Vol 60, No 3 (2018)

Cubic, tetragonal, and monoclinic phases of zirconium oxide with oxygen vacancies and polyvacancies are studied by quantum chemical modeling of the atomic and electronic structure. It is demonstrated that an oxygen vacancy in ZrO₂ may act as both an electron trap and a hole one. An electron added to the ZrO₂ structure with an oxygen vacancy is distributed between two neighboring Zr atoms and is a bonding orbital by nature. It is advantageous for each subsequent O vacancy to form close to the already existing ones; notably, one Zr atom has no more than two removed O atoms related to it. Defect levels from oxygen polyvacancies are distributed in the bandgap with preferential localization in the vicinity of the oxygen monovacancy level.

Steady state leakage currents have been investigated in capacitor structures with ferroelectric solgel films of lead zirconate titanate (PZT) formed on silicon substrates with a lower Pt electrode. It is established that Pt/PZT/Hg structures, regardless of the PZT film thickness, are characterized by the presence of a rectifying contact similar to p–n junction. The steady state leakage current in the forward direction increases with a decrease in the film thickness and is determined by the ferroelectric bulk conductivity.

The effect of restricted geometry and thermal prehistory of sample preparation on phase transitions in nanocomposites on base of porous glasses with average pore diameters 7 and 46 nm filled by potassium nitrate have been studied by X-rays and neutron diffraction. The nanoparticle sizes have been determined and phase diagrams (on cooling) for these nanoparticles have been described. It is shown that there is a critical nanoparticle size in the interval from 30 till 20 nm, at which in nanocomposite the ferroelectric phase is realized only regardless of preparation method.

The results of the experimental studies and numerical simulation of the switching channel development dynamics in planar structures on the basis of vanadium dioxide are reported. The obtained data on the variation of the temperature in the channel with time and of the current arisen after the pulsed load, and on the times of transition from the high-resistance to the low-resistance state and back are analyzed in order to determine the switching mechanism and to predict the functional characteristics of the switchable vanadium- oxide structures as promising materials for the creation of relaxation generators that can serve as prototypes of neural oscillators. It is shown that the switching behavior is associated with the metal–semiconductor phase transition in vanadium dioxide, which is stimulated by the emission of Joule heat.

The results of experimental examination of galvanomagnetic properties of thin bismuth films subjected to plane tensile strain resulting from the difference in thermal expansion coefficients of the substrate material and bismuth are presented. The resistivity, the magnetoresistance, and the Hall coefficient were studied at temperatures ranging from 5 to 300 K in magnetic fields as strong as 0.65 T. Carrier densities were calculated. A considerable increase in carrier density in films thinner than 30 nm was observed. This suggests that surface states are more prominent in thin bismuth films on mica substrates, while the films themselves may exhibit the properties of a topological insulator.

A new structural model of the high-temperature ordered β-TiO phase of titanium monoxide with vacancies in metallic and nonmetallic sublattices has been proposed. The model suggests that the vacancies are simultaneously distributed over sites of two partially disordered superstructures: monoclinic M₅X_5(mon) (space group C2/m (A2/m)) and cubic M₅X_5(cub) (space group Pm\(\overline 3 \)m). The hybrid superstructure belongs to space group P1m1 of the monoclinic system. The proposed notation M_(5–11/18)X_(5–11/18) of the hybrid superstructure takes into account an expansion of the vacancy sublattice due to the superposition of initial \({M_5}{X_{{5^ - }}}\) type superstructures. It is shown that the arrangements of the superstructure reflections in the diffraction spectra of the M_(5–11/18)X_(5–11/18) and M₅X_5(cub) superstructures are identical.

The temperature dependences of the resistivity of granular high-temperature superconductor YBa₂Cu₃O_7–δ ρ(T) are measured at various transverse external magnetic fields 0 ≤ H_ext ≤ 100 Oe in the temperature range from the resistivity onset temperature T_{ρ = 0} to the superconducting transition critical temperature T_c at the transport current density from 50 to 2000 mA/cm². The effect of the external magnetic field and transport current density on the kinetics of phase transitions in both subsystems of granular two-level HTSC (T = T_c2J, T_c1g, T_c) is determined. The feasibility of the topological phase transition, i.e., the Berezinsky–Kosterlitz–Thouless transition, in the Josephson medium at T_c2J < T_BKT < T_c1g “in transport current” is established, and its feasibility conditions are studied.

An improved parameter-free method of joint Argand diagrams was used to expand the permittivity spectrum of gallium arsenide in a region of 19–26 eV into 12 bands of optical transitions with determining their maximum and half-width energies, as well as the oscillator strengths. The values of oscillator strengths of the obtained bands lie within an interval from 0.0009 to 0.06. In the regions of 19.2–21.3 and 24–26 eV, the permittivity spectrum was preliminarily calculated on the basis of experimental reflectance spectra with the use of the Kramers–Kronig integral relation method. The nature of the obtained transition bands is suggested in terms of the band-to-band and exciton transitions.

