Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe12-x Al (x) O-19 (0.1 ae x ae 1.2) have been performed by powder neutron diffraction in the temperature range 300-730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe12-x Al (x) O-19 (x ae 1.2) with a collinear ferromagnetic structure.
A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.
First-principles calculations of phonon spectra based on the density functional theory are carried out for calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with a perovskite structure. By analyzing unstable modes in the phonon spectrum, the possible types of lattice distortion are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn concerning the nature of ferroelectric phenomena in the crystals studied. It is shown that the main factors determining the possible appearance of off-center atoms in the A position are the geometric size and electronic configuration of these atoms.
The methods of the density functional theory were used for the first time for the simulation of discrete breathers in graphene. It is demonstrated that breathers can exist with frequencies lying in the gap of the phonon spectrum, induced by uniaxial tension of a monolayer graphene sheet in the “zigzag” direction (axis X), polarized in the “armchair” direction (axis Y). The found gap breathers are highly localized dynamic objects, the core of which is formed by two adjacent carbon atoms located on the Y axis. The atoms surrounding the core vibrate at much lower amplitudes along both the axes (X and Y). The dependence of the frequency of these breathers on amplitude is found, which shows a soft type of nonlinearity. No breathers of this type were detected in the gap induced by stretching along the Y axis. It is shown that the breather vibrations may be approximated by the Morse oscillators, the parameters of which are determined from ab initio calculations. The results are of fundamental importance, as molecular dynamics calculations based on empirical potentials cannot serve as a reliable proof of the existence of breathers in crystals.
New schemes have been proposed for the structural classification of carbon phases and nanostructures. These schemes are based on the types of chemical bonds formed in materials and on the number of nearest neighbors with which each atom forms covalent bonds. The classification schemes allow one to describe the already known phases and form the methodological basis for the prediction of new phases and nanostructures.
Silicon dioxide amorphous films are the key insulators in silicon integrated circuits. The physical properties of silicon dioxide are determined by the electronic structure of this material. The currently available information on the electronic structure of silicon dioxide has been systematized.
We consider two transient thermal processes in uniformly heated harmonic crystals: (i) equalibration of kinetic and potential energies and (ii) redistribution of the kinetic energy among the spatial directions. Equations describing these two processes in two-dimensional and three-dimensional crystals are derived. Analytical solutions of these equations for the square and triangular lattices are obtained. It is shown that the characteristic time of the transient processes is of the order of ten periods of atomic vibrations. The difference between the kinetic and potential energies oscillates in time. For the triangular lattice, amplitude of the oscillations decays inversely proportional to time, while for the square lattice it decays inversely proportional to the square root of time. In general, there is no equipartition of the kinetic energy among spatial directions, i.e. the kinetic temperature demonstrates tensor properties. In addition, the covariance of velocities of different particles is nonzero even at the steady state. The analytical results are supported by numerical simulations. It is also shown that the obtained solutions accurately describe the transient thermal processes in weakly non-linear crystals at short times.
The heat capacity, thermal conductivity, thermal diffusivity, and thermal expansion of Bi4Ge3O12 single crystals have been measured over a wide temperature range.
For the first time, zinc oxide epitaxial films on silicon were grown by the method of atomic layer deposition at a temperature T = 250°C. In order to avoid a chemical reaction between silicon and zinc oxide (at the growth temperature, the rate constant of the reaction is of the order of 1022), a high-quality silicon carbide buffer layer with a thickness of ~50 nm was preliminarily synthesized by the chemical substitution of atoms on the silicon surface. The zinc oxide films were grown on n- and p-type Si(100) wafers. The ellipsometric, Raman, electron diffraction, and trace element analyses showed that the ZnO films are epitaxial.
