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This paper discusses ways to search for lead-free functional materials for various applications. Using the example of the solid solution systems based on alkali metal niobates, the influence of the position on the phase diagram of the corresponding systems, the number of the components in them, and modification with mono- and combined metal oxides on their characteristics is shown. It has been established that the most effective in terms of piezoelectric characteristics are the solid solution systems in or near the morphotropic region, with 3 or 4 components. A number of materials have been developed to create highly sensitive electromechanical transducers, ultrasonic delay lines and other applications.

C–V characteristics of ZnO-based ceramic structures used in manufacturing high-voltage and low-voltage varistors of different chemical compositions and manufacturing techniques have been investigated. A correlation between the intensity of electric field corresponding to transition of the C–V characteristics to the negative capacitances and average sizes of grains of a varistor structure has been established. Obtained data have been interpreted with the use of notions of the percolation theory of electric conductivity. The Shklovskii–De Gennes model has been used. It has been shown that on the highly nonlinear segment of C–V characteristics of a varistor structure, the size of an infinite cluster are limited to several intercrystallite potential barriers. This result is observed in all kinds of investigated varistor ceramics.

In this paper, we investigated the influence of rare earth ($\mathrm{\text{RE}}=\mathrm{\text{La}}$ and Gd) doped Ba${}_{0.9575}$RE${}_{0.04}$Ca${}_{0.0025}$Ti${}_{0.80685}$ Mn${}_{0.002475}$Nb${}_{0.002475}$Zr${}_{0.1782}$O₃ (BCTMNZ) ceramics were fabricated by using a conventional solid-state reaction method. The doping effects of La and Gd on the structural and magnetic properties were studied. The structural pattern of the ceramic samples were investigated by X-ray diffraction and the results indicated that both samples shows an orthorhombic structure with pure phase. Strain and crystalline size values for Gd and La doped were 0.31–0.33% and $0.154$–$0.181\mu$m, respectively. The room temperature hysteresis loops were obtained by using a vibrating sample magnetometer. La doped ceramic showed the higher value of magnetization i.e., $0.369\mu$B/f.u. as compared to Gd doped BLTMNZ ceramics.

Breakthroughs can be expected in multi-component ceramics by adjusting the phase assembly and the micro–nanostructure. Controlling the architecture of multi-materials at different scales is still challenging and provides a great opportunity to broaden the range of functionalities in the field of ferroelectric-based ceramics. We used the potentialities of Spark Plasma Sintering (SPS) to control a number of key parameters regarding the properties: anisotropy, interfaces, grain size and strain effects. The flexibility of the wet and supercritical chemistry routes associated with the versatility of SPS allowed designing new ferroelectric composite ceramics at different scales. These approaches are illustrated through various examples based on our work on ferroelectric/dielectric composites.

Multiferroic ${\mathrm{BiFeO}}_3$ (BFO) and Y, Zr codoped BFO $\left({\mathrm{Bi}}_{1-x}{\mathrm{Y}}_x{\mathrm{Fe}}_{0.95}{\mathrm{Zr}}_{0.05}{\mathrm{O}}_3\right)$ ceramics were prepared and the influence of codoping on the crystal structure and magnetic properties were investigated in this work. Confirmed by the evolution of X-ray diffraction and Raman modes, the codoping has changed the crystal structure from rhombohedral to tetragonal in bulk BFO ceramics. The enhancement of magnetic behaviors is demonstrated by the damage of space-modulated spiral spin structure, and it can be attributed to the crystal structure change and size effects. Meanwhile, Raman spectra from 300 to 800 K demonstrates that lower frequency phonon modes show rapid softening near the Neel temperature.

A lead-free relaxor ferroelectric, Sr${}_{0.15}$Ba${}_{0.85}$Bi₂Nb₂O₉, was synthesized via solid-state reaction and the temperature-dependence of the heat capacity was measured in a wide temperature range. The dielectric permittivity was also measured between 500Hz and 5MHz in the same temperature range. No anomaly has been detected in the heat capacity curve for the whole temperature range covered in the present experiments, while broad peaks have been observed in the dielectric permittivity with high frequency dispersion. A typical relaxor behavior has been observed from the dielectric analysis. The Debye’s temperature has showed a minimum value near the freezing temperature. The results are discussed considering the spin-glass model and the high frequency dispersion, which has been observed for the studied relaxor system.

