Vol 59, No 9 (2017)

We establish the dependence of the electron binding energy in a separate isolated Wigner–Seitz cell of the metal crystal lattice on the average number of electrons located in this cell. The calculation is made using the modified Hellmann–Feynman theorem, which allows relating the eigenvalue of the steady-state Hamiltonian to the variation in its parameters that do not affect the degree of freedom of a system. As one of these parameters, we choose the average number of electrons in the cell. According to the calculated data, removal of 10–30% of electrons in monovalent metals leads to the crystal lattice fracture. The results obtained using the Hellmann–Feynman theorem are directly compared with the data of the jellium model.

MgB₂ superconductors subjected to deformation under pressure on Bridgeman anvils and subsequent annealings at 800 and 950°C were investigated by X-ray diffraction, scanning electron microscopy, and microanalysis. It was shown that at these temperatures crystals of MgB₂ phase along the residual boron dissolute in the residual liquid magnesium with the formation of both dense and loose areas of MgB₂ phase. The latter has a negative influence on critical current. At the same time, after these treatments, the samples become more uniform by the phase and chemical composition.

The possibility of efficient second-harmonic generation in the optical range in a planar dielectric waveguide with the active region in the form of a one-dimensional photonic crystal has been theoretically shown. The true phase matching can be achieved by controlling wave dispersion in the photonic crystal. The dispersion equations for the photonic crystal and three-layer waveguide have been self-consistently solved. It is shown that the coherence length may exceed 10 mm.

Electrical properties of single crystals of α-LiIO₃ superionic conductor (hexagonal symmetry, sp. gr. P6₃22, z cut) have been studied by the methods of impedance spectroscopy and thermally induced depolarization. The mechanism of thermally induced charge relaxation in α-LiIO₃ crystal is related to “hopping” migration of mobile Li⁺ ions via crystallographic sites in the channels of the [IO₃]^– crystalline framework oriented along the c crystallographic axis.

The 5 × 5 square lattices of magnetic dipoles with cubic crystallographic anisotropy were investigated by the computer simulation method. The conditions for implementing the random orientation of lattice configurations, each of which are characterized by a certain response to the influence of an external magnetic pulse, as well as by the established regime of the oscillation of the total magnetic moment under the influence of an alternating field, are revealed. Regular vibration modes with a doubled frequency and quasi-periodic and chaotic modes are detected. The dependence of the system response on the parameters of the magnetic field pulse is studied.

The Ising model with the frustrated exchange interaction for strongly anisotropic antiferromagnetic ultrathin film is investigated in the mean field approximation at low temperatures. It is shown that a spatially inhomogeneous state can be accomplished in the system at a certain relation of material constants values, along with homogeneous states: ferromagnetic, quadrupolar or supersolid magnetic phases. The phase diagram of the system is built on the basis of stability lines analysis.

We studied the structure and magnetic properties of porous multilayered Co/Pd films deposited on the templates of anodized Al₂O₃ with a specific surface morphology that is characterized by a cellular–porous structure with several pores inside each cell. X-ray diffraction analysis and reflectometry are used to study the peculiarities of the formation of phases in deposited films. The effect of morphological features of porous Co/Pd films on their magnetoanisotropic properties and magnetization reversal processes (magnetization reversal mechanisms, domain structure of films, and coercive field H_c) is revealed by SQUID magnetometry and magnetic force microscopy.

The structure and electrical properties of BiFeO₃ ceramics obtained by spark plasma sintering of a nanopowder are investigated. The nanopowder was synthesized by burning of an organic nitrate precursor. The ac conductivity was measured in a frequency range of 1 kHz–10 MHz in a temperature interval of 25–500°C. It is established that the temperature conductivity coefficients above and below ~350°C significantly differ with both alternating and direct currents. The frequency dependence of the conductivity obeys the Jonscher power law σ ~ ω^s, where s < 1. The interpretation of this behavior is given in the framework of the model of correlated hops of charge carriers over potential barriers. It is assumed that the hopping mechanism is realized between Fe²⁺ and Fe³⁺ ions in ceramic grains. The role of oxygen vacancies in the conduction is also discussed.

Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ_3ω of composites prepared by introducing ferroelectrics SC(NH₂)₂ into matrices of porous aluminum oxide Al₂O₃ with pore sizes 60 and 100 nm are determined. It is found that temperature T_c of the ferroelectric phase transition and the temperature T_i of the phase transition from incommensurable phase to the paraphrase increase significantly. The transition shifts increase as pore diameters decrease.

The response of a material with a random uniform distribution of pores to a sound impulse was studied. The behavior of the numerical characteristics of the recurrence plots (RP) of the normal displacement vector component depending on the degree of damage was investigated. It was shown that the recurrence quantification analysis (RQA) parameters could be very informative for sonic fault detection.

Multilayer nanostructured coatings consisting of alternating MoN and CrN layers were obtained by vacuum cathode evaporation under various conditions of deposition. The transition from micron sizes of bilayers to the nanometer scale in the coatings under investigation leads to an increase in hardness from 15 to 35.5 GPa (with a layer thickness of about 35 nm). At the same time, when the number of bilayers in the coating decreases, the average Vickers hardness increases from 1267 HV_0.05 to 3307 HV_0.05. An increase in the value of the potential supplied to the substrate from–20 to–150 V leads to the formation of growth textures in coating layers with the [100] axis, and to an increase in the intensity of reflections with increasing bilayer thickness. Elemental analysis carried out with the help of Rutherford backscattering, secondary ion mass spectrometry and energy dispersion spectra showed a good separation of the MoN and CrN layers near the surface of the coatings.

The ESR spectrum of Gd³⁺ and Eu²⁺ monoclinic centers, which substitute for yttrium ions in YAlO₃ crystals doped with europium and zirconium, has been investigated. Parameters of the fine structure of these centers are determined. The hyperfine-interaction parameters are determined for the centers with ¹⁵¹Eu isotope, and the hyperfine structure of the centers with ¹⁵³Eu isotope is discussed.

A modification of the Gibbs distribution in a thermally insulated mechanically deformed solid, where its linear dimensions (shape parameters) are excluded from statistical averaging and included among the macroscopic parameters of state alongside with the temperature, is proposed. Formally, this modification is reduced to corresponding additional conditions when calculating the statistical sum. The shape parameters and the temperature themselves are found from the conditions of mechanical and thermal equilibria of a body, and their change is determined using the first law of thermodynamics. Known thermodynamic phenomena are analyzed for the simple model of a solid, i.e., an ensemble of anharmonic oscillators, within the proposed formalism with an accuracy of up to the first order by the anharmonicity constant. The distribution modification is considered for the classic and quantum temperature regions apart.

The structural, electrotransport, and thermodynamic properties of the (1–x)CsH₂PO₄−xBa(H₂PO₄)₂ system in a wide range of compositions (x = 0.1–0.4) were firstly studied to develop the highly conductive proton electrolytes within the medium-temperature range. At x = 0—0.1, formation of disordered substitutional solid solutions, isostructural to CsH₂PO₄ (P2₁/m), with a decrease of the unit cell parameters and considerable increase of proton conductivity as a result of formation of vacancies in the cesium sublattice and weakening of the system of hydrogen bonds, was observed. At x = 0.15–0.4, the heterophase highly conductive systems demonstrating high values of proton conductivity ~10^–2 S/cm at x = 0.15—0.2, stable under the long-term isothermal exposures and low humidity (T ~ 200—210°C, RH ~ 15%), are formed. The phase transition disappears, the energy of activation of conductivity decreases from 0.9 to 0.55 eV at x = 0.2. The conductivity of high-temperature phase does not vary with Ba(H₂PO₄)₂ fraction increase to x = 0.2. The mechanisms of transfer of protons were discussed. It has been shown that when x > 0.10 the contribution to proton conductivity of molecules of the water adsorbed on the phase boundary of the composite systems increases.

