Volume 59, Nº 8 (2017)

The electronic structure, Fermi surface, Sommerfeld and Pauli paramagnetic susceptibility coefficients, cohesive energies, phase and point defect formation energies, elastic constants, bulk, shear, and Young moduli, Poisson ratios, and Vickers microhardness of niobium boronitride Nb₂BN are determined by the ab initio FLAPW-GGA full-potential method. The obtained values are discussed in comparison with similar data for Mo₂BC and other related binary carbides, nitrides, and borides of transition metals, and with available experimental data.

Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa₂Cu₃O_{7 – δ} are measured at various transverse external magnetic fields at 0 < H_ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T_c2J to temperatures slightly exceeding the superconducting transition temperature T_c. Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T_c1g(H_ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T_c1g: the Josephson vortex decreases, and the Abrikosov vortex density increases.

The effect of composition and temperature on the electrical conductivity and the permittivity of the Tl(GaS₂)_{1 – x}(InSe₂)_x has been studied. It is found that the permittivity and the conductivity increase with temperature and vary linearly as the InSe₂ concentration increases: in this case, the conductivity decreases and the permittivity increases. It is confirmed that the system under study contains two series of solid solutions. It is shown that the commensurate–incommensurate phase transition temperatures shift to lower values as the InSe₂ concentration increases.

The local environment and the charge state of a nickel impurity in cubic Ba_0.8Sr_0.2TiO₃ are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is ~2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that Ni²⁺ ions are in a high-spin state at the B sites of the perovskite structure and the difference of charges of Ni²⁺ and Ti⁴⁺ is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO_{3 − δ}. The measurements of the lattice parameter show a decrease in the unit cell volume upon doping, which can indicate the existence of a small amount of Ni⁴⁺ ions at the B site.

The effects of charging of dielectric targets irradiated with moderate-energy electrons in a scanning electron microscope are examined. Considerable differences in the kinetics of charging of the reference samples and the samples preirradiated with ions and electrons are reported. These differences are attributed to the processes of radiation-induced defect formation in Al₂O₃ (sapphire) and SiO₂ that are, however, dissimilar in nature. The contributions of surface structure modification and changes in the electrophysical parameters of the surface (specifically, the charge spreading effect) are revealed. Critical doses of irradiation with Ar⁺ ions and electrons inducing active defect formation in dielectric targets and critical values of internal charge fields producing a significant contribution to the temporal parameters of Al₂O₃ and SiO₂ charging are determined.

The magnetic properties of the multiferroics obtained upon isovalent substitution of samarium cations for bismuth cations in BiFeO₃. The samples have been synthesized by solid-phase reactions under conditions of a cold pressing at high (4 GPa) pressure. The correlation between the structure and the magnetic properties of the multiferroics has been found based on analyzing the experimental data.

The magnetic properties of antiferromagnetic NiO nanoparticles prepared by thermal decomposition of nickel hydroxocarbonate are investigated. According to the data of magnetization measurements in fields of up to 250 kOe, the magnetic moment linearly grows in strong fields, which is caused by the contribution of the antiferromagnetically ordered nanoparticle core, and the antiferromagnetic susceptibility corresponds to that of bulk polycrystalline NiO. This allowed the antiferromagnetic and ferromagnetic contributions to the total magnetic response of a sample to be quantitatively determined. The latter occurs due to the incomplete spin compensation in an antiferromagnetic nanoparticle caused by defects on its surface. It is demonstrated that to correctly determine the superparamagnetic blocking temperature, it is necessary to take into account the antiferromagnetic susceptibility of the particle core.

The Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_{1 – x}Cr_xO₃ (x = 0.05, 0.10, and 0.20) have been performed at room temperature. The multiferroics BiFe_{1 – x}Cr_xO₃ (x = 0.05, 0.10, and 0.20) with the rhombohedral R3c structure have been prepared by solid-state synthesis under high pressures. The effect of substitution of Cr cations for Fe cations on the spatial spin-modulated structure, and also hyperfine electrical and magnetic interactions of ⁵⁷Fe nuclei has been studied. The substituted ferrites demonstrate an anharmonic modulated spin structure of cycloid type, in which iron atoms with different cation environments take part. The anharmonism parameter of the cycloid linearly increases from m = 0.10 at x = 0 to m = 0.78 ± 0.02 at x = 0.20. The constants of magnetic uniaxial anisotropy K_u are estimated at room temperature: K_u ≈ 0.36 × 10⁶ erg/cm³ at x = 0 and K_u ≈ 4.22 × 10⁶ erg/cm³ at x = 0.20.

The fine structure of a breakdown channel from a positive electrode in KCl single crystal is studied in the multipulse exposure at a voltage of 140 kV. The dimensions and the shapes of breakdown structures are established as the functions of the applied voltage. The velocity of propagation of a crack vertex forming the breakdown structure, as well as the pressure in the breakdown channel, are estimated. It is shown that under certain conditions the mechanical destruction structure near the breakdown channel is retained even after several dozen pulses are applied.

The mathematical model of dislocation glide movement is used to calculate the signal of acoustic emission that accompanies the defect overcoming in a crystal. The elastic stress in the emitted signal is estimated. It is established that the acoustic emission signal in the case of dislocation separation from a defect substantially exceeds the signal in the case of deceleration of a gliding dislocation on a defect. The shapes of these signals also differ.

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.

Electron spin resonance spectra of non-Kramers bivalent iron (Fe²⁺) ions have been detected in synthetic and natural beryl crystals with an iron impurity. The observed ESR spectra have been attributed to resonance transitions of Fe²⁺ ions from the ground (singlet) state to excited (doublet) levels with the splitting Δ = 12.7 cm^–1 between the levels. The experimental angular and frequency dependences of the resonance field of the ESR signal have been described by the spin Hamiltonian with the effective spin S = 1. The analysis of the ESR data and optical absorption spectra indicates that the Fe²⁺ ions are situated in tetrahedral positions and substitute Be²⁺ cations in the beryl structure.

The reflection R(ħω), transmission t(ħω), absorption α(ħω), and refraction n(ħω) spectra of polycrystalline In₂O₃–SrO samples with low optical transparency, which contain In₂O₃ and In₂SrO₄ crystallites with In₄SrO_{6 + δ} interlayers, are examined. In the region of small ħω values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n(ħω) and α(ħω) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t(ħω) spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap E_gn in the density of states with a width from–0.12 to–0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In₂O₃ phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap E_gd = 1.3 eV corresponding to the In₂SrO₄ phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.

At oversaturations exceeding the inert range end for face {101} due to the presence of admixture Al(NO₃)₃ · 9H₂O, a new phase is observed during the growth of this face in the form of filamentary crystals. Some experimental dependences of the growth rate of filamentary potassium dihydrophosphate (KH₂PO₄) crystals on the oversaturation have been obtained at different admixture concentrations. The growth of filamentary crystals occurs by the mechanism of two-dimensional nucleation. Their formation is governed by the effect of [AlHPO₄]⁺ complexes in the form of Cabrera and Vermilyea stoppers.

The method of obtaining nanoclusters α-Fe₂O₃ in the pores of monodisperse spherical particles of mesoporous silica (mSiO₂) by a single impregnation of the pores with a melt of crystalline hydrate of ferric nitrate and its subsequent thermal destruction has been proposed. Fe₃O₄ nanoclusters are synthesized from α-Fe₂O₃ in the pores by reducing in thermodynamically equilibrium conditions. Then particles containing Fe₃O₄ were annealed in oxygen for the conversion of Fe₃O₄ back to α-Fe₂O₃. In the result, the particles with the structure of the core-shell mSiO₂/Fe₃O₄@mSiO₂/α-Fe₂O₃ are obtained. The composition and structure of synthesized materials as well as the field dependence of the magnetic moment on the magnetic field strength have been investigated.

