Advances in bismuth sodium titanate-based single crystals: Crystallographic anisotropy and phase boundary design for high-performance piezoelectricity and electrostrain

Bismuth sodium titanate [(Bi1/2Na1/2)TiO3, BNT]-based relaxor ferroelectrics (RFEs) have emerged as promising candidates for lead-free piezoelectric materials owing to their excellent electromechanical performance. Single-crystallization strategies, enabled by engineered domain configurations and crystallographic anisotropy, provide an effective pathway to overcome the performance limitations of BNT-based ceramics. This review systematically summarizes recent advances in BNT-based single crystals, with particular emphasis on crystal growth, the correlation between phase-boundary engineering and piezoelectric response, giant electrostrain, and crystallographic anisotropy. Recent progress in the growth of BNT-based single crystals using various techniques is comprehensively reviewed. The structural origins of weak piezoelectricity in BNT are then analyzed from the perspectives of relaxor behavior, oxygen octahedral tilting, and domain dynamics. On this basis, the effects of different phase boundaries, including rhombohedral–tetragonal and ferroelectric–relaxor boundaries, on the piezoelectric properties of BNT-based single crystals are systematically discussed. Furthermore, the underlying mechanisms of large electrostrain in BNT-based single crystals, together with the role of crystallographic anisotropy in governing electromechanical performance, are summarized. Finally, we outline the key challenges and future directions for the development of high-performance BNT-based RFE single crystals.