The influence of the heating rate of a solid solution of CuCl in glass on the size distribution of the produced CuCl nanoparticles is studied. The distribution curves of CuCl nanocrystals are determined by the method of exciton-thermal analysis. It is established that the concentration of CuCl nanoparticles increases by ten times upon slowing the sample heating process from 2 to 60 min, while the mean radius of particles decreases almost twice. The concentration of CuCl nanoparticles passes through a maximum in the process of heating the sample. The numerical simulation of the nucleation upon slow heating of a solid solution showed that the formation of the concentration maximum of the new phase clusters is determined by a rapid increase in the critical radius owing to an increase in temperature and decrease in the solution supersaturation. As a result, the formation of new phase nuclei ceases at a certain temperature, and a part of the previously formed clusters dissolves.

The atomic and electronic structure of the three surfaces of Ti₃Al alloy—(0001), (\(1\bar 100\)), and (\(11\bar 20\))—is calculated by the projector augmented-wave method in the framework of the electron density functional theory. The surface energies are estimated as a function of the chemical potential of aluminum, which made it possible to construct a stability diagram for the surfaces under study. Adsorption of oxygen on differently oriented surfaces of the alloy is studied. It is found that the most preferred positions for oxygen adsorption are hollow positions on the (0001) and (\(11\bar 20\))_Ti–Al surfaces and bridge positions on the (\(1\bar 100\))_Ti‒Al-1 surface. Structural and electronic factors that determine these energy preferences are discussed. It is shown that regardless of the orientation of the surface, oxygen “prefers” titanium-enriched positions. The effect of oxygen on the atomic and electronic structure of low-index surfaces is discussed. It is found that at low concentrations of oxygen, the formation of its chemical bond with titanium and/or aluminum atoms in the surface and subsurface layers leads to the appearance of low-lying states split off from the bottom of the valence bands of metals, which is accompanied by the formation of a pseudogap and the weakening of Ti‒Al metal bonds in the surface layers.

Influence of magnetic field on orientation and magnetic properties of a compensated ferrocholesteric, a suspension of needle-like ferromagnetic particles in a cholesteric liquid crystal, was studied theoretically. A phase transition from a ferrocholesteric to a ferronematic state in a magnetic field oriented normally to the axis of the helical structure was considered. The dependences of the transition field to the ferronematic phase on the material parameters of the suspension and of the helical structure pitch and magnetization on the field strength were investigated. A possibility of existence of a reentrant ferrocholesteric phase was shown.

The structural state in nanoscaled SiO₂ is probed experimentally via X-ray diffraction and the simulation method. The aerosil nanoparticles and nanoparticles synthesized via the electron beam evaporation are compared. The nanoparticles for all samples are shown to be in the amorphous state. The amorphous state of a SiO₂ unit lattice is simulated via the molecular dynamics. The full-profile refinement of parameters for a simulated SiO₂ phase (the Rietveld method) has allowed the complete structural information to be established at varying the specific surface. The unit cell parameters, the spatial atomic distribution and the degree of cell node occupation are determined, as well. The specific surface area is shown to decrease in aerosil nanoparticles and to increase in tarkosil nanoparticles with the increasing binding energy of atoms in a cell.

The heat capacity (C_P), the thermal diffusion (η), the thermal conductivity (κ), and the electrical resistance of the La_0.825Sr_0.175MnO₃ single crystal have been measured in the temperature range 80–350 K in magnetic fields to 40 kOe. Dependences C_P(T), κ(T), and η(T) have anomalies near T_C, which are suppressed in magnetic field. The minima in dependences κ(T) and η(T) near T_C are explained by the phonon scattering on fluctuations of the magnetic order parameter. Dependences κ(T) and η(T) have anomalies near T_S = 200 K related to the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase.

The thermal expansion and the heat capacity of coarse-crystalline and nanocrystalline silver sulfide Ag₂S were studied by dilatometry and differential scanning calorimentry for the first time in the temperature range 290–970 K. It is found that the thermal expansion coefficient and the heat capacity of nanocrystalline silver sulfide in this temperature range are higher than those in the case of the coarse-crystalline sulfide. It is revealed that the transformation of α-Ag₂S acanthite to β-Ag₂S argentite and β-Ag₂S argentite to γ-Ag₂S phase are the first-order phase transitions; the temperatures and the enthalpies of these transformations have been determined.