Vol 68, No 5 (2023)

Acousto-optic (AO) interaction in optically biaxial crystals is of great interest due to the variety of phase matching geometries caused by the low symmetry of the acoustic, optical, and photoelastic properties. The current state of the research in the field of acousto-optics of biaxial crystals is presented. Some promising biaxial crystals of the orthorhombic and monoclinic systems, which are of practical interest as materials for AO devices, are considered. The unique geometries of isotropic and anisotropic diffraction that exist in only biaxial crystals are analyzed, and the AO devices based on these crystals are briefly reviewed.

The found effect of lowering the symmetry of the crystal structure of thin (less than 1 mm) plates and bars of strontium titanate (SrTiO3) single crystals is discussed. This symmetry lowering manifests itself in the electron spin resonance (ESR) spectra of impurity centers (Fe³⁺ and Mn⁴⁺ ions, used as paramagnetic probes). It is shown that symmetry lowering is observed at temperatures T > 105 K, which generally correspond to the cubic phase of SrTiO3; it leads to the formation of a tetragonal nonpolar structure, differing from the antiferrodistortive (AFD) phase D4ℎ18, which is characteristic of strontium titanate at T < 105 K and was not observed previously in SrTiO3. The factors determining the distortion value are found to be the geometry and ratio of sample sizes, surface finishing quality, and crystallographic orientation of plates.

The structure of the relaxor ferroelectrics PbNi1/3Ta2/3O3 (PNT) was studied by powder X-ray diffraction. The measurements were performed at 313.5 ± 1 K using a powder, which was prepared by grinding PNT single crystals grown by the spontaneous crystallization. The structure refinement and the fitting of the simulated diffraction pattern to the experimental data were performed by the Rietveld method. It was demonstrated that the grown crystals of PNT have a perovskite structure (sp. gr. Pm3m (221), a = 4.02679(2) Å). Polarized Raman spectra of PNT were recorded at room temperature. The main modes of the light scattering spectra were assigned to the Е1 and А1 components of the transverse optical phonon (ТО1) and the А1 component of the longitudinal optical phonon (LO3). The temperature dependence of the dielectric susceptibility shows a broad frequency-dependent anomaly with a maximum at 89 К at a frequency of 1 kHz.

The polarization reversal of Rb2ZnCl4 crystals in the incommensurate phase near a ferroelectric phase transition (PT) under the action of harmonic electric field has been studied using harmonic analysis. During polarization reversal of a test sample under the action of harmonic electric field, a PT induced by the electric field from the incommensurate phase to the ferroelectric phase and a PT from the field-induced ferroelectric phase back to the incommensurate phase occur. The study of the active and reactive contributions to the amplitudes of current density harmonics made it possible to determine the current values of harmonic electric field intensity at which the corresponding PTs occur.

The influence of a permanent magnetic field on macro- and nanoscopic properties of triglycine sulfate (TGS) crystals with impurity chromium (TGS-Cr) has been investigated. This work continues the previous studies of magnetically induced effects in ferroelectrics. A specific feature of TGS crystals is the presence of a nanoscale relief on a polar (010) cleavage; this nanorelief is a qualitative characteristic of the crystal defect structure. It is shown that the exposure of a crystal in a magnetic field of 2 T leads to a change in its dielectric properties, accompanied by a long-term transformation of the nanorelief. The results obtained are indicative of a magnetically induced change in the defect crystal structure. A qualitative difference is found between the magnetically induced effects in TGS-Cr crystals and undoped TGS crystals. The relationship between the magnetically induced effects and structural defects is discussed.

One from the most important features of relaxors is the possibility of inducing a stable ferroelectric phase in them by applying an electric field. This induced phase transition has been multiply investigated; in particular, the structural changes during the transition and its kinetics were traced. However, the processes occurring under electric field switching remain little-studied, and the structural transformation during switching generally has not been investigated at all. To fill in this gap, the temporal evolution of the pattern of Bragg and diffuse X-ray scattering in classical relaxor PbMg1/3Nb2/3O3 at T = 175 K during multiple switching of the direction of a dc external field E = ± 6.25 kV/cm has been traced. It is shown that the glasslike state decays and a mixed ferro-glass phase is formed after switched on a field. Switching the field sign leads to a rise of dipole-glass correlations; however, no signs of occurrence of an inhomogeneous state with limited regions of dipole-glass and ferroelectric phases were found. Switching the field to the initial direction causes rapid formation of the ferroelectric phase, without any rise in the dipole-glass correlations. A repeated switching leads to a decrease in the Bragg scattering intensity, related to the long-range order. This effect is presumably due to the occurrence of random weakly correlated ionic displacements, violating the long-range order.