We present the results of studies of the dielectric properties of nanocomposites based on Al2O3 oxide films with a pore size of 330 and 60 nm with particles of an organic ferroelectric diisopropylammonium bromide (C6H16BrN, DIPAB) introduced into the pores, aimed at determining the size dependences of phase transition parameters. A shift in the phase transition to low temperatures and diffusion of the transition are found, which become more significant for smaller pores. A broadening of the temperature hysteresis of the dielectric constant of nanocomposites during the phase transition was also observed. The decrease in the phase transition temperature in nanocomposites with DIPAB nanoparticles is consistent with theoretical models of the size effects on the structural phase transition.
The regularities of the effect of the shape of interphase boundaries in small volume systems on the separation of solutions with an upper critical solution temperature (UCST) are described by the example of Bi–Sb alloy particles with a core–shell configuration. The change in the shape of interphase boundaries is simulated in general by introducing a parameter corresponding to the degree of deviation of the shape of the boundaries from the spherical one. An analysis of the extrema of the Gibbs function revealed regularities in the effect of the shape of the core and shell phases on phase equilibria, the thermodynamic stability of heterogeneous states, and the phase separation diagram. The deviation of the shape of the interphase boundaries from the spherical shape changes the UCST and the mutual solubility of the components. The deformation of the shell of a core–shell particle increases the thermodynamic stability of the heterogeneous state, which contributes to the separation of the solution. The deformation of the core lowers the thermodynamic stability of the heterogeneous state and expands the range of metastable states.
Mesoporous nanocrystalline CaF2 powder was produced by pulsed electron beam evaporation (PEBE) in vacuum. The specific surface area (SSA) of CaF2 nanopowder (NP) reached 88.7 m2/g. The effect of in-air thermal annealing in the temperature range of 200–900°C on the particle size, morphology, textural, thermal, magnetic, and luminescence properties of NPs is studied. A strong deviation from stoichiometry is observed in produced nanoparticles and a significant increase in the SSA after annealing at 200°C. The obtained CaF2 NP shows ferromagnetic (FM) behavior. The FM response appearance can be explained by the formation of structural and radiation defects. An analysis of pulsed cathodoluminescence (PCL) and magnetization curves of CaF2 NPs allows conclusions about their interrelation.
The “conductivity logarithm–composition” correlation logσdc(x) = ax + b has been revealed for the strongly nonstoichiometric fluorite Sr1 – x R x F2 + x (x is the RF3 molar fraction, 0.15 ≤ x ≤ 0.47) and tysonite R1 – y Sr y F3 – y (y is the SrF2 molar fraction, y = 1 – x, 0.85 ≤ x ≤ 0.97) phases in the SrF2–RF3 systems (R = La, Ce, Pr, Nd). The conductivity σdc of the eutectic 70LaF3 + 30SrF2 (mol %) composite fits well with the dependence logσdc(x) for the nonstoichiometric Sr1 – x La x F2 + x and La1 – y Sr y F3 – y phases.
ZnO together with TiO2 is a main photocatalyst for various redox reactions to convert light energy into a chemical one and to purify the environment. Intrinsic surface defects in ZnO—the vacancies in anionic and cationic sublattices (F-type and V-type centers)—allow creation of long-lived (up to 103 s) photocatalysis centers and, therefore, tenfold increase in quantum yield of reactions. Slow surface states—the photocatalysis centers—appear via diffusion of electrons and holes generated during the interband transitions in the bulk of a photoactivated sample. The transfer efficiency, however, decreases sharply because of recombination of charge carriers and losses during overcoming the surface Schottky barrier. Neutral energy carriers—excitons—were used in this work to decrease these losses during the energy transfer to a surface. High exciton binding energy in ZnO (60 meV) allows it to move at room temperature without decay. The exciton energy loss for radiation is effectively decreased in our experiments via formation of a 2D surface structure. The results confirm high efficiency of exciton channel to form surface long-lived photocatalysis F-centers and V‑centers during the photoadsorption and photodesorption processes of oxygen, which simulate full cycle of a redox photocatalytic reaction.
