Том 59, № 10 (2017)

Ti–V–Cr alloys are hydrogen storage materials, but their characteristics, which are important for practical applications, depend strongly on composition. The search for an optimal composition with given characteristics requires the support of theoretical calculations of the electronic structure of alloys and their hydrides. In this paper, the interstitial energy and energy of hydrogen dissolution in the hydride of a ternary disordered Ti_0.33V_0.27Cr_0.4H_1.75 alloy with a face-centered cubic lattice were calculated within the framework of the density functional theory using the pseudopotential method. The deviation of the dissolution energy distribution from the Gaussian distribution is shown. Based on the data obtained for a particular hydride, the energy distributions of hydrogen dissolution in a number of hydrides of alloys (Ti_0.8Cr)_1–xV_x with x = 0.9, 0.8, 0.7, and 0.6 have been derived. A correlation was found between the theoretically calculated width of the energy distribution of hydrogen dissolution and the experimental slope of the pressure “plateau.”

The previously proposed model of pearlite transformation develops taking into account the possible interaction between carbon and lattice dilatations arising in austenite near the pearlite colony. The normal stresses caused by the colony stimulate autocatalysis of plates, and tangential stresses promote the stabilization of the transformation front. The mechanism of ferrite branching, which can play an important role in the kinetics of pearlite and bainite transformations, is discussed.

The Andreev reflection spectra of high-quality superconducting Sn_1–xIn_xTe crystals with critical temperature T_c = 3.6–3.8 K were examined. The two-band nature of the superconducting state of this compound was established. The energy gaps have s-symmetry and were measured to be 2Δ₁/kT_c = 3.5 ± 0.9 and 2Δ₂/kT_c = 7 ± 1. Neither of the contacts with a resistance of 1.5–23 Ω revealed a peak at zero voltage typical of the topological superconducting state.

We present the results of theoretical analysis of structural varieties of compounds for which polytypism is important. As a result, the possibility of isomeric polytype varieties existing has been established, as well as regularities in their formation, depending on the period of repeatability of layers in packaging. In addition, the methods of density functional theory are used to calculate the structure and properties of various polytypic modifications of diamond. Dependences of the interlayer distances and the difference total energy on the hexagonality of diamond polytypes and their isomeric varieties have been found. The theoretical powder X-ray diffraction patterns of diamond polytypes are calculated and their comparative analysis is performed.

The structure and mechanical properties of the crystals of solid solutions of zirconium dioxide, which are stabilized by yttrium and cerium oxides, have been studied. The electron paramagnetic resonance technique has been used to identify Ce³⁺ ions and to determine their relative concentration in the crystals. It is shown that the presence of Ce³⁺ ions in the crystals is the main factor responsible for their high fracture toughness. The annealings carried out during investigations, which lead to a decrease in the concentration of Ce³⁺ ions, show that a change in the valence state of cerium ions lowers the fracture toughness of the crystals.

The cross-breeding problem of the temperature dependence of the antiferromagnetic susceptibility of ferrihydrite nanoparticles is considered. Iron ions Fe3+ in ferrihydrite are ordered antiferromagnetically; however, the existence of defects on the surface and in the bulk of nanoparticles induces a noncompensated magnetic moment that leads to a typical superparamagnetic behavior of ensemble of the nanoparticles with a characteristic blocking temperature. In an unblocked state, magnetization curves of such objects are described as a superposition of the Langevin function and the linear-in-field contribution of the antiferromagnetic “core” of the nanoparticles. According to many studies of the magnetization curves performed on ferrihydrite (and related ferritin) nanoparticles in fields to 60 kOe, dependence χ_AF(T) decreases as temperature increases, which was related before to the superantiferromagnetism effect. As the magnetic field range increases to 250 kOe, the values of χ_AF obtained from an analysis of the magnetization curves become lower in magnitude; however, the character of the temperature evolution of χ_AF is changed: now, dependence χ_AF(T) is an increasing function. The latter is typical for a system of AF particles with random orientation of the crystallographic axes. To correctly determine the antiferromagnetic susceptibility of AF nanoparticles (at least, ferrihydrite) and to search for effects related to the superantiferromagnetism effect, it is necessary to use in experiments the range of magnetic field significantly higher than that the standard value 60 kOe used in most experiments. The study of the temperature evolution of the magnetization curves shows that the observed crossover is due to the existence of small magnetic moments in the samples.

