxy on sapphire substrates of different orientations have been estimated using Raman spectroscopy. It is found for the first time that the peak position of the phonon mode E₂(high) in the Raman spectra of gallium nitride films at a wave vector of 572 cm^–1 depends on the initial orientation of sapphire substrate and is low-frequency shifted when passing from Ga-polar to partially N-polar orientation. Additional modes are found in the spectra of GaN layers grown on substrates with m and r orientations. It is shown that a decrease in the composition deviation from stoichiometry, caused by reducing the HCl flow through the gallium source during the growth of GaN layers, leads to an increase in the phonon-mode intensity in the Raman spectrum.

Semi-empirical and ab initio theoretical investigation of crystal structure geometry, interatomic distances, phase densities and elastic properties for some CaAl₂O₄ phases under pressures up to 200 GPa was performed. Two independent simulation methods predicted the appearance of a still unknown super-dense CaAl₂O₄ modification. In this structure, the Al coordination polyhedron might be described as distorted one with seven vertices. Ca atoms were situated inside polyhedra with ten vertices and Ca–O distances from 1.96 to 2.49 Å. It became the densest modification under pressures of 170 GPa (density functional theory prediction) or 150 GPa (semi-empirical prediction). Both approaches indicated that this super-dense CaAl₂O₄ modification with a “stuffed α-PbO₂” type structure could be a probable candidate for mutual accumulation of Ca and Al in the lower mantle. The existence of this phase can be verified experimentally using high pressure techniques.

The microhardness on the (010) and (001) planes in K₂Co(SO₄)₂ · 6H₂O crystal has been measured by the Vickers indentation method. Its values are, respectively, 11500 and 1300 MPa. No microhardness anisotropy of the first kind is revealed on either plane. The fracture geometry under indentation by a spherical indenter and Vickers and Knoop indenters is studied. The crystal has lower brittleness than the isomorphic Cs₂Ni(SO₄)₂ · 6H₂O crystal.

A classification of acoustic-beam reflection resonances in orthorhombic crystals under conditions where a proximity to conversion is implemented in the vicinity of total internal reflection is proposed. In this case, the energy from the incident pump beam falls almost entirely into a narrow intense reflected beam propagating at a small angle with respect to the surface. The crystal boundary is parallel to one of the elastic symmetry planes, and the excited beam propagates near one of axes 2 in this plane. Depending on the relations between the elastic moduli and the chosen propagation geometry, 18 types of resonances may occur, but no more than three in each crystal. The developed theory combines an approximate analytical description and accurate computer analysis. The relations between the elastic moduli providing minimum energy loss over the parasite reflected wave are determined. Some crystals with resonant excitation very close to conversion are revealed.

The ionic conductivity σ of the crystallization products of Ba_{1 - x}Yb_xF_{2 + x} melts with 10, 20 and 25 mol % YbF₃ has been studied. A Ba_0.9Yb_0.1F_2.1 sample is a solid solution with the CaF₂ structure type, sp. gr. Fm\(\overline 3 \)m. A Ba_0.8Yb_0.2F_2.2 sample contains two cubic forms with sp. gr. Fm\(\overline 3 \)m and Pm\(\overline 3 \)m. The σ values for Ba_0.9Yb_0.1F_2.1 and Ba_0.8Yb_0.2F_2.2 coincide and are equal to ~3 × 10^–5 S/cm at 500 K. A Ba_0.75Yb_0.25F_2.25 sample is heterogeneous, despite its monolithic nature and transparency. A greater part of its volume has a cubic lattice with sp. gr. Pm\(\overline 3 \)m, while the smaller part is a phase crystallizing in the orthorhombic system. A change of composition from x = 0.2 to 0.25 leads to a change in the symmetry group and type of the cluster defects in the Ba_{1 - x}Yb_xF_{2 + x} phase. The sp. gr. Fm\(\overline 3 \)m is replaced by the sp. gr. Pm\(\overline 3 \)m, and octahedral‒cubic {Ba₈Yb₆F₆₉} clusters are transformed into “inverse” octahedral‒cubic {Yb₈Ba₆F₇₁} clusters. These changes in the defect structure lead to an increase in conductivity by a factor of about 100. The fluorine-ionic conductivity of Ba_0.75Yb_0.25F_2.25 is 2.5 × 10^–3 S/cm at 500 K. This value exceeds the conductivity of Ba_0.69La_0.31F_2.31 crystal by a factor of 15 (Ba_0.69La_0.31F_2.31 has the best conducting properties among the fluorite phases of the Ba_{1 - x}R_xF_{2 + x} family, for which σ was found to increase with a decrease in the atomic number of rare earth element (REE)).

