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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.

Ternary solid solutions of BiFeO₃, GdFeO₃ and BaTiO₃ have been prepared by solid-state reaction method. Dielectric properties of these ceramics have been characterized in the temperature range between room temperature and 673 K and magnetic properties between 5 K and 300 K. For the understanding of the multiferroic property, the relation between the crystal structures, magnetic transition and ferroelectric transitions with increasing temperature have been analyzed. All ceramic samples show single perovskite phase. When Ba content exceeds 0.2 wt%, the sintering ability is weakened and the phase structure of Bi_1-x-yGd_xBa_yFe_1-yTi_yO₃ (with x = 0.2 and y = 0.1, 0.2, 0.3, 0.4, 0.5) solid solution changes from rhombohedral to tetragonal phase. The maximum ferroelectric transition temperature (T_e of this system was in the range 154–165°C with the dielectric constant peak of 6000 for y = 0.5 at 100 kHz. Well-saturated piezoresponse hysteresis loops were observed for all compositions indicating room temperature ferroelectricity. With increasing Ba content (up to 0.3 wt%) the remanent magnetization M_r increased and the coercive magnetic field decreased.

Polycrystalline samples of samarium barium tungstate [Sm₂(Ba_0.5W_0.5)₂O₇: (SBW)] pyrochlore structure type oxides have been prepared by a solid-state reaction technique. X-ray diffraction (XRD) patterns of this compound at room temperature suggest the formation of a single phase compound with orthorhombic structure. Studies of the dielectric constant and dielectric loss of compound as a function of frequency (4 kHz–1 MHz) at room temperature, and as a function of temperature (23–350°C) at 20 and 50 kHz frequencies suggest that the compound does not have dielectric anomaly. The variation of dc resistivity suggests the semiconductor characteristics of the material. The value of activation energy (E_a ~ 0.43 at 20 kHz and E_a ~ 0.29 at 50 kHz) of the above mentioned compound has been calculated from the slope of the ln σ_ac versus 1/T graph in the high temperature region (> 240°C). The low value of activation energy supports the superionic nature of the compounds in the high temperature region.

This paper presents a study of the structure and microwave properties of substituted into the Ti⁴⁺ site of calcium titanate ceramics. The structural and dielectric properties of solid solutions in CaTi_1-x(Nb_1/2Fe_1/2)_xO₃, was done. The microwave dielectric properties of solid solutions in CaTi_1-x(Nb_1/2Fe_1/2)_xO₃, (CNFTOX with x = 0.1 to x = 1) is discussed. The modified CaTiO₃ (CTO) ceramics were prepared by a new procedure in the solid-state method. A study of the variations of the ball-milling process was done. The calcinations were done at 900°C for 3 and 5 h respectively, and the sintering at 1100°C, for 3 h. The structural property studies of the ceramics were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The X-ray analysis showed that all samples have an orthorhombic structure. The refinement analysis of all samples were also duly performed and discussed. The nine scattering bands centered at 183, 227, 247, 288, 338, 470, 490 and 805 cm^-1 were observed. Several peaks presented a small shift as a function of the x value. All Raman spectra in the studied samples showed a small band at 805 cm^-1, which is a function of the x value. The dielectric permittivity and loss at microwave frequencies (MW) were investigated. For both calcination treatments (900°C for 3 and 5 h), the dielectric permittivity decreased with a decrease of the titanium content. Dielectric permittivity values in the range of 20 to 80 were obtained. It was also observed that a higher number of balls in the milling process contribute to the increase of the ε_r values. The CNFTO has an excellent microwave property at x = 0.6, with a temperature coefficient of resonant frequency (τ_f) near zero (τ_f = 2.8 ppm/°C).

The influence of (Li, Ce) doping on the electrical properties of bismuth layer Sr_2-x(Li, Ce)_x/2Bi₄Ti₅O₁₈ [abbreviated to SBTi-(Li, Ce)_X/2] ceramics was investigated. X-ray diffraction analysis showed that all the ceramic samples were single-phase compounds. The (Li, Ce) modification significantly decreased the dielectric loss of Sr₂Bi₄Ti₅O₁₈ ceramics and greatly improved the piezoelectric activity. At x/2 = 0.0125, the SBTi-(Li, Ce)_x/2 ceramics exhibited the excellent properties with high remnant polarization (P_r = 9.3 μC/cm²) and high Curie temperature (T_c = 299°C). Meanwhile, the SBTi-(Li, Ce)_0.0125 ceramics had the largest piezoelectric constant (d₃₃ = 26 pC/N). The results showed that the SBTi-(Li, Ce)_x/2 ceramic was a promising lead-free piezoelectric material.

In this paper, we present impedance spectroscopy of Sodium Bismuth Titanate-based materials belonging to (1-x)Na_1/2Bi_1/2TiO_3-xBaTiO₃(x = 0.04) (NBT–BT) system. NBT–BT ceramics are prepared by high temperature solid-state reaction method. X-ray diffraction technique showed single-phase polycrystalline sample with an ABO₃ perovskite structure. Dielectric behavior and the impedance relaxation were investigated in a wide range of temperature (room temperature (RT) –500°C) and frequency (1 kHz–1 MHz). A broad dielectric constant peak was observed over a wide temperature range around the phase transition temperature. The complex impedance plot exhibited one impedance semicircle identified over the frequency range of 1 kHz–1 MHz, which is explained by the grain effect of the bulk. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole–Cole type relaxation.

Ca_5-xZn_xNb₄TiO₁₇ ceramics with 0 ≤ x ≤ 0.4 were prepared through a solid-state reaction method. Effects of zinc substitution on sintering behavior and microwave dielectric properties of Ca₅Nb₄TiO₁₇ ceramics were investigated. The sintering temperature was significantly lowered from 1480°C for pure Ca₅Nb₄TiO₁₇ to 1260°C for x = 0.4. The microwave dielectric properties are strongly correlated with the composition. It is worth noting that the temperature coefficient of resonant frequency (τ_f) displays a tendency toward positive value, ranging from −126.4 ppm/°C to −8.6 ppm/°C. A temperature stable microwave ceramic with dielectric constant of 52 and Q × f value of 9937 GHz is achieved at x = 0.4 and is a potential candidate for application as cores in dielectrically loaded antennas.

Three different SrFe_xTi_1-xO₃(x = 0.001, x = 0.005, x = 0.01) ceramics were prepared by the conventional solid-state reaction. The crystalline structure, surface morphology and dielectric properties were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and Agilent 4294A impedance analyzer, respectively. It is shown that both the sintering temperature and doping concentration influence the lattice constant, grain size, dielectric constant and the dielectric loss. When the sintering temperature is higher than 1390°C, the lattice constant, grain size and dielectric constant all decrease with the increase of the doping concentration, except the dielectric loss tangent which shows the opposite trend. Leakage current tests show that the leakage current density falls down with the increase of Fe doping concentration in the given region.

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.