Vol 87, No 9 (2023)

The results of the group-theoretical analysis of phase transitions in the LiRh₂O₄ are presented. It is found that the critical irreducible representation inducing phase transitions and multiorder in this substance is the eight-dimensional representation \(~{{k}_{{11}}}{{{\text{\tau }}}_{5}} + {{k}_{{10}}}{{{\text{\tau }}}_{2}}\). It is shown that the multiorder and the structural mechanism of formation of the tetragonal phase of lithium rhodonite are related to the displacements of oxygen atoms, the tilts of the [RhO]₆ octahedra, and the ordering of the rhodium t_2g orbitals.

Using the model of Gaussian subchains “beads on a thread”, the reduced parameters of the state equation for porous polypropylene films in various solvents are found. It is shown that to satisfy to experimental data, the coil-globule transition is shifted towards higher values of the Flory–Huggins interaction parameter.

We presented the results of the study of solid solutions based on Bi_0.5La_0.5MnO_{3 ± σ} modified Nb₂O₅. Structural, microstructural, magnetic-dielectric and magnetic-resistive correlation in a wide range of frequencies and temperatures are investigated. Correlations between the structure of ceramics, their dielectric properties and behavior in a magnetic field are established.

The dielectric properties of (1 – x)BaTiO₃–xSrTiO₃ (x = 0.0; 0.25; 0.50; 0.75 and 1.00) mixed nanocomposites, obtained by sintering the initial components at a temperature of 1273 K for 5 hours, have been studied within the temperature range of 100–500 K. X-ray analysis, performed at room temperature, showed that the composites are formed by BaTiO₃ (P4mm) and SrTiO₃ (Pm-3m) crystallites and do not contain Ba_{(1 – х)}Sr_хTiO₃ solid solutions. A decrease in the temperatures of the Amm2 → P4mm → Pm-3m phase transitions in barium titanate particles with an increase in the SrTiO₃ concentration in the composite was found.

It is shown that the energy of nonequilibrium nonthermalized holes in ZnS crystals with a cubic unit cell does not depend on temperature. The nature of the temperature dependence of the nonequilibrium nonthermalized holes mobility in ZnS crystals depends on the average relaxation time upon scattering by ions and neutral impurity atoms, dislocations, and microstrains.

The results of dielectric hysteresis loops and dielectric permittivity studying of 0.725NaNbO₃–0.20KNbO₃–0.075CdNb₂O₆ ceramics in strong (5–40 kV · cm^–1) and weak (0–0.3 kV · cm^–1) alternating electric field are presented. The parameters of the reversible and irreversible contributions to the dielectric permittivity are determined. Based on the Preisach model, a diagram of the density of domain switching is constructed. It is shown that the distribution function of domains over local coercive fields is characterized by a sharp peak, which indicates a high degree of homogeneity of the domain structure.

The temperature behavior of the LaMnO₃/Bi₄Ti₃O₁₂/Ba_0.4Sr_0.6TiO₃/MgO heterostructure resistance was studied. Within the temperature range of 120–180 K, an anomalous region with two resistance maxima was found. Below 120 K, a transition to a metallic character of conductivity occurs. It was revealed the significant increase of the heterostructure resistance under the influence the green light radiation.

The effect of a weak static magnetic field on the low-frequency dielectric permittivity of the crystals of nominally pure triglycine sulfate (TGS): and TGS containing cobalt ions (TGS + Co²⁺) is investigated. In TGS, the magnetic effect is essentially anisotropic: it is completely absent when the vectors \(\vec {B}\) and \({{\vec {P}}_{{\text{s}}}}\) are collinear and is most pronounced when \(\vec {B} \bot {{\vec {P}}_{{\text{s}}}}\) and the magnetic field is oriented along the crystallophysical axis \(\vec {a}\). In TGS + Co²⁺ magnetic effect is noticeably stronger than in TGS, but practically does not depend on the mutual orientation of vectors \(\vec {B}\) and \({{\vec {P}}_{{\text{s}}}}\).

The method of formalized modeling revealed the complex nature of the dependence of the parameters of the process of rearrangement of the biocomposite microstructure on the temperature gradient. Based on the analysis of the dynamics of changes in the potential difference formed along the thickness of the birch wood microcut, the parameters of the biocomposite polarization process for various temperature gradients have determined.

As a result of comparative studies of the electric and dielectric properties of sodium-lithium niobate ceramics with a percentage of Li equal to 10 and 40 mass %, it was found that both materials have a hopping conduction mechanism. At frequencies from 500 Hz to 100 kHz, thermal ionic polarization predominates. In the low-frequency region, migratory polarization predominates. The revealed difference in the temperature behavior of the complex conductivity of the studied samples made it possible to explain the presence of ferroelectric properties in the sample with 10% Li and their absence in the sample with 40% Li.

A structure based on a thin-film nanocrystalline zinc oxide obtained by the sol-gel method on a flexible Kapton substrate has been developed. It has been established that its electrical resistance increases significantly under the influence of moisture contained in the air. When irradiated with ultraviolet radiation, the resistance of the structure decreases by almost two orders of magnitude. After the UV exposure finish, a long-term process of restoration of electrical conductivity is observed, which is described by the fractional-exponential Kohlrausch function.

We study the processes of piezoelectric relaxation that occur in piezoelectric ceramics under the influence of weak constant electric fields. Precise measurements of the piezoresonance spectra of the ferroelectric ceramics of the PZT system at various directions of the applied constant electric field and polarization were performed using the method and program for the analysis of piezoelectric resonance for the radial and thickness resonant vibration modes of piezoceramic disks. A qualitative analysis of the time dependences of the real and imaginary parts of the piezoelectric constants of the studied piezoceramic disks obtained because of mathematical processing of sequentially measured piezoresonance spectra was carried out, and a physical interpretation of the results was proposed.