Vol 64, No 5 (2019)

Based on the results of authors' studies, an approach to the analysis of [2 + 2] photocycloaddition (PCA) topochemical reactions of unsaturated compounds, occurring in a single crystal with either its retention or decomposition under exposure to visible light, has been developed. The main crystal packings, favorable for photoreaction in crystal, are revealed. Conditions for PCA reaction with single crystal retention are established. The factors increasing the probability of implementing crystal packing motifs that are favorable for this reaction (by chemical modification of structural units) are analyzed. The fact of extraordinary implementation of both direct and back (under UV irradiation) photoreactions in the same single crystal is explained.

Internal periodicity of phase transitions (fusion and polymorphism) has been revealed in trifluorides of Y, La, and 14 lanthanides. This periodicity is determined by the internal periodicity of filling of the 4f-electron subshell of rare-earth elements (REEs, R), in which cerium ⁵⁸Се–⁶⁴Gd and terbium ⁶⁵Тb–⁷¹Lu subsets can be selected. The lanthanide compression of ionic radii R³⁺ (15% towards the smaller one) induces the formation of three structure types: LaF₃, β-YF₃, and α-YF₃ (α-UO₃). The formula volumes V_form of REE trifluorides of the LaF₃ and β-YF₃ structure types only partially obey the internal periodicity of lanthanides with an increase in Z. The internal periodicity of the fusion and polymorphism of REE trifluorides (without ScF₃) divides RF₃ into four structural subgroups: A (R = La–Nd), B (R = Pm–Gd), C (R = Tb–Ho), and D (R = Er–Lu, Y), strictly specifying the percentage and elemental composition of each subgroup. The periodicity of the RF₃ polymorphism in the products manifests itself in the crystallization of the melts prone to supercooling in the form of a periodic change of single crystals with coarse- or fine-grained blocks of the low-temperature forms.

Orientation relationships {110}_Nb||{100}γ and 〈111〉_Nb||〈001〉γ between the crystal lattices of the Nb solid solution and particles of γ-silicide Nb₅Si₃ were determined by transmission electron microscopy. The following two types of matching planes were found: {110}_Nb||{100}γ and {111}_Nb||{001}γ. The former planes form steps at an angle of 120°. The correctness of the calculated orientation relationships was confirmed and these relationships were compared with experimental electron diffraction patterns using methods of mathematical modeling, which enable one to obtain superimposed electron diffraction patterns of several phases and tilt the model crystal. An interface model in two sections was constructed. The morphology of the interface between the particle and the matrix was explained in terms of structural features. The lattice misfit between the matrix and γ-silicide was calculated.

The structure of single-crystal dodecaboride TmB₁₂, which possess metallic properties, has been studied by X-ray diffraction at nine temperatures in the range of 85–293 K. Jahn–Teller distortions of the cubic lattice are revealed. They are found to cause an asymmetric electron density distribution in crystal. Static components of atomic displacements are selected, and characteristic Einstein and Debye temperatures are determined for thulium and boron atoms, respectively. Temperature dependences of interatomic distances are obtained. The problem of localizing conduction electrons by means of X-ray diffraction analysis is stated.

A mutant of bacterial carboxypeptidase T from Thermoactinomyces vulgaris (CPT5) with amino-acid substitutions in the S1' specificity pocket of the active site for the residues corresponding to the S1' region of pancreatic carboxypeptidase B (Gly215Ser, Ala251Gly, Thr257Ala, Asp260Gly, Thr262Asp) was crystallized in complex with N-BOC-L-Leu. The latter occupies the S1 subsite of the active site and is a reaction product analog. The X-ray diffraction data set was collected from a crystal of CPT5 at the SPring 8 synchrotron facility. The three-dimensional structure of CPT5 was determined at 1.40 Å resolution and refined to Rfact = 13.6%, Rfree = 12.5%. The binding of N-BOC-L-Leu to the S1 subsite of CPT5 leads to conformational changes in the protein concerning primarily the Сα atoms of five residues of the flexible loop at the interface between the S1 and S1' subsites and the side chains of the residues Tyr255 and Leu254 involved in the induced fit. Besides, conformational changes are observed in the active-site residues Glu277 and Asp262 and the side chains of residues located on the surface of the protein molecule. The observed changes differ from the conformational changes, which occur upon binding of N-BOC-L-Leu to wild-type CPT. This fact is indicative of the mutual influence of the S1 and S1' subsites in metallocarboxypeptidase T.

