Lead-free Na_0.5Bi_0.5TiO₃ -BaTiO₃ ceramics have been prepared in the whole range of concentrations and studied at room-temperature by means of X-ray, Raman scattering and infrared techniques. X-ray measurements revealed rhombohedral, rhombohedral-tetragonal boundaries and tetragonal modifacations depending on the contents of BaTiO₃. The distinct changes of the Raman and infrared spectra with increasing of BaTiO₃ content, which were correlated with X-ray results, were observed. The broad phonon spectra indicated the disorder in the A site of Na_0.5Bi_0.5TiO₃ -BaTiO₃ system.

The measurements of time dependencies of dielectrics and structural properties of Na_0,5Bi_0,5TiO₃ have been performed. The results of this investigations confirm the prior data, that phase transition in Na_0,5Bi_0,5TiO₃ from tetragonal to rhombohedral phase takes place in wide temperature range, in which both phases coexist. Moreover, this phase transition indicate the features of isothermal phase transformation.

The purpose of the present study was to learn the morphological, structural, ferroelectric, dielectric, electromechanical, magnetoelectric, and magnetic properties, and DC conductivity of BaTiO₃-Ni_0.64Zn_0.36Fe₂O₄ (BT-F) multiferroic composites compacted via the free sintering method. The influence of the ferrite content in ceramic composite materials on the functional properties is investigated and discussed. X-ray diffraction studies confirmed the presence of two main phases of the composite, with strong reflections originating from BaTiO₃ and weak peaks originating from nickel-zinc ferrite. BT-F ceramic composites have been shown to exhibit multiferroism at room temperature. All studied compositions have high permittivity values and low dielectric loss, while the ferroelectric properties of the BT component are maintained at a high level. On the other hand, magnetic properties depend on the amount of the ferrite phase and are the strongest for the composition with 15 wt.% of F (magnetization at RT is 4.12 emu/g). The magnetoelectric coupling between BT and F phases confirmed by the lock-in technique is the largest for 15 wt.% ferrite. In the present work, the process conditions of the free sintering method for obtaining BT-F multiferroic composite with good electrical and magnetic properties (in one material) were optimized. An improved set of multifunctional properties allows the expansion of the possibilities of using multiferroic composites in microelectronics.

Lead-free Ba_1−xSr_xTiO₃ (BST) (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.45) ceramics were successfully prepared via the solid-state reaction route. A pure perovskite crystalline structure was identified for all compositions by X-ray diffraction analysis. The basic phase transition temperatures in these ceramics were studied over a wide temperature range. A change in symmetry from a tetragonal to cubic phase was detected, which was further proven by phonon anomalies in composition/temperature-dependent Raman spectra. The incorporation of Sr²⁺ into BaTiO₃ (BT) lead to a shift in the phase transitions to lower temperatures, suppressing the ferroelectric properties and inducing relaxor-like behavior. Therefore, it was reasonable to suppose that the materials progressively lack long-range ordering. The initial second-harmonic generation (SHG) measurements demonstrated that the cubic phase of BST ceramics is not purely centrosymmetric over a wide temperature interval. We discussed the possible origin of the observed effects, and showed that electric field poling seems to reconstruct the structural ordering destroyed by the introduction of Sr²⁺ to BT. In the first approximation, substitution of Sr for larger Ba simply reduced the space for the off-central shift in Ti in the lattice and hence the domain polarization. A-site cation ordering in BST and its influence on the density of electronic states were also explored. The effect of doping with strontium ions in the BST compound on the density of electronic states was investigated using ab initio methods. As the calculations showed, doping BT with Sr²⁺ atoms led to an increase in the bandgap. The proposed calculations will also be used in the subsequent search for materials optimal for applications in photovoltaics.

Na_0.5Bi_0.5TiO₃ (NBT) and Fe- and Mn-modified NBT (0.5 and 1 mol%) ceramics were synthesized by the solid-state reaction method. The crystal structure, dielectric and thermal properties of these ceramics were measured in both unpoled and poled states. Neither the addition of iron/manganese to NBT nor poling changed the average crystal structure of the material; however, changes were observed in the short-range scale. The changes in shapes of the Bragg peaks and in their 2Θ-position and changes in the Raman spectra indicated a temperature-driven structural evolution similar to that in pure NBT. It was found that both substitutions led to a decrease in the depolarization temperature T_d and an increase in the piezoelectric coefficient d₃₃. In addition, applying an electric field reactivated and extended the ferroelectric state to higher temperatures (T_d increased). These effects could be the result of: crystal structure disturbance; changes in the density of defects; the appearance of (Fe_Tiˈ-), (Mn′_Ti-V^••_O) and (Mn″_Tii-V^••_O )—microdipoles; improved domain reorientation conditions and instability of the local polarization state due to the introduction of Fe and Mn into the NBT; reinforced polarization/domain ordering; and partial transformation of the rhombohedral regions into tetragonal ones by the electric field, which supports a long-range ferroelectric state. The possible occupancy of A- and/or B-sites by Fe and Mn ions is discussed based on ionic radius/valence/electronegativity principles. The doping of Fe/Mn and E-poling offers an effective way to modify the properties of NBT.

The low-lead (1-x)Pb(Mg_1/3Nb_2/3)O₃-xBaTiO₃ (x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15) ceramics were fabricated by a two-step solid-state reaction route. The X-ray diffraction characterization confirms a perovskite crystal structure with tetragonal symmetry at room temperature (except for x = 0.15). SEM images indicate that all ceramic samples are dense and compact, with the grain size of 2–6 μm. Along with the increase of Pb(Mg_1/3Nb_2/3)O₃ content, the electrical conductivity increases, and the Seebeck coefficient and the thermal conductivity decrease. With increasing temperature, σ and α increase and κ decreases. In general, adding Pb(Mg_1/3Nb_2/3)O₃ to BaTiO₃ leads to an improvement of the thermoelectric figure of merit ZT. The results are explained on the basis of structural, morphological and compositional changes.