Narrow Results

Solid solutions (SS) with a quasi-binary cross-section of a four-component system of the composition $(1-x)$(Na${}_{0.5}$K${}_{0.5}$)NbO₃–xPb(Ti${}_{0.5}$Zr${}_{0.5}$)O₃, based on compositions with fundamentally different physical responses (Na, K)NbO₃ (KNN), Pb(Ti, Zr)O₃ (PZT), have been prepared by a two-stage solid-phase synthesis followed by sintering using conventional ceramic technology. The influence of thermal cycling on the dielectric properties of ceramic SS with $x=0.00,\dots ,1.00$ has been studied. It has been shown that thermally induced fatigue does not have a significant effect on the Curie temperature and the diffusing of the phase transition (PT) of SS containing only extreme components. It has been found that when the concentration of PZT in the studied system is varied, the ${\epsilon }^{\prime }$/${\epsilon }_0(T)$ dependencies take on a form characteristic of ferroelectrics with a diffuse PT. When the repolarisation cycles are varied, the diffusing parameter and the Curie and Burns temperatures show a wave-like behavior. The observed effects are explained by the interaction of different defect types. A conclusion is drawn on the possible practical applications of the studied SS.

Pure and superstoichiometric modified by 2wt.% Bi₂O₃ and 2wt.% manganese oxide (Mn₂O₃) ceramic samples of the binary system 0.71BiFeO₃-0.29BaTiO₃ were fabricated using conventional ceramic technology by double solid-phase synthesis with following sintering. Mechanical activation of synthesized powders was carried out at the stage of manufacturing press powders prepared for sintering. X-ray analysis at room temperature showed that all studied ceramics have pseudo-cubic symmetry. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The modification increased the homogeneity of the material, dielectric constant and piezomodulus $d_{33}$ and also resulted in a decrease in the dielectric loss tangent. The highest piezoelectric coefficient $d_{33}=105$ pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 740–770K.

TlInSe₂ crystals were grown by Bridgman method. The thermal conductivity coefficient was studied in the temperature range of 80–600K, along and perpendicular to the parallelepiped sample layers. The corresponding parameters of the TlInSe₂ crystal were calculated based on the performed experiments. The results of the performed analysis have shown that the theoretical calculations and experimental results are of the same order, while the obtained and calculated values differ from each other by a factor of two. The temperature dependence of the thermal conductivity coefficient of TlIn${}_{1-x}$Dy_xSe₂ solid solutions is also studied. Moreover, effect of the gamma irradiation on the temperature dependence of the thermal conductivity of the TlInSe₂ crystals were studied.

Luminescence spectra of Ce³⁺-ion in Ca_1-xSr_xS solid solutions have been investigated. It has been shown that the evolution of Ce³⁺ excitation and emission spectra through the concentration range may be interpreted in correlation with the variations, with the host lattice composition, of the lattice constant, the local structure in the activator neighborhood and the band gap. Under activator excitation, in addition to the two conventional bands, an additional band peaking at about 650 nm was observed (mainly in SrS) and was related to the contamination of the samples as a consequence of extended storage. Except for this band, the luminescence properties did not change relatively to those obtained previously on the same samples. The most drastic modification affected SrS:Ce³⁺, despite the fact that all the samples have been stored in the same conditions.

Recently first principles microscopic calculations, using the generalized gradient approximation, appeared for the solid mixed system AgCl_xBr_1-x at various compositions. Here, we suggest a model that can estimate the compressibility of the mixed crystals in terms of the compressibilities of the end members alone. This model makes use of a single parameter, i.e. the compressibility of a defect volume, when considering the volume variation produced by the addition of a "foreign molecule" to a host crystal as a defect volume.

The first-principles calculations of band gaps and bulk moduli of cesium lead halide perovskite solid solutions, $\mathrm{\text{CsPb}}{({\mathrm{\text{I}}}_{1-x}{\mathrm{\text{Cl}}}_x)}_3$ and $\mathrm{\text{CsPb}}{({\mathrm{\text{I}}}_{1-x}{\mathrm{\text{Br}}}_x)}_3$, are performed at the level of general gradient approximation of the density functional theory. We use supercell approach for computational modeling of disordered systems, which gives a description of the properties of the structure baasing on the average over a set of multiple configurations, namely distributions of different species over a given set of atomic positions. The calculations were performed with the CRYSTAL14 program package. The dependence of the band gap and bulk modulus on the content $x$ are investigated over the whole range $0\le x\le 1$.

