Vol 60, No 9 (2018)

The atomic structure of iron–aluminum alloy samples containing about 7 at % of aluminum (α region of the phase diagram) was investigated by X-ray diffraction. The samples were annealed in the paramagnetic (T > ^T_C) and ferromagnetic (T < T_C) states. In the first case, the structural state was fixed by quenching from the annealing temperature in water, and, in the second case, by slow cooling. Diffraction patterns of single-crystal samples were taken on an X-ray four-circle diffractometer. It is shown that local ordering, regardless of the prehistory of a sample, is a combination of B2-phase clusters (the CsCl type structure), which were previously found in iron–silicon alloys with Si content up to 10%, and small regions with D0₃ short-range order. The former consist of two B2-cells having a common face, while the latter consist predominantly of one unit cell of the D0₃-phase. Within the accuracy of the experiment, no significant difference in the structural states in the paramagnetic and ferromagnetic phases was observed.

The electronic structure and the exchange interactions in EuNi₄Co and YbNi₄Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi₄Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.

The complex methods of the physicochemical analysis are used to study TlGaTe₂–Te and TlInTe₂–Te alloys in which the tellurium solubility region up to 5.0 at % is observed. The temperature dependences of the lattice parameters and the electrical conductivity of TlGaTe_{2 + x} and TlInTe_{2 + x} have been studied in different crystallographic directions. The TlGaTe_{2 + x} and TlInTe_{2 + x} solid solutions undergo a phase transition at a temperature of 498 K. The transition nature is interpreted.

Taking into account the inexhaustible interest in studying the peculiarities of physical properties in the neighborhood of phase transitions and the growth of experimental investigations of cobalt fluoride, we have studied the peculiarities of magnetic susceptibility in the vicinity of the critical field H_C at which cobalt fluoride performs the second-order phase transition from the antiferromagnetic phase to the angular phase. It is discovered that in the magnetic field H ‖ C₄, the magnetic susceptibility becomes infinite at H → H_C. It is shown that as the magnetic field direction deviates from the C₄ axis, the magnetic susceptibility in the critical field H_C proves to be finite. It is also shown that the change in the magnetic susceptibility with the change in the magnetic field considerably decreases at extremely insignificant deviations of the field H from the C₄ axis. Since the calculations are performed in terms of the Landau theory of phase transitions, we pay attention to the similarity and difference between the obtained results and those in the vicinity of the Curie point obtained by using the Landau theory of phase transitions.

Some specific features of the crystal and magnetic structures of granular powder spinel-like ferrites Mn_0.160Mg_0.404Zn_0.448Fe₂O₄, Mn_0.676Zn_0.227Fe_0.09Fe₂O₄, Mn_0.5792Zn_0.2597Fe_0.1612Fe₂O₄, and Ni_0.32Zn_0.68Fe₂O₄ have been studied by neutron diffraction. It has been established that the crystal structure of all the studied compounds has a cubic symmetry with space group Fd\(\bar 3\)m. Ferrimagnetic ordering is observed in all the studied structures. Based on the experimental data, the unit cell parameters and interatomic bond lengths of the studied compounds are determined alongside with the distribution of cations between octahedral and tetrahedral crystallographic positions in their cubic crystal structure. Corresponding average magnetic moments are calculated for different positions in their cubic structure. Some structural mechanisms of the formation of magnetic properties depending on the level of doping and the size of powder grains are discussed.

The structural and magnetic properties of thin-film Fe/poly(diphenylene phthalide) (PDP)/Fe systems are studied, along with the behavior of these systems in magnetic fields. The mean surface roughness of the studied samples is around 5–8 nm, and local near-surface magnetic properties show variation within 10%. The samples are characterized by two-step hysteresis loops, the step size depending on the thickness of the polymer layer and the difference in the thickness between the magnetic layers. The results are explained by the effects the exchange interaction between the magnetic layers, mediated by the PDP interlayer, has on the behavior of our samples in a magnetic field.

Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe₂O₄) is a core, and an amorphous silicon dioxide (silica SiO₂) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe₂O₄/SiO₂ MNC has the core/shell structure formed by the Fe‒O–Si chemical bond. After coating with silica, the MgFe₂O₄/SiO₂ MNC saturation magnetization significantly decreases in comparison with MgFe₂O₄ particles without a SiO₂ shell. Spherical particles agglomerated from MgFe₂O₄ nanocrystallites ∼9.6 and ∼11.5 nm in size function as cores coated with SiO₂ shells ∼30 and ∼50 nm thick, respectively. The total size of obtained CS MNCs is ∼200 and 300 nm, respectively. Synthesized CS MgFe₂O₄/SiO₂ MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.