The pressure dependences of thermal emf (a parameter that ranks among the most sensitive to phase transformations) are studied for the purpose of identifying baric phase transitions in the 10–50 GPa interval in the Cd₃As₂ + MnAs (44.7% MnAs) structure formed by ferromagnetic MnAs granules in a semiconductor Cd₃As₂ matrix.

An epitaxial cubic 350-nm-thick cadmium selenide has been grown on silicon for the first time by the method of evaporation and condensation in a quasi-closed volume. It is revealed that, in this method, the optimum substrate temperature is 590°C, the evaporator temperature is 660°C, and the growth time is 2 s. To avoid silicon etching by selenium with formation of amorphous SiSe₂, a high-quality ~100-nm-thick buffer silicon carbide layer has been synthesized on the silicon surface by substituting atoms. The powder diffraction pattern and the Raman spectrum unambiguously correspond to cubic cadmium selenide crystal. The ellipsometric, Raman, and electron diffraction analyses demonstrate high structural perfection of the cadmium selenide layer and the absence of a polycrystalline phase.

The comparative study of the magnetoelectric properties and magnetostriction of HoGa₃(BO₃)₄ and HoAl₃(BO₃)₄ single crystals has been carried out. The investigated compounds exhibit qualitatively similar magnetodielectric and inverse magnetoelectric ME_E effects with the close absolute values, which is indicative of the weak effect of a nonmagnetic metal ion. On the contrary, the magnetostriction of the galloborate has been found to be threefold higher than that of the alumoborate. In addition, the difference between the qualitative behaviors of magnetostriction has been established: the magnetic-field dependence of magnetostriction for the alumoborate has the maximum near 70 kOe at T = 4.2 K, while the galloborate magnetostriction has no maximum and does not saturate in a field of 140 kOe.

We report on the magnetostriction of hexagonal HoMnO₃ and YMnO₃ single crystals in a wide range of applied magnetic fields (up to H = 14 T) at all possible combinations of the mutual orientations of magnetic field H and magnetostriction ΔL/L. The measured ΔL/L(H, T) data agree well with the magnetic phase diagram of the HoMnO₃ single crystal reported previously by other authors. It is shown that the nonmonotonic behavior of magnetostriction of the HoMnO₃ crystal is caused by the Ho³⁺ ion; the magnetic moment of the Mn³⁺ ion parallel to the hexagonal crystal axis. The anomalies established from the magnetostriction measurements of HoMnO₃ are consistent with the phase diagram of these compounds. For the isostructural YMnO₃ single crystal with a nonmagnetic rare-earth ion, the ΔL/L(H, T) dependences are described well by a conventional quadratic law in a wide temperature range (4–100 K). In addition, the magnetostriction effect is qualitatively estimated with regard to the effect of the crystal electric field on the holmium ion.

Using the method of impedance spectroscopy and optical density measurements, the diffusion of oxygen in single crystals of lithium niobate of the congruent composition after the reductive thermochemical processing is studied. The parameters describing the diffusion of oxygen in the temperature range 493–693 K are established.

A comparative study of the dielectric properties and electric polarization of multiferroics GdMn₂O₅ and Gd_0.8Ce_0.2MnO₅ has been carried out in the temperature range 5–330 K. The polarization properties in the ferroelectric state that forms due to a charge ordering and exchange striction have been studied at T ≤ T_C = 30 K. The properties of the restricted polar phase separation domains formed in the crystals containing ions Mn³⁺ and Mn⁴⁺ have been studied, too. These domains exhibit the electric polarization in the temperature range from 5 K to some temperatures T_f ≫ T_C. Such a high-temperature polarization is due to the frozen superparaelectric state of the restricted polar domains.

A new method of two-stage crystallization of lead zirconate–titanate (PZT) films using a seed sublayer with a low excess lead content has been proposed and realized. A seed layer with a strong texture of perovskite Pe(111) grains is formed from a solution with a lead excess of 0–5 wt %; the fast growth of the grains is provided by the deposition of the main film from a solution with high lead content. As a result, a strong Pe(111) texture with complete suppression of the Pe(100) orientation forms. An analysis of current–voltage dependences of the transient currents and the distributions of the local conductivity measured by the contact AFM method reveals two various mechanisms of current percolation that are determined by traps in the bulk and at the perovskite grain interfaces.

The problem of normal collision of a spherical particle with a half-space is considered with allowance for nonlocal plastic deformation in the case where the strength limit depends on the contact radius, as well as for the strengthening effect in the deformed material. The dimensionless coefficient of normal velocity restitution has been calculated numerically as a function of the initial velocity of the spherical particle. The obtained data coincide well with experimental results available in the literature.