We analyzed magnetic force microscopy images of the surface of samples cut parallel and perpendicular to the texture created by applying a field in the process of pressing a magnet. The distribution of the linear size (width) of the domains correlates with the distribution of the projection area of grains onto the shear plane in sintered (PrDy)(FeCo)B magnets with radial magnetization and in prismatic (NdDy)(FeCo)B magnets. The effects of grain size and direction of the axes of easy magnetization of grains on the width of the domains, as the main factors controlling the domain structure, are discussed.
Based on the solution of the hydrodynamic equations and Maxwell’s equations, we show that an external quasi-homogeneous magnetic field leads to the emergence of a secondary electric field that is resulted from a nonlinear effect over magnetic potential A. This field is proved to exist in the region with a depth of $$\delta {\text{/}}2$$ , where δ is the London penetration depth. The hydrodynamic flow velocity is estimated.
Results of the study of the effect of various types of plastic deformation on microstructural features and change in physical and mechanical properties of the nonstoichiometric Heusler alloy Ni47Mn42In11 are shown. It was demonstrated that the deformation by rolling and upsetting leads to an increase in the microhardness and to an embrittlement of the investigated alloy. Severe plastic deformation by torsion under high pressure of 8 GPa at room temperature was found to strongly refine initially coarse grain and to contribute to the formation of a nanocrystalline structure with grain fragments up to 10 nm. In this case, the fraction of viscous constituent on the fracture and the microhardness increased, while the magnetic susceptibility decreased.
Two-dimensional hexagonal boron nitride (h-BN) as a graphene-like material was investigated due to its impending applications in electronics. The h-BN band gap E g as an important factor and its variation between bilayer ZrSe2 sheets were explored under an external electric field. The initially indirect band gap is found to convert to direct band gap by means of density functional theory. Additionally, the band gap is modulated by van der Waals corrections from 0.21220 to 0.01770 eV. Based on the results, the proposed heterostructure is converted to the direct band gap, and band gap smoothly decreased from 0.25440 to 0.0436 eV following the application of external electric field from 0.2 to 0.6 eV. Moreover, ZrSe2|h-BN|ZrSe2 is investigated under the applied biaxial compressive strain from 1 to 4%. The findings demonstrated that the gap was decreased by any compressive strain amplification, while the semiconducting behavior in the heterostructure attained to the semi-metallic performance under the increasing strain.
The independent components of the tensor of piezoelectric moduli are calculated for various 2D nanoallotropes of boron nitride. The principle of the proposed approximation calculation method consists in the fact that the effective dipole moment of the unit cell of the 2D structure normalized to the unit area is expressed through the tensor of elastic rigidities and relative cell deformations. It is shown that, in addition to well-known graphite-like boron nitride h-BN, its other hexagonal and tetragonal nanoallotropes possessing higher piezoelectric properties in comparison to h-BN can be of practical interest as well.
A crystallophysical model of ion transfer in the superionic Pb1 – x Sn x F2 conductor with a fluorite (CaF2) structure is proposed. The concentration dependence of the ionic conductivity of Pb1 – x Sn x F2 single crystals and poly- and nanocrystals is analyzed. The single-crystal form of the superionic conductor is characterized by the highest conductivity. The mobility and concentration of anionic charge carriers in a single crystal and ceramics of Pb1 – x Sn x F2 (x = 0.2) is calculated on the basis of structural and electrophysical data. The mobility of carriers μmob = 2.5 × 10–6 cm2/s V (at 293 K) in a single crystal is seven times higher than in nanoceramic. The concentration of carriers n mob = 1.7 × 1021 and 3.6 × 1021 cm3 (4.5 and 9.5% of the total number of anions) for a single crystal and nanoceramic, respectively. The comparison of isostructural Pb0.8Sn0.2F2, Pb0.67Cd0.33F2, and Pb0.9Sc0.1F2.1 single crystals shows that anionic carriers have a maximum mobility in the β-PbF2 and SnF2 based solid solution.
