Nonstoichiometric effect on dielectric and large-signal electromechanical properties of environmentally friendly BNT-6BT ferroelectric ceramics

Although the nonstoichiometric influence on the small-signal dielectric and piezoelectric properties of (Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectrics has been studied extensively over the past decade, the features of large-signal electric field-induced strain (electrostrain), which are of particular importance to actuator devices, have not been thoroughly investigated. In this study, we used the solid-state reaction method to manufacture nonstoichiometric 0.94(Bi_0.5+xNa_0.5−x)TiO₃-0.06BT (BNTx-6BT) ceramics, where x = 0.0–0.05, and investigated the nonstoichiometric effect on the dielectric and large-signal electromechanical properties, with special emphasis on the electrostrain properties. Our results suggest that the room-temperature phase structures of BNTx-6BT ceramics changed from a regular ferroelectric phase to a relaxor ferroelectric phase as the Bi/Na ratio increased from stoichiometric 50/50 to nonstoichiometric 55/45 owing to the nonstoichiometric effect on the long-range ferroelectric order. In the x = 0.02 nonstoichiometric composition, an ultrahigh and electrostrictive-type electrostrain of 0.53% was identified. Compared to their stoichiometric counterparts, nonstoichiometric compositions have stronger temperature stability during polarization, resulting in good temperature stability of the electrostrain. Our findings not only reveal the nonstoichiometric effect on the phase evolution and its impact on the dielectric and large-signal electromechanical properties of BNTx-6BT ceramics but also provide a new method for tailoring the large-signal electrostrain properties of BNT-based ceramics.