The processes of charge transport on alternating current in ${({\mathrm{\text{TlGaSe}}}_2)}_{1-x}{({\mathrm{\text{TlInSe}}}_2)}_x$ solid solutions have been studied. It has been established that in weak alternating electric fields, there is a hopping mechanism of charge transfer over localized states in the vicinity of the Fermi level. A quantitative assessment of parameters was made in the framework of the effective medium theory and the Mott approximation. The use of impedance spectroscopy methods in ${({\mathrm{\text{TlGaSe}}}_2)}_{1-x}{({\mathrm{\text{TlInSe}}}_2)}_x$ solid solutions in the frequency range of 25–106 Hz, at temperatures of 180, 240 and 300 K charge transfer processes has been investigated. It was found that at 300 K in the low-frequency region, there are additional contributions to the conductivity, which, apparently, is associated with diffuse ion transfer near the boundary of the solid electrolyte and the electrode. Impedance locus curves, at low frequencies and at a temperature of 300 K, are characteristic of Warburg diffuse impedance.

Research study of volt-amperage properties (VAP) of $\mathrm{\text{Ca}}{({\mathrm{\text{Al}}}_{0.1}{\mathrm{\text{Ga}}}_{0.9})}_2{\mathrm{\text{S}}}_4:{\mathrm{\text{EuF}}}_3$ crystals determined the mechanism of current flow though the studied samples and this mechanism is based on barrier Schottky emission and emission of Franklin–Paul. The dielectric constant of the material, height of the potential barrier on metal–semiconductor border, concentration of the traps and the effective mass of electrons are calculated.

The solid solutions of ${\mathrm{\text{Ca}}}_x{\mathrm{\text{Ba}}}_{1-x}{\mathrm{\text{Ga}}}_2{\mathrm{\text{S}}}_4(x=0,0.1,\dots ,1)$ were synthesized by solid-phase reactions from powder components of CaS, BaS, and Ga₂S₃. The temperature-concentration dependences of the Gibbs free energy of formation of ${({\mathrm{\text{CaGa}}}_2{\mathrm{\text{S}}}_4)}_x{({\mathrm{\text{BaGa}}}_2{\mathrm{\text{S}}}_4)}_{1-x}$ solid solutions from ternary compounds and phase diagrams of the CaGa₂S₄–BaGa₂S₄ were determined by a calculation method. It was revealed that continuous solid solutions are formed in these systems. The spinodal decomposition of ${({\mathrm{\text{CaGa}}}_2{\mathrm{\text{S}}}_4)}_x{({\mathrm{\text{BaGa}}}_2{\mathrm{\text{S}}}_4)}_{1-x}$ solid solutions into two phases is predicted at ordinary temperatures.

The influence of the $B$-site ion substitutions in ($1-x)$(Bi${}_{1∕2}$Na${}_{1∕2})$TiO₃–$x$BaTiO₃ system of solid solutions on the relative stability of the antiferroelectric (AFE) and ferroelectric (FE) phases has been studied. The ions of zirconium, tin, and (In${}_{0.5}$Nb${}_{0.5})$, (Fe${}_{0.5}$Nb${}_{0.5})$, (Al${}_{0.5}$V${}_{0.5})$ ion complexes have been used as substituting elements. An increase in the concentration of the substituting ion results in a near linear variation in the size of the crystal lattice cell. Along with the cell size variation, a change in the relative stability of the AFE and FE phases takes place according to the changes of the tolerance factor of the solid solution. An increase in the tolerance factor leads to the increase in the temperature of the FE–AFE phase transition, and vice versa. Obtained results indicate the way for raising the temperature of the FE–AFE phase transition in (Bi${}_{1∕2}$Na${}_{1∕2})$TiO₃-based solid solutions.

Lead-free sodium–potassium niobate-based piezoelectric materials are most intensively studied in order to replace the widely used Pb-based ones. In this work, the effects of modification of compositions by donor and acceptor dopants in the A- and B-sites of perovskite lattice on structure, dielectric, ferroelectric, and piezoelectric properties of ceramics from Morphotropic Phase Boundary in the ($1-x$)(K${}_{0.5}$Na${}_{0.5}$)NbO₃–$x$BaTiO₃ system and in compositions with $x=0.05$ and 0.06 additionally doped by Ni${}^{3+}$ cations have been studied.

