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.

Using the free (pressureless) sintering method, multiferroic ceramic composites based on two ferroelectric materials, i.e., BaTiO₃ (B) and Pb_0.94Sr_0.06 (Zr_0.46Ti_0.54)_0.99Cr_0.01O₃ (P), and magnetic material, i.e., zinc–nickel ferrite (F) were obtained. Three composite compositions (BP-F) were obtained with a constant 90/10 content (ferroelectric/magnetic) and a variable content of the ferroelectric component (B/P), i.e., 70/30, 50/50, and 30/70. Crystalline structure, microstructural, DC electrical conductivity, dielectric, and ferroelectric properties of multiferroic composites were investigated. The concept of a composite consisting of two ferroelectric components ensures the preservation of sufficiently high ferroelectric properties of multiferroic composites sintered by the free sintering method. Research has shown that the percentage of individual ferroelectric components in the composite significantly affects the functional properties and the entire set of physical parameters of the multiferroic BP-F composite. In the case of the dielectric parameters, the best results were obtained for the composition with a more significant amount of BaTiO₃; i.e., permittivity is 1265, spontaneous polarization is 7.90 µC/cm², and remnant polarization is 5.40 µC/cm². However, the most advantageous set of performance parameters shows the composite composition of 50BP-F.

Lead-free K_0.5Bi_0.5TiO₃ (KBT) ceramics were prepared using a finely tuned convectional solid-state reaction method. Their phase transitions in unpoled and poled states were examined. The temperature-dependent evolution of the reflections sensitive to structural changes and their 2Θ-positions indicated two temperature-driven phase transitions: tetragonal–tetragonal at about 200 °C, and tetragonal–cubic at around 400 °C. These structural transformations are further corroborated by studies examining Raman spectroscopy, dielectric properties, and mechanical properties. It was demonstrated that a prior E-field poling process significantly influences the polar state, causing an increase in the local degree of order, as well as the transformation of the cubic phase into the tetragonal one. This stabilizes and widens the temperature range of the ferroelectric phase. It was found that phase transformations in KBT are accompanied by a softening of the mechanical behavior similarly to improper ferroelastic transformations. The results demonstrate that KBT possesses favorable structural, dielectric, and mechanical characteristics, making it a potential candidate for electronic applications. The present study provides a clear understanding of the multi-scale structural behavior in multi-phase KBT, bridging micro-heterogeneity behaviors and macro-properties, and demonstrates an effective method of tuning the properties of KBTs by E-poling with a low electric field.