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.

Neutron diffraction was used to study the temperature evolution of the structure of (1–x)Pb · (Fe_2/3W_1/3O₃)–(x)PbTiO₃ solid solutions of two compositions x = 0.2 and 0.3, in which the existence of the morphotropic phase boundary (MPB) is observed, in the temperature range 90–400 K. It is shown that the system is in the two-phase state, in which the cubic and the tetragonal phases coexist, even at temperatures higher than MPB. The temperature dependences of the phase percentages were obtained. The static displacements of lead ions from the crystallographic position (000) are estimated in terms of a multiwell potential.

The influence of annealing on the microstructure and mechanical properties of ultrafine-grained (UFG) commercially pure aluminum preliminarily subjected to severe plastic deformation by high pressure torsion has been studied. It is found that annealing of the UFG samples in the temperature range 363–473 K for 1 h leads to increases in the conventional yield strength and ultimate tensile strength, which attained maximum values (50 and 30%, respectively) after annealing at 423 K. A key role of nonequilibrium high-angle grain boundaries in the strengthening effect of UFG-Al due to annealing is discussed. The increase in the strength of UFG-Al is accompanied by a significant decrease in its ductility. A new approach of increasing the ductility of UFG-Al with retaining a high strength is proposed. It is an introduction of additional dislocation density to a UFG structure relaxed by annealing.

For the first time a theoretical analysis of scale effects upon the shock plastic compression of nanocrystals is implemented in the context of a dislocation kinetic approach based on the equations and relationships of dislocation kinetics. The yield point of crystals τ_y is established as a quantitative function of their cross-section size D and the rate of shock deformation as τ_y ~ ε^2/3D. This dependence is valid in the case of elastic stress relaxation on account of emission of dislocations from single-pole Frank–Read sources near the crystal surface.

The electron paramagnetic resonance (EPR) spectra of iron-doped ZnSe single crystals were studied. In addition to cubic Fe³⁺ and Mn²⁺ centers and also Fe²⁺, and Cr²⁺ centers, monoclinic Fe³+ complexes locally compensated by Cu+ ions were revealed. Some trigonal centers with a spin of 3/2 were also found and studied. The zero-field splittings of monoclinic centers were measured, and the parameters of the monoclinic and trigonal spin Hamiltonians were determined. The nature of trigonal centers was discussed.

Thermal expansion and structural and magnetic phase transitions in alloys of the Ni–Mn–Sn system have been investigated. The spontaneous martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) alloys is found to be accompanied by high jumps in the temperature dependences of the linear thermal expansion. The relative change in the linear sizes of these alloys at the martensitic transformation is ~1.5 × 10^–3. There are no anomalies in the magnetic-ordering temperature range in the temperature dependences of the coefficient of linear thermal expansion. The differences in the behavior of linear thermal expansion at the martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) and Ni₄₇Mn₄₀Sn₁₃(x = 4) alloys have been established.

It is shown that the Imry–Ma theorem stating that in space dimensions d < 4 the introduction of an arbitrarily small concentration of defects of the “random local field” type in a system with continuous symmetry of the n-component vector order parameter (O(n) model) leads to long-range order collapse and to the occurrence of a disordered state is not true if the anisotropic distribution of the defect-induced random local field directions in the space of the order parameter gives rise to the effective anisotropy of the “easy axis” type. In the case of a weakly anisotropic field distribution, in space dimensions 2 ≤ d < 4 there exists some critical defect concentration, above which the inhomogeneous Imry–Ma state can exist as an equilibrium one. At a lower defect concentration, long-range order takes place in the system. In the case of a strongly anisotropic field distribution, the Imry–Ma state is suppressed completely and long-range order state takes place at any defect concentration.

The properties of heterophase core/shell/shell Ag/FeCo/Ag nanoparticles synthesized via a plasma method that are promising for biological applications are studied. As is established, the core/shell/shell Ag/FeCo/Ag nanoparticles exhibit a superparamagnetic state at room temperature that allows one to manage the hyperthermia process. The magnetic characteristics of core/shell/shell Ag/FeCo/Ag nanoparticles are interpreted by assuming partial oxidation of the surface layer of a ferromagnetic FeCo shell and formation of the antiferromagnetic Co_xFe_1–xО layer on the FeCo surface. The interaction between the surface antiferromagnetic Co_xFe_1–xО layer and the ferromagnetic FeCо shell causes the emergence of the exchange bias in Ag/FeCo/Ag nanoparticles.