Passivation of a silicon–ytterbium nanofilm interface with СО and О₂ molecules chemisorbed on the opposite side of films is studied. The transfilm inhibition of silicides is found to be caused by the Coulomb interactions between the localized electrons forming the donor–acceptor bonding of molecules with films and the conductivity electrons of ytterbium (6s-band). This interaction increases the energy of the chemisorbed molecules–ytterbium films system. At a given amount of chemisorbed molecules this increase is higher for the thinner rather than thicker films. This correlation with the film thickness favors the lack of chemical interaction between silicon and ytterbium, when СО and О₂ molecules are chemisorbed on the nanofilm surface.

A molecular model describing translational diffusion in freely suspended smectic films (FSSFs) in air is proposed. This model is based on the random walk theory and allows calculation of the translational diffusion coefficient (TDC) across smectic layers (along the director). All values necessary for calculating the TDC are obtained within the generalized mean-field model considering not only anisotropic interactions between nearest neighbors of molecules forming FSSFs, but also the stabilizing effect of the smectic/air interface. The spatial inhomogeneity of order parameters over the FSSF section, arising in this case, results in the fact that the surface tension at the smectic/air interface not only suppresses thermal fluctuations in surface layers, but also completely suppresses translational diffusion of molecules from the FSSF to air. The results of calculations of dimensional translational diffusion in the bulk of the FSSF formed by 5-n-alkyl-2-(4-n-(perfluoroalkyl-metyleneoxy))pentyl molecules during its thinning show that the TDC monotonically increases as the smectic film is thinned.

Yttrium-containing endohedral metallofullerenes are synthesized in an RF arc discharge in the helium flow with embedded Y₂O₃. It is shown that the formation of the metallofullerenes depends on the helium pressure in a chamber; however, this dependence cannot be explained using the model of formation of conventional fullerenes without a guest atom in a molecule. The results of extraction of Y@C₈₂ by pyridine and carbon disulfide are reported. The pressure corresponding to the maximum yttrium-containing endohedral metallofullerene content is shown to be 60 kPa; under this pressure, extraction by carbon disulfide allows obtaining 27.1 wt % of the endohedral metallofullerene, while extraction by pyridine yields its amount of 17.3 wt %.

For single-layer graphene placed on a metal substrate, the influence of intra- and interatomic Coulomb repulsion of electrons (U and G, respectively) on its phase diagram is considered in the framework of an extended Hartree-Fock theory. The general solution of the problem is presented, on the basis of which special cases allowing for analytical consideration are analyzed: free and epitaxial graphene with and without regard for the energy of the electron transition between neighboring atoms of graphene. Three regions of the phase diagram are considered: spin and charge density waves (SDW and CDW, respectively) and the semimetal (SM) state uniform in the spin and charge. The main attention is paid to undoped graphene. It is shown that the allowance for the interaction with a metal substrate expands the SM existence domain. However, in all the considered cases, the boundary between the SDW and CDW states is described by the equation U = zG, where z = 3 is the number of nearest neighbors in graphene. The widening of the SM state region also results from the doping of graphene, and the effect is independent of the sign of free carriers introduced into epitaxial graphene by the substrate. According to estimates made, the only state possible in the buffer layer is the metal-type SM state, whereas, in epitaxial graphene, the CDW state is possible. The influence of temperature on the phase diagram of epitaxial graphene is discussed.

The SmBiGeO₅ compound is synthesized from Sm₂O₃, Bi₂O₃, and GeO₂ by solid-state synthesis with subsequent annealing at 1003, 1073, 1123, 1143, 1173, and 1223 K. The metastable Bi₂GeO₅ compound is prepared from melt. Temperature dependences of specific heat of Bi₂GeO₅ (350–1000 K) and SmBiGeO₅ (370–1000 K) are measured by differential scanning calorimetry. Basing on the experimental dependences C_P = f(T), the thermodynamic functions of the oxide compounds are calculated.