We report the review on the physical properties of the liquid crystal freely suspended films. The importance of the freely suspended films for the study of the fundamental problems of the self-confined systems as well as their practical implementations are demonstrated.

Photosemiconductor thin films based on two organic porphine derivatives have been investigated. These compounds have different pendent groups; the film morphology, along with the specific fabrication technique, is determined to a great extent by these groups. The films have been fabricated by vacuum sputtering and using the Langmuir−Schaefer method. According to the atomic force microscopy (AFM) data, the Langmuir−Schaefer films are more homogeneous than the sputtered ones. It is shown that the sputtered films based on substituted porphine have a looser stacking than the initial analog. A spectroscopy study revealed a bathochromic shift of the Soret band in the Langmuir−Schaefer films–sputtered films series. This shift is explained by the increase in the concentration and size of molecular aggregates in sputtered films. It is shown that a polycrystalline C₆₀ fullerene film deposited onto an amorphous substituted porphine layer improves the photoelectric characteristics of the latter. Both the time stability of the photodiode structure and its ampere‒watt sensitivity increase (by a factor of 10 in the transition regime). The steady-state current does not change. The effect of polarity reversal of the photovoltaic signal is observed in a planar С₆₀‒substituted metalloporphine heterostructure, which is similar to the pyroelectric effect. The polarity reversal can be explained by the contribution of the trap charge and discharge current at the interface between the amorphous photosemiconductor and crystalline photosemiconductor to the resulting photoelectric current.

We have investigated the conductivity of some representatives of different technological forms of fluoride-conducting solid electrolytes R_1–yM_yF_3–y (M = Ca, Sr, Ba; R are rare earth elements) with an LaF₃ structure: single crystals, cold- and hot-pressing ceramics based on a charge prepared in different ways (mechanochemical synthesis, solid-phase synthesis, and fragmentation of single crystals), polycrystalline alloys, etc. It is shown (by impedance spectroscopy), that different technological forms of identical chemical composition (R, M, y) exhibit different electrical characteristics. The maximum conductivity is observed for the single-crystal form of R_1–yM_yF_3–y tysonite phases, which provides (in contrast to other technological forms) the formation of true volume ion-conducting characteristics.

The stability function of the solid‒liquid interface for PbF₂–CdF₂ solid solution with respect to constitutional supercooling is calculated using the phase diagram of the system. The calculated curve is typical of the systems with continuous solid solutions, having minima points in the liquidus and solidus curves. This dependence can be used to estimate the technological parameters of the process which are required for growing crystals with the high optical quality.

A two-dimensional mathematical model of a polycrystalline sample and an experiment on electron backscattering diffraction (EBSD) is considered. The measurement parameters are taken to be the scanning step and threshold grain-boundary angle. Discrete pole figures for materials with hexagonal symmetry have been calculated based on the results of the model experiment. Discrete and smoothed (by the kernel method) pole figures of the model sample and the samples in the model experiment are compared using homogeneity criterion χ², an estimate of the pole figure maximum and its coordinate, a deviation of the pole figures of the model in the experiment from the sample in the space of L₁ measurable functions, and the RP-criterion for estimating the pole figure errors. Is is shown that the problem of calculating pole figures is ill-posed and their determination with respect to measurement parameters is not reliable.