Lead-free Na_0.5Bi_0.5TiO₃(NBT) ceramics were prepared via a conventional oxide-mixed sintering route and their electrical transport properties were investigated. Direct current (DC, σ_DC) and alternating current (AC, σ_AC) electrical conductivity values, polarization current (first measurements) and depolarization current, current–voltage (I–U) characteristics (first measurements), and the Seebeck coefficient (α) were determined under various conditions. The mechanism of depolarization and the electrical conductivity phenomena observed for the investigated samples were found to be typical. For low voltages, theI–Ucharacteristics were in good agreement with Ohm’s law; for higher voltages, the observed dependences wereI–U²,I–U⁴, and thenI–U⁶. The low-frequency σ_ACfollowed the formula σ_AC–ω^s(ω is the angular frequency andsis the frequency exponent). The exponentswas equal to 0.18–0.77 and 0.73–0.99 in the low- and high-frequency regions, respectively, and decreased with temperature increasing. It was shown that conduction mechanisms involved the hopping of charge carriers at low temperatures, small polarons at intermediate temperatures, and oxygen vacancies at high temperatures. Based on AC conductivity data, the density of states at the Fermi-level, and the minimum hopping length were estimated. Electrical conduction was found to undergo p–n–p transitions with increasing temperature. These transitions occurred at depolarization temperatureT_d, 280 ℃, and temperature of the maximum of electric permittivityT_mis as typical of NBT materials.

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.

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.

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.

Low-lead (1−x)BaTiO₃–xPbMg_1/3Nb_2/3O₃ ceramics (x = 0, 0.025, 0.05, 0.075, 0.1, and 0.15) were prepared by the conventional oxide mixed sintering process, and their optical band gap, Seebeck coefficient, ac (σ_ac) and dc (σ_dc) conductivities as a function of temperature were investigated for the first time. It was found that all samples have p-type conductivity. The low-frequency (20 Hz–2 MHz) ac conductivity obeys a power law σ_ac~ω^s, which is characteristic for disordered materials. The frequency exponent s is a decreasing function of temperature and tends to zero at high temperature. σ_ac is proportional to ω^0.07–ω^0.31 in the low-frequency region and to ω^0.51–ω^0.98 in the high-temperature region. The temperature dependence of the dc conductivity showed a change in slope around the temperature at which the phase transition appeared. Both ac and dc conductivities showed a thermally activated character and possessed linear parts with different activation energies and some irregular changes. It was found that the hopping charge carriers dominate at low temperature and small polarons and oxygen vacancies dominate at higher temperature. (1−x)BaTiO₃–xPbMg_1/3Nb_2/3O₃ ceramics are expected to be promising new candidate for low-lead electronic materials.