The vibrational and thermal properties of dolomite and alkali chlorates and perchlorates were studied in the gradient approximation of density functional theory using the method of a linear combination of atomic orbitals (LCAO). Long-wave vibration frequencies, IR and Raman spectra, and mode Gruneisen parameters were calculated. Equation-of-state parameters, thermodynamic potentials, entropy, heat capacity, and thermal expansion coefficient were also determined. The thermal expansion coefficient of dolomite was established to be much lower than for chlorates and perchlorates. The temperature dependence of the heat capacity at T > 200 K was shown to be generally governed by intramolecular vibrations.

The energy distributions and the properties of the lower vacant electronic states in cobalt and nickel tetraphenylporphyrin complexes CoTPP and NiTPP are studied by X-ray absorption spectroscopy. Quasimolecular analysis of the experimental absorption spectra measured in the region of the 2p and 1s ionization thresholds of complexing metal atoms, as well as the 1s thresholds of ligand atoms (nitrogen and carbon), is based on the comparison of the corresponding spectra with each other and with the spectra of the simplest nickel porphyrin NiP. It has been established that, despite a general similarity of the spectra of nitrogen and carbon in CoTPP and NiTPP, the fine structure of the 2p and 1s absorption spectra of cobalt and nickel atoms are radically different. The observed differences in the spectra of cobalt and nickel are associated with the features of the energy distribution of vacant 3d electron states. The presence in CoTPP of the partially filled valence 3db_2g molecular orbital (MO) results in the appearance in the cobalt spectra of a low-energy band, which is absent in the spectrum of nickel in NiTPP and leads to a doublet structure of transitions to b_1g and e_g MOs due to the exchange interaction between 3d electrons in partially filled 3db_2g and 3db_1g or 3de_g MOs. The spectrum of vacant states in CoTPP differs from that in NiTPP also due to the smaller energy distance between 3db_1g and e_g MOs and the different positions of nonbonding MOs with the C2p character of the porphine ligand.

Nanosized films of stabilized zirconia with Au nanoparticles formed by implanting Au ions are studied by X-ray photoelectron spectroscopy and transmission electron microscopy. The effect of irradiation of films with Au ions and postimplantation annealing on the distribution of chemical elements and zirconium- containing ZrO_x compounds over the depth of the films is studied. Based on the data on the dimensional shift of the Au 4 f photoelectron line, the average value of the nanoparticle size is determined.

ZnO films obtained by high-frequency magnetron sputtering and doped with a Fe⁵⁷ metallic 3d impurity by the diffusion method are studied. The type of local environment of Fe⁵⁷ impurity atoms on varying the deposition parameters of ZnO films is determined by Mössbauer spectroscopy. It is established that the ground state of Fe⁵⁷ impurity atoms corresponds to metallic iron in the magnetically ordered state and there is a small fraction of Fe⁵⁷ atoms with a local environment corresponding to the complex oxide Fe₃O₄, having the magnetically ordered state; there is also a fraction of iron atoms in the paramagnetic state. The magnetic and magnetooptical parameters of the films were measured using magnetooptic Kerr effect. The spectral dependences of the polar magnetooptic Kerr effect in ZnO(Fe⁵⁷) films are measured in a photon energy range of 1.5–4.5 eV and simulated by the effective-medium method. It is established that ZnO(Fe⁵⁷) possess an easy-plane magnetic anisotropy with a magnetization lying in the film plane.

A new mode of reorientation of the field of director \(\hat n\) of a nematic liquid crystal (LC) encapsulated into a rectangular cell under strong crossed electric (E) and magnetic (B) fields has been proposed. Numerical calculations performed within the linear generalization of the classical Ericksen–Leslie theory show that transition periodic structures facilitating a decrease in the effective rotational viscosity of a nematic, formed by 4-n-pentyl-4'-cyanobiphenyl (5CB) molecules, may arise during reorientation of \(\hat n\) at certain ratios of the moments and momenta per unit volume of the LC phase and when E ≫ E_th. The calculations conducted for 5CB also indicate that the periodic structures formed in a LC cell facilitate a decrease in reorientation time τ_on of the director field.

Properties of surface plasmon polaritons of the TM type at the interface of an isotropic insulator and a periodic graphene–insulator structure have been investigated. It is established that the presence of graphene in this structure allows one to obtain (in certain frequency ranges) negative effective permittivity and implement the condition for the existence of a surface wave that is practically unabsorbed. The influence of the graphene content in the structure on the characteristics of plasmon polaritons (in particular, the possibility of their significant slowing-down) is demonstrated.

The thermal expansion coefficient (TEC) of Co₂MAl (M = Ti, V, Cr, Mn, Fe, Ni) band ferromagnets is measured in the temperature interval of 80–900 K. The magnetic-fluctuation contribution α_m, _sf(T) = α_m(T) + α_sf(T) to TEC comprised of negative Stoner α_m(T) and positive spin-fluctuation α_sf(T) contributions is presented. It is assumed that the contribution of α_m(T) component is proportional to the square of spontaneous magnetization. The contribution of α_sf(T) is stipulated by the effects of inducing local magnetic moments by spin fluctuations and is proportional to the Pauli susceptibility.