Effect of BiMeO ₃ on the phase structure, ferroelectric stability, and properties of lead-free Bi_0.5(Na_0.80K _0.20)_0.5TiO₃ ceramics

The effects of BiMeO₃ (Me = Fe, Sc, Mn, Al) addition on the phase transition and electrical properties of Bi_0.5(Na _0.80K_0.20)_0.5TiO₃ (BNKT20) lead-free piezoceramics were systematically investigated. Results showed that addition of BiFeO₃ into BNKT20 induces a phase transition from tetragonal-rhombohedral coexisted phases to a tetragonal phase with the observation of enhanced piezoelectric properties (d₃₃ = 150 pC/N for 0.02BiFeO₃). BiScO₃, BiMnO₃, and BiAlO ₃ substitutions into BNKT20 induce a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long-range ferroelectric order, and correspondingly adjusts the ferroelectric-relaxor transition point T _F-R to room temperature. Accordingly, large accompanying normalized strains of 0.34%-0.36% are obtained near the ferroelectric-relaxor phase boundary, and the mergence of large strain response can be ascribed to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation. Moreover, our study also revealed that the composition located at the ferroelectric-relaxor phase boundary where the strain response is consistently derivable shifts to a BNKT20-rich composition as the tolerance factor t of the end-member BiMeO₃ increases, and this relationship is expected to provide a guideline for designing high-performance (Bi_0.5Na _0.5)TiO₃-based materials by searching the ferroelectric-relaxor phase boundary.