Data on a new SRS-active Ca₂Ga₂SiO₇ crystal, its Stokes and anti-Stokes χ⁽³⁾-nonlinear lasing in the visible region, and prospects of its use in self-SRS laser converters are reported.

The temperature evolution of the dielectric spectra of TGS–TGS + Cr crystals (TGS stands for triglycine sulfate) with a periodic distribution of chromium impurity (Cr³⁺) has been studied in comparison with nominally pure TGS crystals in the frequency range from 10 to 10⁷ Hz upon heating and cooling. It is shown that impurity crystals are characterized by the absence of dielectric dispersion in both measurement modes. An approach is proposed to explain the peculiarities of the dielectric dispersion of crystals with a periodic distribution of chromium impurity from the standpoint of mobile activity of certain elements of the TGS crystal structure.

An effect of uniaxial pressure on the spectral and temperature dependences of birefringence of K_1.75[NH₄]_0.25SO₄ single crystals has been studied. Different changes in the birefringence are found for different compression directions. The possibility of the occurrence of isotropic points in the crystal under uniaxial compression is discussed. A decrease in the birefringence under crystal heating is established, and the positions of two isotropic points in the range of 290–630 K are determined. The partial isomorphic substitution of NH₄ ions for K ions was found to result in a shift of isotropic points to lower temperatures.

Vanadium-doped diamond-like silicon-carbon films have been studied by X-ray reflectometry and transmission electron microscopy. A comparison of the data obtained has shown that the layered structure of the films is retained over an area of about 100 mm² at a total film thickness of 3–4 µm and layer thicknesses of 10–40 nm. The film surface has been examined by semi-contact probe microscopy. It is demonstrated that the root-mean-square roughness decreases by an order of magnitude (from 2.0 to 0.2 nm) with an increase in the vanadium content in the film from 12 to 17 at %.

The processes of fabrication of nonpolar (11\(\bar {2}\)0) ZnO films with a thickness of up to 7.25 µm on (1\(\bar {1}\)02) sapphire (Al₂O₃) substrates with a terrace-step surface nanostructure by magnetron sputtering at a growth rate of up to 4.5 nm/s has been studied. The effect of steps in the initial growth stages has been found, which leads to the formation of isolated ZnO nanostructures decorating the steps. In all next growth stages, a continuous (11\(\bar {2}\)0) epitaxial ZnO film with the longitudinal [0001] axis of crystallites, oriented parallel to the step edge on the sapphire surface has been observed. It is established that the interplanar spacing in the ZnO films in the [11\(\bar {2}\)0] direction, determined from the X-ray diffraction data, depends on the thickness and deposition rate and approaches the value of X-ray diffraction references for ZnO with an increase in thickness to 7.25 µm and a decrease in the growth rate to 2.1–2.3 nm/s.

The formation of thin-film coatings from detonation nanodiamond (DND) suspensions on atomically ordered conducting Ni–W substrates by self-assembly in an evaporating aqueous solution using the spin coating technique has been investigated. It is shown that, in the absence of a polymer matrix, the DND distribution over the substrate is nonuniform and the adhesion to the substrate is weak, which prevents obtaining a uniform and stable emission current. The use of DND–water-soluble polymer (chitosan and carrageenan polysaccharides, polyacrylic acid, and sulfonated polyaniline (SPANI)) coatings makes it possible to enhance the film adhesion to the substrate and provide a disaggregated state of nanocrystallites. The DND–SPANI coatings have demonstrated the best emission characteristics. Based on the analysis of the emission characteristics, a new model of emission center in the DND–polymer–substrate system has been proposed.

Mixed crystals of potassium‒cobalt sulfate hexahydrate/potassium‒nickel sulfate hexahydrate (K₂Co(SO₄)₂ · 6H₂O/K₂Ni(SO₄)₂ · 6H₂O (KCNSH)) have been grown from a solution with an isomorphic-component ratio of KCSH : KNSH = 1 : 2. The presence of mosaic microheterogeneity in KCNSH crystals at a level of ∼2 at % is confirmed, and its dependence on supercooling is studied. The radial heterogeneity of KCNSH crystals is investigated as a function of the growth direction and rate. The heterogeneity of Co concentration in different samples is shown to vary from 0.2 to 0.8 wt %.

The medieval Russian bronze reliquary cross pendant was investigated using a complex of nondestructive methods in order to determine the degree its integrity and identify the material filling its internal cavity. The metal composition is established. It is found that the cross material was subjected to spatially inhomogeneous corrosion and that the cavity is filled with soil. It is shown that application of complementary nondestructive methods provides the most reliable information on historical heritage objects.