Ceramic samples of (Bi${}_{1-x}$Pb_x)NbO₄ ($x=0$, 0.025, 0.05, 0.10, 0.15, 0.20) with 0.75wt.% V₂O₅ addition sintered at 920${}^{\circ }$C, 940${}^{\circ }$C and 960${}^{\circ }$C are investigated. Pb is selected as a substitute for Bi${}^{3+}$ in BiNbO₄ ceramics as it exists in two stable valence states of $+$2 and $+$4 and the average valency matches to that of Bi${}^{3+}$. The average Shannon radius (for octahedral coordination) of Pb${}^{2+}$ (1.19Å) and Pb${}^{4+}$(0.775Å) cations is 0.9825Å, which is similar to that of Bi${}^{3+}$ ion (1.03Å). The dense ($>94$%) polycrystalline (Bi${}_{1-x}$Pb_x)NbO₄ samples fabricated mostly reveal orthorhombic (Pnna) phase ($\alpha$-BiNbO₄, Sp. Gp. 52) by powder XRD. Presence of satellite Pb₂Nb₂O₇ phase, the amount of which is increasing with increase in Pb content, is also noticed. The microwave dielectric constant (${\epsilon }_r^{\prime }$) values of the niobates are found to increase from 42 to 71, whereas the quality factor (Q_u.f) values are found to decrease from 5400 to 550 GHz with increasing substitution of Pb. The compositions so synthesized are important as hardly there are any microwave dielectric ceramics available with $45<{\epsilon }_r^{\prime }<75$.

Lead free 0.94(Bi${}_{0.5}$Na${}_{0.5})$TiO₃–0.06BaTiO₃ ceramics were prepared by conventional solid-state mixed oxide route with the A-site stoichiometry modified to incorporate donor-doping (through Bi-excess) and acceptor-doping (through Na-excess). Both stoichiometric and nonstoichiometric ceramics exhibited a single perovskite phase with pseudo-cubic symmetry. A significant improvement in the dielectric properties was observed in Bi-excess compositions and a deterioration in the dielectric properties was observed in Na-excess compositions. Impedance spectroscopy was utilized to analyze the effects of A-site nonstoichiometry on conduction mechanisms. Compositions with Bi-excess resulted in an electrically homogeneous microstructure with an increase in resistivity by $\sim$3–4 orders of magnitude and an associated activation energy of 1.57eV which was close to half of the optical bandgap. In contrast, an electrically heterogeneous microstructure was observed in both the stoichiometric and Na-excess compositions. In addition, the Na-excess compositions exhibited low resistivities ($\rho \sim 10^3\mathrm{\Omega }$-cm) with characteristic peaks in the impedance data comparable to the recent observations of oxide ion conduction in (Bi${}_{0.5}$Na${}_{0.5})$TiO₃. Long term annealing studies were also conducted at 800${}^{\circ }$C to identify changes in crystal structure and electrical properties. The results of this study demonstrates that the dielectric and electrical properties of 0.94(Bi${}_{0.5}$Na${}_{0.5})$TiO₃–0.06BaTiO₃ ceramics are very sensitive to Bi/Na stoichiometry.

The solid solutions of ($1-x$) LaFeO₃–$x$BaTiO₃ (0.0$\le x\le$0.25) have been synthesized successfully by the conventional solid-state reaction method. Room temperature (RT) X-ray diffraction studies reveal the stabilization of orthorhombic phase with Pbnm space group. Complete solubility in the perovskite series was demonstrated up to $x=0.25$. The dielectric permittivity shows colossal dielectric constant (CDC) at RT. The doping of BaTiO₃ in LaFeO₃ exhibit pronounced CDC up to a composition $x=0.15$, further it starts to decrease. The frequency-dependent dielectric loss exhibits polaronic conduction, which can attribute to presence of multiple valence of iron. The relaxation frequency and polaronic conduction mechanism was shifted towards RT as function of $x$. Moreover, large magnetic moment with weak ferromagnetic behavior is observed in doped LaFeO₃ solid solution, which might be the destruction of spin cycloid structure due to insertion of Ti in Fe–O–Fe network of LaFeO₃.

