Large reversible strains in the vicinity of the ferroelectric/antiferroelectric morphotropic phase boundary of a (0.97–x)Bi0.5Na0.5TiO3–0.03BaTiO3–xNaNbO3 ternary system

Abstract

In spite of a great deal of attention to the outstanding electrostrains of (Bi_0.5Na_0.5)TiO₃-based lead-free ceramics, the structure–property relationship is still under debate. In this work, the phase structure evolution in (0.97–x)(Bi_0.5Na_0.5)TiO₃–0.03BaTiO₃–xNaNbO₃ ceramics was investigated as a function of composition and electric field, in which a successive phase transition from a ferroelectric (FE) monoclinic (Cc) phase to a FE rhombohedral (R3c) phase and finally to an antiferroelectric (AFE) tetragonal (P4bm) phase can be observed with increasing NN content. The dielectric-temperature measurement indicates that this is closely related to the variation of the phase transition temperature from a normal FE to an ergodic relaxor FE and from an ergodic relaxor FE to a relaxor AFE (T_R–T). Most interestingly, an asymmetric strain–field curve with a strain gap was observed in a relatively wide composition range of 0 ≤ x ≤ 0.11. The results demonstrate that the strain gap in the composition range of 0 ≤sx ≤ 0.08 is mainly due to a field-induced irreversible phase transition from Cc to R3c. However, for compositions with 0.09 ≤.x ≤ 0.11, it is caused by an irreversible field induced P4bm to R3c phase transition, although these samples become a complete ergodic relaxor, indicating that they should be an AFE instead of an FE. A completely reversible phase transition from relaxor AFE P4bm to FE R3c accompanying a low-hysteresis giant strain of 0.44% can be obtained as the composition with x h 0.12 is close to the AFE side of the R3c-P4bm morphotropic phase boundary, demonstrating great application potential for ceramic actuators.