Phase structure and properties of sodium bismuth titanate lead-free piezoelectric ceramics

The lead-free sodium bismuth titanate (BNT) system has been extensively investigated in the past decade due to its multi-functional electro-active properties. Here, we present a comprehensive review that encompasses the fundamentals and state-of-the-art in the development of BNT-based ceramics, with attention to the underlying composition, microstructure, and macroscopic properties. The phase structure, phase transitions, and relaxor characteristics of BNT and BNT-based solid solutions are described carefully, with a series of proposed phase diagrams. The attractive functional properties of BNT-based ceramics include piezoelectricity, electric-field-induced strain, and energy storage performance for applications in sensors, actuators, and dielectric capacitors. The focus of this review is on the microscopic origin of the macroscopic behavior, the proposed strategies for optimization of functional properties, and current challenges. Moreover, the potential applications of BNT-based ceramics in the areas of electrocaloric, oxide-ion conduction, and luminescence are briefly introduced. Finally, future perspectives are provided to highlight new and emerging research directions in this growing area.