Dielectric properties of the nanostructured multiferroic composite on the basis of silicate porous glass simultaneously filled with ferromagnetic (cobalt oxide CoO) and ferroelectric (sodium nitrite) materials have been investigated in wide temperature (270–570 K) and frequency (10^–1–10⁷ Hz) ranges. The mean diameter of pores in the matrix is 7 ± 1 nm. The magnetic material particles are synthesized directly in the pores of the glass matrix and occupy about 10% of the pore volume. The porous glass is well wetted with NaNO₂. The latter easily infiltrates into the glass and occupies 90% of the remaining unfilled pore volume. The dielectric response of matrices filled with both the components together and with each component separately is studied. An analysis of the obtained data makes it possible to reveal the contributions of individual components into the dielectric response of the composite and the influence of the confined geometry on their dielectric properties. It is found that the incorporation of CoO nanoparticles leads to an order of magnitude increase in the dielectric permittivity and electrical conductivity of the two-component composite in comparison with these values for the composite filled solely with sodium nitrite and to a decrease in the activation energy over the entire studied temperature range. These studies are of interest not only as a preliminary investigation prior to the study of the effect of a magnetic field on the dielectric properties of the synthesized composite, but are of independent physical interest as well, since they allow one to determine the influence of the confined geometry on the dielectric properties of magnetic metal oxides and on the of their phase transition parameters.

The intercalation of cobalt under a graphene monolayer grown on a Ni(111) single crystal film is studied. The experiments are conducted in ultrahigh vacuum. Samples are characterized in situ by low energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and magnetic linear dichroism in photoemission of Co 3p electrons. New data are obtained on the evolution of the atomic and electronic structure and magnetic properties of the system with increasing thickness of the intercalated cobalt layer in the range up to 2 nm. It is shown that a pseudomorphic epitaxial film of Co(111) having magnetization perpendicular to the surface is formed under the grapheme layer during intercalation in an anomalously wide range of thicknesses.

The ab initio calculations of the electronic structure of low-dimensional graphene–iron–nickel and graphene–silicon–iron systems were carried out using the density functional theory. For the graphene–Fe–Ni(111) system, band structures for different spin projections and total densities of valence electrons are determined. The energy position of the Dirac cone caused by the p_z states of graphene depends weakly on the number of iron layers intercalated into the interlayer gap between nickel and graphene. For the graphene–Si–Fe(111) system, the most advantageous positions of silicon atoms on iron are determined. The intercalation of silicon under graphene leads to a sharp decrease in the interaction of carbon atoms with the substrate and largely restores the electronic properties of free graphene.

Phase transitions in normal alkanes, such as tricosane (C₂₃H₄₈), tetracosane (C₂₄H₅₀), and pentacosane (C₂₅H₅₂), were studied by differential scanning calorimetry. The elimination of some procedural errors provided the possibility to obtain true values for the thermodynamic parameters of phase transitions and ascertain their nature. The comparative analysis of heat capacity jumps was performed on the basis of the theory of diffuse first-order phase transitions.

The melting processes of various Pt–Pd nanoparticles (binary alloy, core–shell, D ≤ 4.0 nm) with different percent platinum atom content are investigated via the molecular dynamics using the embedded atom method potential in order to establish the thermal stability of simulated particle structure. In accordance with the data obtained, the most thermally stable are Pt–Pd nanoalloys with a diameter above 2.0 nm and core–shell Pd@Pt particles. As is shown, heating of binary Pt–Pd cluster alloys with the particle diameters less than 2.0 nm may cause the transition to pentagonal symmetry structures and core–shell-like complex formations.

For the creation of new promising chemical sensors, it is very important to study the influence of the interface between graphene and aqueous solutions of acids and alkalis on the transistor characteristics of graphene. Transistor structures on the basis of graphene grown by thermal decomposition of silicon carbide were created and studied. For the interface of graphene with aqueous solutions of acetic acid and potassium hydroxide in the transistor geometry, with a variation in the gate-to-source voltage, the field effect corresponding to the hole type of charge carriers in graphene was observed. It is established that an increase in the concentration of molecular ions in these solutions leads to an increase in the dependence of the resistance of the transistor on the gate voltage.

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T_C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS_M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.