Switching of morphotropic phase boundary and large strain response in lead-free ternary (Bi0.5Na0.5)TiO3–(K0.5Bi0.5)TiO3–(K0.5Na0.5)NbO3 system

Abstract

In this work, we report the phase diagram of lead-free ternary (1 − y)[(1 − x)(Bi0.5Na0.5)TiO3–x(Bi0.5K0.5)TiO3]–y(K0.5Na0.5)NbO3 (BNT–BKT–KNN) system and study the switching characteristics of the morphotropic phase boundary (MPB). The addition of KNN intrinsically changes the structural nature of the system with the shift of MPB from MPB(I) between ferroelectric rhombohedral and ferroelectric tetragonal phases to MPB(II) between ferroelectric rhombohedral and relaxor pseudocubic phases. As the MPB(I) switches to MPB(II), large piezoelectric response with d33 ∼ 150pC/N that obtained for BNT–0.20BKT near MPB(I) almost disappears. Instead, a significant jump of electric-filed-induced strain S up to 0.32%–0.46% (Smax/Emax = 400–575 pm/V) is achieved near MPB(II) due to the shift of the ferroelectric-relaxor transition temperature TF-R down to room temperature. In this study, giant strain ∼0.46% occurs in a very narrow region in the BNT-BKT-KNN system with x = 0.20, y = 0.01, which lies on an underlying tricritical triple point of a rhombohedral (R), tetragonal (T), and pseudocubic (Pc) phases. In-situ high-energy X-ray scattering experiments with external electric field reveal an initial electric-field-induced distortion from the Pc structure for the MPB(II) compositions, while those with single R phase shows no such distortion, which suggests that the large strain achieved near the MPB(II) is likely to be induced by the electric-field-induced structure distortion due to its relative instability structure. We believe that the discovery of a compositional line in the pseudo-ternary system, where the strain response is consistently derivable, should be useful for designing high-performance piezoelectric materials in other BNT-based systems by searching MPBs.