Achieving ultrahigh strain in lead-free piezoelectric ceramics of Bi0.5(Na0.41K0.09)TiO3-(Ba0.5Sr0.5)HfO3 via A/B-site co-doping strategy

BNT-based piezoelectric ceramics have garnered considerable attention owing to their potential to exhibit large electrostrain. However, irreversible phase transitions and domain structure alterations lead to limited effective strain and poor temperature stability, which restricts their application in high-precision displacement devices, representing a major bottleneck in current research. In this study, 0.82Bi 0.5 Na 0.5 TiO 3 –0.18Bi 0.5 K 0.5 TiO 3 (BNKT) was selected as the base composition, and an A/B-site co-doping strategy was employed to develop a (Ba 0.5 Sr 0.5 )HfO 3 (BSH) composite modification system. By precisely adjusting the BSH doping concentration ( x  = 0–0.03), the ferroelectric – relaxor phase transition temperature ( T F-R ) was effectively shifted to below room temperature, thereby significantly lowering the field-induced phase transition energy barrier. Finally, the BNKT-0.02BSH ceramic exhibits a high piezoelectric strain coefficient ( d 33 ∗) of 648 pm/V under an electric field of 65 kV/cm, along with a strain response of 0.475 % at 80 kV/cm. Meanwhile, the strain stability remains above 85 % over the temperature range of 30–100 °C. Multi-scale characterization techniques, including X-ray diffraction and temperature-dependent Raman spectroscopy, confirmed that the improved performance enhancement is attributed to BSH modification, which increases local structural heterogeneity and significantly promotes electric field-induced reversible phase transitions between the relaxor to ferroelectric states. This study offers novel insights into the understanding of the composition-structure-property relationship.