Ceramic solid solutions GdNb_x${\text{Ta}}_{1-x}$O₄ ($x=0$–1) were synthesized by sol–gel. Morphological particularities of microstructure were studied, strength characteristics (Young’s modulus) and stress intensity factor for mode I $K_{\mathrm{\text{IC}}}$ were evaluated. The latter is a criterion of crack resistance of the material. Luminescent properties of solid solutions GdNb_x${\text{Ta}}_{1-x}$O₄ were researched under UV exciting radiation. A comparatively small addition of Ta ($x=0.9$) was shown to increase luminescence intensities of centers ${\text{Nb}}^{4+}$–O⁻. An addition of Ta to GdNbO₄ leads to creation of solid solution ${\text{GdNb}}_{0.9}$${\text{Ta}}_{0.1}$O₄. Intensity of these centers is 3–6.5 times larger than that of GdNbO₄ and GdTaO₄.

Some of the known binary ABO₃perovskites with ferro- (FE) or antiferroelectric (AFE) at $T_{\text{C}}$ and different magnetic phase transitions (PT): ferro-and antiferromagnetic at $T_{\text{N}}$ and also some of their solid solutions are considered. Some correlations between their FE or AFE and/or magnetic PTs temperatures, on the one hand, and their interatomic bond $A$–O strains, on the other hand, have been constructed. It is shown that in the plotted diagrams these temperatures change with a change in ${\delta {}_A}_{\text{O}}$ values as follows: classical FEs are followed by multiferroics, starting with BiFeO₃ and ending with YVO₃, followed by classical AFEs. At the same time, the temperatures $T_{\text{C}}$ and $T_{\text{N}}$ experience maxima at the corresponding points for BiFeO₃, then quickly decrease, and the difference between them, $T_{\text{C}}$-$T_{\text{N}}$, basically also decreases, slightly increasing along the way to the point EuTiO₃. which made it possible to systematize these $T_{\text{C}}$ and $T_{\text{N}}$.

Ceramics of quasi-binary concentration section ($x$ = 0.50, 0.1 $\le y\le$ 0.2, $\mathrm{\Delta }y$ = 0.025) of the ternary solid solution system (1$-x-y)$BiFeO₃–$x$${\text{PbFe}}_{0.5}$${\text{Nb}}_{0.5}$O₃–$y$PbTiO₃ were prepared by the conventional solid-phase reaction method. By using X-ray diffraction technique, the phase diagram of the system was constructed, which was shown to contain the regions of cubic and tetragonal symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of the selected solid solutions were investigated. The highest piezoelectric coefficient $d_{33}$= 260 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior, a region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 350–500 K.

Solid solutions (SS) of 3- and 4-component systems based on lead titanate-zirconate were prepared by the method of solid-phase reactions and uniaxial hot pressing. The dependences of the relative permittivity of polarized samples on the electronegativity (EN) of their constituent cations have been studied. The ferro-hardness of the SS (the stability of the domain structure to external influences) is shown to be directly dependent on the EN of elements B in the corresponding oxidation states, i.e., the degree of covalence of the B–O bond. The deviation from this dependence in SS with Ni and Cd is explained by their individual features, which result in changes in the degree of bond covalence in both cationic sublattices. The conducted crystal-chemical analysis made it possible to choose promising SS when creating ferroelectric materials, including textured piezoelectric ceramic materials for piezoelectric transducers for various purposes: Piezotransformers, piezoelectric motors, ultrasonic emitters, filter devices, ultrasonic flaw detectors, accelerometers, etc.

Ceramic samples of the ($1-x$)BaTiO₃⋅xPbFe${}_{2/3}$W${}_{1/3}$O₃, $0\le x\le 1$ (($1-x$)BT⋅xPFW) system were synthesized by solid-state reactions method. The samples were characterized by X-ray diffraction (XRD) and dielectric studies, as well as by the measurements of the thermally stimulated depolarization currents (TSDC). It was found that the predominant phase in the samples is presented by the (Ba${}_{1-x}$Pb_x)(Ti${}_{1-x}$Fe${}_{2x/3}$W${}_{x/3}$)O₃ solid solutions with a perovskite structure, herewith the samples with $0\le x<0.25$ are practically single-phase, and with $0.25\le x<1$ contain the impurity phase BaWO₄ (up to 15 mass.% at $x=0.60$–0.90). Information has been obtained about the changes in the structural and dielectric characteristics of the solid solutions with the change of their composition. It is established that the solid solutions crystal lattice symmetry at 296 K changes from tetragonal at $x\le 0.04$ to cubic at $x\ge 0.05$. An increase in the PFW content in solid solutions causes a gradual change in their properties from ferroelectric at $0\le x<0.10$ to relaxor ferroelectric at $0.10\le x\le 0.25$, and then to properties similar to those of the dipole glass with weak or zero correlation between dipoles at $0.25<x\lesssim 0.90$. The addition of BT to PFW leads to rather quick degradation of the relaxor ferroelectric properties of PFW in the region $x=0.9$–1.0.