Domain evolution and coercive field reduction in rhombohedral (Na0.5Bi0.5)TiO3-based crystals by alternating electric field

(Na0.5Bi0.5)TiO3 (NBT)-based materials have been widely studied for their large electric-field-induced strains. However, a high coercive field (E-c) has long hindered the applications of NBT-based materials. Here, we propose a simple method to significantly reduce the E-c of rhombohedral NBT-based crystals through domain engineering. By applying an alternating current (AC) electric field along the [001] direction, the E-c of Mn-doped (Na0.485K0.015Bi0.5)TiO3 single crystal can be reduced from 70 to 20 kV/cm after about 25 cycles without sacrificing the ferroelectric polarization. Meanwhile, the piezoelectric coefficient d(33) and the optical transparency of the crystals are also enhanced compared with those after direct-current electric field poling. The domain structure characterization shows that the AC cycles can form a laminar domain configuration, in which the 109 degrees domain walls are parallel to (001) planes. It is demonstrated that in the laminar domain configuration, almost only 71 degrees polarization switching occurs when the external electric field is reversed. The required energy for polarization reversal is significantly lower than that of the "4R" domain configuration; thus, the E-c is reduced greatly. The low E-c is maintained after depolarization at 250 degrees C, evidencing good thermal stability of the laminar domain configuration. Furthermore, this method is also applicable to other rhombohedral single crystals and may be applied to [001]-textured polycrystalline ceramics in the future; thus, it may indeed benefit the practical applications of NBT-based piezoelectric devices.