Using the conventional solid-state reaction method, 0.5wt.% of MnO₂-, NiO-, Cr₂O₃-, Fe₂O₃- and Co₂O₃-added 0.94${\text{Na}}_{0.5}$${\text{Bi}}_{0.5}$TiO₃ (NBT)–0.06BaTiO₃ (BT) ceramics were prepared. The perovskite nature of the prepared ceramics was analyzed by powder XRD and the surface morphology was studied by means of SEM. The dielectric analysis was carried out from room temperature to $350^{\circ }$C at various frequencies and the diffusive transition at the dielectric maxima confirmed the relaxor nature of the ceramics. Creation of oxygen vacancies by the possible substitution of ferromagnetic impurities decreased the dielectric constant and piezoelectric constant ($d_{33})$. The co-existence of ferromagnetism and the ferroelectricity was observed in the Mn, Ni, Co and Cr-added NBT–BT ceramics. The magnetic force microscopy (MFM) analysis was carried out to study the ferromagnetic domains in the prepared ceramic.

Phase diagrams were constructed (at room temperature), the dielectric, piezoelectric, elastic and reversible properties of the Ge-modified $x$PbTiO₃–$y$PbZrO₃–$z$${\text{PbNb}}_{2∕3}$${\text{Mg}}_{1∕3}$O₃ multicomponent system ceramics were studied. The data obtained are compared with the results of studies of solid solutions of basic binary systems, as well as with the results obtained previously for other sections of the system under consideration. A significant increase in the width of the morphotropic region, diffusion of the concentration dependences of the electrophysical characteristics, and an increase in stability under the influence of constant bias fields in the solid solutions of the studied system were established.

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.

The paper presents the results of a comprehensive study of the thermophysical properties (thermal conductivity, thermal diffusivity, heat capacity) of high-temperature multiferroic BiFeO₃ modified with rare-earth elements (REEs) (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu). The regularities of the formation of the mentioned characteristics were established. The assumptions about the nature of the observed phenomena were suggested.

Thin films of polycrystalline (Ba${{}_{1-}}_x$${\text{Sr}}_x)$TiO₃ ($x$ = 0.2 and 0.3) with Perovskite structure were prepared by a dip and dry technique on a platinum-coated silicon substrate. The good quality thin films with uniform microstructure and thickness were successfully produced by dip-coating techniques annealed at 730${}^{\circ }$C for 1 h. The resulting thin film shows a well-developed dense polycrystalline structure with more uniform grain size distribution. The BST thin films were characterized for their structural, Raman spectroscopy, morphological properties, and complex impedance properties. The dielectric constant-frequency curve showed the good dielectric constant and loss dielectric loss with low-frequency dispersion. The BST 0.3 thin film reveals that the dielectric constant and dielectric loss at a frequency of 1 kHz were 578 and 0.02, respectively. The obtained results on dielectric properties can be analyzed in terms of the Maxwell–Wagner model.

(${\text{Y}}_x$Bi${{}_{1-}}_x{)}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ($x=$ 0.00–0.30) ceramics were successfully prepared via the conventional solid-state method. X-ray powder diffraction confirmed the lattice constant gradually decreases with increasing Y${{}^3}^{+}$ content. SEM images displayed Y${{}^3}^{+}$ substitution for Bi${{}^3}^{+}$ gave rise to the large abnormal grains, and the size of abnormal grains became larger with the increase of Y${{}^3}^{+}$ substitution. (${\text{Y}}_x$Bi${{}_{1-}}_x{)}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ceramics presented the relatively high dielectric constant of 7400 with the dielectric loss of 0.055 when $x=$ 0.20. The analysis of complex impedance suggested the grains are semiconductive and the grain boundaries are insulating. For pure ${\text{Bi}}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ceramics, the appearance of additional low-frequency peaks in electrical modulus indicated the grain boundaries are heterogeneous. The investigation of modulus peaks fitting with Arrhenius formula implied that the low-frequency permittivity for all (${\text{Y}}_x$Bi${{}_{1-}}_x{)}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ceramics was ascribed to the Maxwell–Wagner relaxation at grain boundaries. In addition, a set of clear dielectric peaks above ${200}^{\circ }$C associated with Maxwell–Wagner relaxation can be found for all (${\text{Y}}_x$Bi${{}_{1-}}_x{)}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ceramics in the temperature dependence of dielectric constant. This set of clear dielectric peaks showed a tendency to shift to higher temperatures with the increase of Y${{}^3}^{+}$ substitution. Meanwhile, a tiny dielectric anomaly at room temperature was found in Y-doped ${\text{Bi}}_{2/3}$Cu₃Ti₄${\text{O}}_{12}$ ceramics.