The correlation of the chemical composition, the structure, and the microwave characteristic of solid solutions of the BaFe_{12 – x}D_xO₁₉ (0.1 ≤ x ≤ 1.2) barium hexaferrite substituted with diamagnetic Al³⁺ and In³⁺ ions has been studied. The precise data on the crystal structure have been obtained by powder neutron diffraction using a high-resolution Fourier diffractometer (Dubna, JINR). The data on the distribution of the diamagnetic substituting ions in the hexaferrite structure have been obtained by Mössbauer spectroscopy. The microwave properties (the transmittance and the reflectance) have been studied in the frequency range 20–65 GHz and in external magnetic fields to 8 kOe. It is found that the transmission spectra are characterized by a peak that corresponds to the resonant frequency of the electromagnetic energy absorption, which is due to the ferromagnetic resonance phenomenon. The correlation of the chemical composition, the features of the ion distribution in the structure, and the electromagnetic properties has been revealed. It is shown that external magnetic fields shift the absorption peak of electromagnetic radiation to higher frequencies due to an increase in the magnetocrystal anisotropy. The results enable the conclusion that the features of the intrasublattice interactions and the electromagnetic properties should be explained using the phenomenological Goodenough–Kanamori model.

Ferroelectric domain structures formed by an electron beam in lithium niobate crystals are studied using low-voltage SEM microscopy. The structures are formed in crystals with different conductivity, including samples with high-resistance congruent composition (CLN) and samples with conductivity increased by reductive annealing (RLN). The potential nature of the contrast of the domain structures observed in the secondary electron mode depending on the conductivity of the samples and the direction of spontaneous polarization of the domains is analyzed. It is assumed that the domain contrast in CLN crystals is associated with long-lived charges localized near domain walls and in the irradiated areas. The recorded domain structures in the CLN crystals are visualized on polar and nonpolar cuts. In the RLN crystals with improved conductivity compared to CLN, the potential contrast of the periodic domain structures is found only on the polar cuts, where vector P_s of the domains is perpendicular to the irradiated surface. This contrast is likely because the field of the spontaneous electric polarization charges influences the secondary electrons.

We have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr_{1 – x}Ca_xTiO₃ single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr_{1 – x}Ca_xTiO₃ with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO₃ and CaTiO₃, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr_{1 – x}Ca_xTiO₃. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization P_sh = 〈P_fl²〉^1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization P_s (the contribution from the long-range order) has been determined. We have estimated the values of P_sh and P_s, which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of P_s and P_sh (which considerably exceeds the value of P_s in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr_{1 – x}Ca_xTiO₃.

Analytical expressions for the displacement vector, stain tensor, and Eshelby tensor have been obtained in the case where an inclusion in an elastically isotropic infinite medium has a polyhedral shape. The eigenstrain (e.g., the lattice mismatch) is assumed to be constant inside the inclusion but not obligatorily hydrostatic. The obtained expressions describe the strain both inside the inclusion and in its environment. It has been shown that a complex three-dimensional configuration of the elastic strain field (as well as of the displacement vector field) is reduced to a combination of simple functions having an illustrative physical and geometrical interpretation.

The peculiarities of the formation of a main crack in the Westerly granite samples under quasistatic uniaxial compression without any lateral upthrust have been studied using the data of acoustic emission (AE) and X-ray computer microtomography (CT). The multifractal analysis of pauses between acoustic emission events and the analysis of the energy distribution functional forms of the AE signal have been performed. According to the computer tomography data, defects form only near a future main crack; i.e., no stage of disperse accumulation of defects over entire sample volume has been revealed. Two stages of the main crack formation have been observed: the first stage is characterized by an exponential energy distribution of AE signals and the second stage is characterized by a power low of the AE signal energy distribution. The multifractal analysis of the pauses between neighboring AE signals demonstrate the transition from the multifractal dynamics of the acoustic emission to the monofractal dynamics when approaching a fracture moment.

High-resistance photosensitive crystals of Bi₁₂SiO₂₀ (BSO) doped with iron ions were studied. X-ray diffraction analysis reveals the compression of a unit cell in a BSO : Fe crystal with increasing impurity concentration. Electron paramagnetic resonance demonstrates a decrease in the intensity of the EPR signal when the BSO: Fe crystal is exposed to light that generates photocarriers. It is found that the characteristic time of the EPR signal decrease is close to the value of the Maxwellian relaxation time measured with the help of the longitudinal electrooptical effect. A physical model of the mechanism of optical charge exchange of magnetic iron centers is discussed, based on the statement that the nature of the crystal bonds of the iron ion with ligands without structural modification of the crystal lattice changes during the photogeneration of carriers. A physical model is proposed, according to which a trivalent Fe³⁺ ion transforms into a divalent state of Fe²⁺ with a change in the total spin from 5/2 to 2. The compression of the unit cell with increasing iron ion concentration in the framework of the model under discussion is due to the transformation of atomic orbitals upon replacement of silicon ions by iron ions. The transformation process affects the cells unoccupied by iron, which is proved by the absence of a bifurcation of X-ray reflections and indicates the long-range nature of the intracrystalline interactions in sillenites.

