Properties of Na 0.5 Bi 0.5 TiO 3 Ceramics Modified with Fe and Mn.

Na 0.5 Bi 0.5 TiO 3 (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 d 33 . 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 Ti '-), (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.