Dense (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ lead-free ceramics with the simple composition were prepared via sintering in low oxygen partial pressure (pO₂, $\sim$${\text{10}}^{-12}$ atm) atmosphere and adding LiF. All the ceramics have pure orthorhombic structure. Compared to the LiF-added (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics sintered in air and the low pO₂-sintered pure (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics without LiF addition, the present ceramics exhibit improved piezoelectric and ferroelectric properties. The piezoelectric constant $d_{33}$ is 125 pC/N, and the converse piezoelectric constant $d_{33}$* is 186 pm/V. The dielectric constant and dielectric loss of the ceramics at room temperature and 1 kHz are 451 and 0.03, respectively. Under the measured electric field of 70 kV/cm, the remanent polarization is 25.9 $\mu$C/cm² and the coercive field is 13.9 kV/cm. Furthermore, if the base metals such as Cu and Ni powders were mixed into the green pellets and sintered in the low pO₂ atmosphere, the base metals cannot be oxidized, suggesting possibility of using base metals as electrodes.

Polycrystalline ${\text{Na}}_{0.9}$${\text{Ba}}_{0.1}$${\text{Nb}}_{0.9}$(${\text{Sn}}_{0.5}$${\text{Ti}}_{0.5}{)}_{0.1}$O₃ is prepared by the solid-state reaction technique. The formation of single-phase material was confirmed by an X-ray diffraction study and it was found to be a tetragonal phase at room temperature. Nyquist plots ($Z^{\prime \prime }$ versus $Z^{\prime })$ show that the conductivity behavior is accurately represented by an equivalent circuit model which consists of a parallel combination of bulk resistance and constant phase elements (CPE). The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity ${\sigma }_{dc}$ follows the Arrhenius relation. The modulus plots can be characterized by the empirical Kohlrausch–Williams–Watts (KWW), $\phi (t)$ = exp($-(t$/$\tau {)}^{\beta })$ function and the value of the stretched exponent ($\beta )$ is found to be almost independent of temperature. The near value of activation energies obtained from the analyses of modulus and conductivity data confirms that the transport is through an ion hopping mechanism dominated by the motion of the (${\text{O}}^{2-})$ ions in the structure of the investigated material.

Solid solutions of the composition Ba${{}_{1-}{}_x{}_{-}}_y$(Mg, Ln)${}_x$${\text{Sr}}_y$TiO₃ ($x$ = 0.01; 0.025; 0.04; $y$ = 0.20; 0.50; 0.80; Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tu, Yb) were prepared by two-stage solid-phase synthesis followed by sintering using conventional ceramic technology. The influence of rare-earth elements on the microstructure of the prepared ceramic samples was investigated. It was found that regardless of the type of modifiers introduced, the grain landscape of the studied solid solutions with different amounts of SrTiO₃ is refined (in the initial system, the average grain size, $\bar{d}$, at $x$ = 0.20 is 6 $\mu$m; at $x$ = 0.50 is 4 $\mu$m; at $x$ = 0.80 is 18 $\mu$m) to crystallite sizes not exceeding (2-3) $\mu$m, and compacted. The using of mechanical activation procedures leads to an even greater decrease in the size and an increase in the density of ceramics. The increasing in the concentration of modifiers in each group (within the considered range of dopant variation) against the background of such a fine-grained structure has little effect on the dynamics of changes in $\bar{d}$. It is concluded that it is advisable to use the data obtained in the development of functional materials based on BST/(Mg, Ln) and devices with the participation of these compositions.

The paper discusses the features of the effect of modification with lithium carbonate on the dielectric dispersion of lead ferroniobate ceramics. The modifier has been previously shown to change the ways of the recrystallization sintering and therefore reduce the sintering temperature of the ceramics, increase their average grain size, and improve their dielectric and piezoelectric properties. In this study, the emphasis is placed on the analysis of the diffusion effects of the ferroelectric phase transition upon such modification considered from the standpoint of the chemical specifics of the modifier and its location in the structure of the parent compound.

The solid solutions of the (1-x)Na${}_{0.5}$Bi${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ system were produced by the conventional ceramic technology using mechanical activation of the synthesized product. It was found that in the (1-x)Na${}_{0.5}$Bi${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ system at room temperature, a number of morphotropic phase transitions occur: rhombohedral → cubic → tetragonal → monoclinic phases. The introduction of a small amount of Na${}_{0.5}$K${}_{0.5}$NbO₃ leads to an increase in the temperature stability of the dielectric properties of ceramics. A change in the relaxor properties of the solid solutions of the (1-x)Na${}_{0.5}$Bi${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ system was shown. The increase in energy density and energy efficiency was found at additive 10mol.% of Na${}_{0.5}$K${}_{0.5}$NbO₃.