Bismuth sodium titanate and related compounds are promising lead-free ferroelectric materials potentially useful in a wide range of piezoelectric applications. The domain structure plays an important role in determining the piezoelectric and ferroelectric properties and thereby the performance of electromechanical transducers. In this work, piezoresponse force microscopy (PFM) is used to gain insights into the mesoscopic-scale domain structure and its evolution under electric field in the (1−x)Bi_0.5Na_0.5TiO₃-xBaTiO₃ (BNT-BT) piezoceramics with compositions varying from x = 0 to x = 0.08. A phase transition from the rhombohedral phase to the tetragonal phase is observed with increasing BT contents. A relationship is established between the relaxor behavior and the domain structures imaged by PFM, i.e., short-range polar regions without visible domains in relaxor ceramics of pure BNT, while long-range ordered polar states with clear domains in ferroelectric ceramics with the addition of BT content. Distinct micro-domains are observed in the ceramics with compositions close to the morphotropic phase boundary (MPB), but the domain size drops to nanometers in the MPB composition with an increasing domain wall density. An electric field can induce the transformation from the relaxor behavior to a ferroelectric state, accompanied by an increase in domain sizes and a rearrangement/reorientation of the polar domains. This study of domain structure and its evolution in BNT-BT provides a better understanding of the relationship between the crystal structure, mesoscopic-scale domains, and macroscopic properties in these important lead-free piezoelectric ceramics.

Lead-free bismuth sodium titanite, (Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoelectric thin films have enormous potential in applications such as micro-actuators and micro-ultrasonic transducers due to their large strain responses. In this paper, (1 − x)(Bi_0.5Na_0.5)TiO₃-xBaZrO₃ (x = 0.02, 0.04, 0.06 and 0.07, BNT-100xBZ) piezoelectric thin films were deposited on Pt/TiO₂/SiO₂/Si (001) substrates by sol–gel method. All BNT-100xBZ thin films showed good macro-morphology and crystallinity. In addition, no clear phase structure transition was observed by increasing the BZ contents in the BNT-100xBZ solution, which, however, leads to evident variations in the dielectric and piezoelectric properties of the BNT-100xBZ thin films. A giant macro-scale strain value of 1.31% was obtained in the BNT-4BZ thin films, accompanied by a relatively lower dielectric loss of 0.03 for all BNT-100xBZ thin films. According to temperature- and frequency-dependent dielectric properties, it is found that the BNT-4BZ composition has a coexistence of ferroelectric phase and relaxor characteristics, which is different from other compositions. Furthermore, the intensity of the current peak related to the domain switching in the BNT-4BZ thin films is higher than that of other compositions. This work is of great interest for further enhancement of the strain response and reduction of the dielectric loss in BNT-based lead-free thin films for various piezoelectric device applications like micro-actuator systems.