Vol 82, No 3 (2018)

Properties of the dielectric response of (K_0.5Na_0.5)(Nb_{1 – x}Ta_x)O₃ + 0.5 mol % MnO₂ (KNN-xTa) ferroelectric ceramics at temperatures including that of the structural phase transition (SPT) from the orthorhombic phase to tetragonal is studied and compared to the behavior of elastic properties in this region of temperatures. The character is determined of the concentration dependences of such anomalies as the temperature of the maximum of permittivity T_max(x) in the SPT region and temperature T_extr at which the maximum increment of permittivity upon specimen heating in the SPT region occurs. It is found temperature T_extr coincides with the one at which minimum v(T) of the velocity of a longitudinal acoustic wave is observed at SPT. Features of a nonlinear dielectric response associated with the coexistence of different types of domain boundaries in the region of a smeared structural transition in KNN-xTa ceramics are determined in studying the temperature behavior of polarization loops.

Temperature dependences of the order parameter in nanocomposites based on porous borosilicate glasses with mean pore diameters of 20 and 46 nm and filled with sodium nitrite are studied via the diffraction of synchrotron radiation. The mean diffraction sizes of NaNO₂ nanoparticles in the pores of these matrices and the temperatures of the transition of sodium nitrite to the ferroelectric state are determined. It is shown that the phase transition for these nanoparticles remains a first-order phase transition.

Microstructural features and electrophysical parameters of a number of solid solutions based on PbTiO₃ are investigated. Solid solutions (Pb_{1–3/2x + z/2}Nd_x)(Ti_1–y–zMn_yInz)O₃ and Pb_1–xCa_xTi_1–y(W_1/2Co_1/2)_yO₃ with extreme values of the mechanical Q-factor and electromechanical anisotropy are chosen as the objects of investigation. The ferro-piezoceramic samples are obtained through conventional sintering and hot pressing. X-ray structural and microstructural studies are performed, and the complex elastic, dielectric, and piezoelectric parameters of experimental samples are measured. The frequency dependences of complex parameters of the experimental samples have been studied at frequencies of up to 40 MHz. It is established that the extreme values of the mechanical Q-factor and piezoelectric anisotropy of the investigated PbTiO₃-based solid solutions are due to microstructural features associated with the technological regimes of ferropiezoceramic fabrication.

Isofrequency dependences of the spectral intensities of inelastic scattering in LiTaO₃, Pb₅Ge₃O₁₁, BaTiO₃, and KNbO₃ crystals are registered at a fixed frequency of the spectrometer close to that of the exciting radiation. Results from soft mode analysis are presented.

The structure of an M-type lead hexaferrite (magnetoplumbite) crystal is studied via the diffraction of synchrotron radiation. The crystal model contains two iron ions positions with dynamic disorder. The dynamics of the crystal lattice in the temperature range of 80 to 775 K is studied by Raman scattering. Particular attention is given to the discussing the nature of the local disorder of iron ions in the trigonal bipyramidal environment of oxygen ions.

The magnitudes and temperature dependences of the root-mean-square of polarization fluctuations, which characterizes short-range ordering below T_C, are determined for the first time for Sr_0.986Ca_0.014TiO₃ single crystals, using temperature measurements of light refraction made in a manner developed by the authors. Ab initio calculations of the equilibrium structures and total energies of three lowtemperature phases for SrTiO₃ and CaTiO₃ are performed and the symmetry of the ground state of their solid solution is identified on the basis of the obtained results. Features of short- and long-range ordering in the Ca²⁺-induced ferroelectric phase of the Sr_{1 − x}Ca_xTiO₃ system are discussed.

The applicability of the simplest cluster models for semi-quantitative descriptions of the thermodynamics of structural phase transitions in quasi–one-dimensional ferroelectrics is considered. A lead hydrogen phosphate is used as an example. Even such simple models can qualitatively and, in some cases, quantitatively reproduce experimental data.

The influence the surface energy introduced earlier by Kretschmer and Binder has on flexoelectric polarization induced by bending a dielectric plate is studied in the context of phenomenological theory. In contrast to the familiar effect this energy has on permittivity, its influence on the flexoelectric response does not vanish within a thick plate.

