Perovskite ferroelectric materials have been widely investigated for their tunable piezoelectric properties, particularly through defect engineering strategies. Acceptor dopants typically induce a "hardening" effect that improves mechanical quality factors, while donor dopants create "softening" effects that enhance dielectric and piezoelectric performance. In lead-free ferroelectrics, which have dominated piezoelectric research in recent decades, defect engineering shows similar promise for property optimization. However, the field lacks a comprehensive understanding of how specific defect features influence material properties, hindering the development of universal design principles.
A telling example involves KTN perovskite doped with Fe and Mn at the B-site. Despite both being acceptor dopants, they produce markedly different effects: Mn doping increases local structural heterogeneity to boost piezoelectric coefficients, while Fe doping stabilizes polarization to improve mechanical quality factors. Furthermore, dopant-induced local ferroelectric distortions have been shown to interact with phase boundaries, simultaneously enhancing piezoelectric response while maintaining thermal stability - a critical advance in overcoming the traditional performance trade-off. Most remarkably, recently developments on oxygen vacancies and their coupling with A-site vacancies in lead-free systems have been shown to produce unprecedented piezoelectric coefficients reaching into the thousands - a performance level previously considered unattainable without lead.
These findings underscore how dopants fundamentally modify ferroelectric lattice characteristics, from polarization behavior to structural heterogeneity. The specific electronic configurations of defect structures profoundly influence polarization frameworks, creating diverse and significant impacts on piezoelectric properties. This understanding provides a valuable framework for developing advanced lead-free perovskites with tailored performance characteristics.

Shujun Zhang is a Chair Professor at City University of Hong Kong, prior to which, he was a distinguished Professor at University of Wollongong, Australia, Senior Scientist and Professor at the Pennsylvania State University, USA. Presently, his focus lies in exploring the intricate relationship between design & fabrication, microstructure, properties, and device performance of electronic materials, particularly with applications in piezoelectric transducers and energy storage/harvesting. He has received several accolades from various societies, including an Academician of World Academy of Ceramics, an IEEE Fellow of the Ultrasonics, Ferroelectric and Frequency Control Society (UFFC-S); a Fellow of the American Ceramic Society; and a Future Fellow of Australian Research Council. Additionally, he is a recipient of IEEE UFFC-S Ferroelectrics Recognition Award; a winner of the New South Wales Premiers Prizes for Science & Engineering; the Ross Coffin Purdy Award at the American Ceramic Society; and Vice-Chancellor’s Research Excellence Award for Researcher of the Year at UOW. He is the Editor-in-Chief for Microstructures, section Editor-in-Chief for Crystals, as well as an Associate Editor for the IEEE Transactions on UFFC, Journal of the American Ceramic Society, and Science Bulletin. He has also served as an elected IEEE UFFC AdCom member (2016-2018) and Vice President for Ferroelectrics (IEEE UFFC, 2021-2023).