Modulation of phase boundary and domain structures to engineer strain properties in BNT-based ferroelectrics

Bismuth sodium titanate (BNT) ceramics exhibit outstanding strain responses but are unfavorable for application in high-sensitivity displacement actuators due to the large negative strain resulting from irreversible changes in their phase transition and domain structure. Here, (1−x)Bi_0.50Na_0.41K_0.09TiO₃–xNaNbO₃ (BNKT−xNN) solid solutions were prepared to improve the strain properties through the strategy of modulating the phase boundary and domain structures. The introduction of sodium niobate could effectively regulate the relative content of the tetragonal (P4bm) and rhombohedral (R3c) phases in the phase boundary region. The ferroelectric-to-relaxor phase transition (T_F−R) was reduced, and the ergodic relaxor (ER) state was nurtured at room temperature. Excellent zero-negative strain properties of S = 0.41% and d₃₃* = 742 pm/V were achieved from the reversible transition between the ER and ferroelectric states under an applied electric field (x = 0.04). Additionally, understanding the domain states via piezoelectric force microscopy (PFM) and first-order reversal curve (FORC) revealed that the superior strain responses originated from the reversible inter-transformation of substable macrodomains and polar nanoregions (PNRs) in the phase boundary. This study provides new insight into the interplay between the evolution of phase boundaries and domain structures and the strain properties of BNT-based ceramics.