Experimental data on fluctuations of leakage currents over time in samples of polycrystalline ferroelectric ceramics (SBPZT-1, ceramics based on sodium bismuth lead zirconate–titanate) upon the application of a constant electric field are considered. Variations in the type of leakage current dependence on time with an increase in electric field strength on samples with different values of the loss-angle tangent are determined. Exponential decay is accompanied by random jumps, i.e., variations in the values of the leakage current.

The electrophysical properties of ceramic–crystal matrix composites are studied. Piezoelectrically active PZT/LiNbO₃ ceramic matrix composites with LiNbO₃ concentrations of 0–20 vol % are fabricated. Complex elastic, dielectric, and piezoelectric parameters are measured, and the microstructural characteristics of the obtained samples are studied experimentally. It is found that the extreme electrophysical properties of ceramic–crystal matrix composites depend on the properties and structure of the piezoceramic matrix and crystalline filler, and on the matrix microporosity produced during sintering. The electrophysical parameters of ceramic–crystal matrix composites as functions of crystalline filler content are established through the competing microporosity growth effects of the ceramic matrix and the increase in crystalline filler content.

Results are presented from measuring the volt–farad characteristics, temperature hysteresis, and current-voltage characteristics of capacitor structures based on Ba_0.55Sr_0.45TiO₃ ceramics with 12 wt % magnesium additive (BST(M)). It is shown that the rate of temperature change has a substantial effect on the type of ferroelectric hysteresis. Values of the pyroelectric coefficients for BST(M) ceramics in the paraelectric phase ((2–8) × 10⁻⁴ C m⁻² K⁻¹) are determined. It is shown that in the absence of an external electric field, the temperature hysteresis could be due to the pyroelectric effect.

Dielectric and elastic properties are studied in the vicinity of a diffuse structural phase transition in a PZT-based soft-grained ceramic material with ferroelectric–relaxor properties. Low-temperature phase transition R3c → R3m between two rhombohedral phases occurs when T < T_C. Anomalies in different parameters are observed in the region of the phase transition.

The spectral dependences of photostimulated currents in single crystals of lead magnoniobate and lead magnoniobate titanate are investigated. The results suggest that the distribution of the density of impurity states in the bandgap of the studied crystals is quite wide. These levels could participate in the formation of diffuse phase transitions.

Results from studying contact phenomena at a PZT–Pt interface are presented. The space charge region (SCR) near the PZT–Pt interface can be detected directly by means of induced current. The dependence of the change in the SCR’s width on the external bias voltage is determined.

Varying the elemental composition of ferroelectric thin films of barium strontium titanate, barium zirconate titanate, and barium stannate titanate solid solutions obtained by ion-plasma sputtering of ceramic targets at different pressures of the working gas is studied through simulation and experimentally. The experimental results are interpreted based on statistical simulation of the transport processes for fluxes of sputtered Ba, Sr, Sn, Zr, and Ti atoms in a gas medium. It is shown that the simulation results describe variations in the elemental composition satisfactorily (within 5%) in a wide range of working gas pressures. Theoretical and experimental studies enable us to choose the optimum values of technological parameters for obtaining ferroelectric thin films with specified elemental compositions that determine their electrophysical properties.

Recent advances in the fields of physical acoustics, imaging and imaging techniques, piezoelectric materials, and ultrasound transducer designs have led to the emergence of new methods and equipment for ultrasound diagnostics, therapy, and aesthetic medicine, and to the development of traditional and emerging new applications. Ultrasound transducers for medical diagnostic equipment, particularly high-intensity focused ultrasound transducers (HIFU), are one promising use of piezoceramic and composite materials. This work is devoted to the development of mathematical models, numerical modeling, and optimizing acoustic fields and transducers of this type made of porous ferroelectric piezoceramics.

The evolution of Raman spectra in a wide range of temperatures that includes the ferroelectric transition point in lithium niobate single crystals is studied for polarization geometry X(ZZ)Y. In this geometry, the soft mode responsible for the phase transition distinguished by 1A₁(TO)-type symmetry should appear in the spectra. Experimental studies show that the 1A₁(TO) mode interacts resonantly with nonfundamental modes in the low-frequency region of the spectrum. Near the ferroelectric phase transition point, an isofrequency opalescence effect is observed that consists of an abrupt increase in Raman signal intensity at fixed frequencies near the excitation line.