The reflection spectra of FeGe₂ single crystal in a wide spectral region and in the temperature range from 80 to 310 K are studied. The energy of plasma oscillations, the relaxation frequency of charge carriers, and phonon frequencies are determined. Anisotropy of the optical properties is studied. It is shown that the phase transition from the collinear antiferromagnetic structure to the spiral one is accompanied by a significant rearrangement of electronic states.

Using the Monte Carlo method, magnetic structures of the ground state and thermodynamic properties of the antiferromagnetic Ising model on a body-centered cubic lattice with competing exchange interactions are studied. The investigations are carried out for the ratio of the exchange interactions of next and nearest neighbors r = J₂/J₁ = 2/3. All possible magnetic structures of the ground state for this ratio of exchange interactions have been obtained. It has been shown that at r = 2/3 the competition of exchange interactions does not lead to the appearance of frustration and degeneracy of the ground state. Based on the histogram data analysis, it has been shown that the phase transition of the second kind is observed in the model under study at r = 2/3.

The kinetics of a spontaneous formation of liquid phase in a stretched (superheated) Lennard-Jones fcc crystal is studied. Molecular dynamics experiments are used to determine the main parameters of the nucleation process: nucleation frequency J, diffusion coefficient of nuclei D_*, nonequilibrium Zel’dovich factor Z, and critical nucleus size R_*. The calculations are performed at negative pressures from the endpoint of the melting line and at positive pressures that are higher by a factor of eight than the critical pressure. The simulation results are compared to the classical homogeneous nucleation theory. It is found that the theory qualitatively correctly reproduces the dynamics of developing the process. The theory and the simulation demonstrate good quantitative agreement for the transition rate of the liquid phase nucleus through the critical size, but there is large difference in the numbers of critical nuclei in the unit volume of the metastable phase. In the case of significant superheatings and negative pressures, the contribution of the energy of elastic stresses to the moving force of the phase transformation is small and it can be neglected in a first approximation. The mismatch between the theory and the simulation results can be eliminated taking that the surface free energy of a curved “crystal–liquid droplet” interface is smaller than that of a plane interface by 30–35%.

A simple cluster model is proposed to describe zigzag and armchair contacts of graphene to a hexagonal two-dimensional binary compound adsorbed on a metal substrate. A graphene–boron nitride heterostructure (HS) is studied in detail. Analytical expressions are obtained for the local densities of states and the occupation numbers of contact atoms. The charge transfer for quasi-free HSs is analyzed. The energy of binding of a HS to a metal substrate is estimated.

Comparative analysis of phase and relaxation transitions in poly(tetrafluoroethylene) by differential scanning calorimetry and dynamic mechanical analysis has been carried out. Elimination of procedural errors in the first case has allowed us to obtain true values of thermodynamic parameters for phase transitions and reveal their nature. Quantitative analysis of heat capacity peak profile was performed on the basis of smeared first-order phase transition theory and relaxation glass transition has been characterized.

The effect of the orientational defect (OD) on the formation process of a vortical flow v(t, r), emerging in a microsized liquid crystal (LC) cell under the action of a focused laser radiation, was studied within the nonlinear generalization of the classical Ericksen–Leslie theory by numerical methods, considering the thermomechanical contributions to both the stress tensor and viscous torque, that acts on the unit volume of the liquid crystal phase (LC phase). The analysis of the obtained results showed that the vortical flow, rotating clockwise, is generated in a “defect” LC cell close to the OD, with the OD, placed on the lower bounding surface, on which the laser radiation was focused. The rotational velocity of this flow is two orders of magnitude greater than the rotational velocity of the vortex, which is generated in a “pure” LC cell at the same conditions and rotates anticlockwise.

Using powerful synchrotron X-ray radiation of the beamline “Belok” operated by the National Research Center “Kurchatov Institute,” we perform X-ray diffraction (XRD) study of an intact, virgin (not subjected to any external mechanical loads) particle isolated from reactor powder of ultrahigh molecular weight polyethylene. Along with the peaks originating from the orthorhombic phase, we detect the peaks characteristic of the monoclinic phase that is stable only under mechanical stress, suggesting that the mechanical stress that leads to the formation of the monoclinic phase and persists at room temperature develops during the polymer synthesis. The monoclinic phase gradually disappears when the particle is heated stepwise in increments of 5 K, and its peaks become undetectable when the temperature reaches 340 K. We contrast the results obtained for the phase composition of the virgin particle to those for a tablet prepared by compaction of the same reactor powder at room temperature. XRD analyses of the tablet were performed on D2 Phaser (Bruker) instrument. The monoclinic phase that originates during the polymer synthesis and the one that forms in the tablet during compaction have different parameters. We discuss the mechanisms by which these two different monoclinic phases originate during the processes involved.