The forward domain growth in polar direction has been investigated on the example of the formation of isolated wedge-shaped domains and arrays of domains on lithium niobate nonpolar cuts under an electric field of a scanning probe microscope. Domain growth occurs due to the generation of steps and motion of charged kinks along charged domain walls (CDWs). A simulation of field spatial distribution showed that the generation of steps near a domain vertex is mainly caused by the effect of external field, whereas the forward growth is due to the kink motion in the field induced by neighboring kinks. Scanning by a probe tip with an applied voltage leads to the self-assembled formation of domain arrays with domain length alternation: doubling, quadrupling, and chaotic behavior under the action of the depolarizing fields formed by three neighboring domains.

The room-temperature X-ray and Mössbauer data on the BiFeO3, Bi0.95Sr0.05FeO3 – y, and Bi0.90Sr0.10FeO3 – y multiferroics obtained by solid-phase synthesis are presented. The samples have a rhombohedral crystal structure with the sp. gr. R3c. The lattice parameter ah is invariable, while the parameter ch decreases with increasing strontium content. The 57Fe Mössbauer spectra recorded at a temperature of 295 K have been interpreted within the cycloid spatial spin-modulated structure model. It has been established that the easy-axis magnetic anisotropy is implemented in the investigated multiferroics. The anharmonicity parameter m of the spin-modulated structure has been measured. Upon replacement of trivalent Bi3+ ions in small amounts (x = 0–0.10) with divalent Sr2+ ions, the parameter m increases by a factor of more than 3: from 0.10(4) at x = 0.00 to 0.36(10) at x = 0.10.

Fe/BaTiO3, Fe/SrTiO3, Co/BaTiO3, and Co/SrTiO3 heterostructures, which exhibit magnetoelectric effect, have been investigated. It is shown that the magnetic properties of thin ferromagnetic films can be controlled using an external electric field. The structural, electronic, and magnetic properties of the heterostructures have been investigated applying ab initio calculation methods. It is shown that, using the inverse piezoelectric effect, one can reduce the absolute value of the ferromagnet magnetization vector. This approach may be a basis for controlling the properties of one of the ferromagnetic layers of a superconducting spin valve and, as a consequence, the superconducting properties of the valve.

Single-phase ceramic samples of new compositions (Na1 – хSrх)0.5Bi0.5TiO3 (x = 0–0.5), including those modified by additives of SiO2 and ZnO oxides, have been obtained by solid-phase synthesis. The crystal structure and microstructure of these samples, as well as their dielectric, nonlinear optical, and local piezoelectric properties, have been studied. The formation of a perovskite-type phase with a pseudocubic unit cell in all synthesized samples and an increase in the cell volume as a result of partial substitution of perovskite structure cations are established. A decrease in the temperature of ferroelectric phase transitions (confirmed by the methods of dielectric spectroscopy and laser second-harmonic generation) to the tetragonal paraelectric phase is revealed. Remanent piezoelectric hysteresis loops are obtained for the synthesized samples in the polarization switching mode; this result confirms the occurrence of ferroelectric polarization switching.

The dielectric properties of ferroelectric composites and the specific features of the phase transitions occurring in them are discussed in comparison with the homogeneous ferroelectrics incorporated in the composites studied. The components incorporated into the dielectric matrix of ferroelectrics are considered to be triglycine sulfate, single crystals of potassium dihydrogen phosphate group, sodium nitrite, and perovskite-type materials. The factors changing the temperature range of polar phase existence in the ferroelectric composites under consideration are revealed and discussed. The results of the studies performed in this field are briefly reviewed. The work with the ferroelectric components incorporated into the aforementioned composites was performed in cooperation and under the guidance of L.A. Shuvalov.