Stable Ferroelectric Behavior of Nb-Modified Bi_0.5K_0.5TiO₃-Bi(Mg_0.5Ti_0.5)O₃ Lead-Free Relaxor Ferroelectric Ceramics

Crystal structure, dielectric, ferroelectric, piezoelectric, and electric field-induced strain properties of lead-free Nb-modified 0.96Bi_0.5K_0.5TiO₃-0.04Bi(Mg_0.5Ti_0.5)O₃ (BKT-BMT) piezoelectric ceramics were investigated. Crystal structure analysis showed a gradual phase transition from tetragonal to pseudocubic phase with increasing Nb content. The optimal piezoelectric property of small-signal d₃₃ was enhanced up to ∼ 68 pC/N with a lower coercive field (E_c) of ∼ 22 kV/cm and an improved remnant polarization (P_r) of ∼ 13 μC/cm² for x = 0.020. A relaxor-like behavior with a frequency-dependent Curie temperature T_m was observed, and a high T_m around 320°C was obtained in the investigated system. This study suggests that the ferroelectric properties of BKT-BMT was significantly improved by means of Nb substitution. The possible shift of depolarization temperature T_d toward high temperature T_m may have triggered the spontaneous relaxor to ferroelectric phase transition with long-range ferroelectric order without any traces of a nonergodic relaxor state in contradiction with Bi_0.5Na_0.5TiO₃-based systems. The possible enhancement in ferroelectric and piezoelectric properties near the critical composition x = 0.020 may be attributed to the increased anharmonicity of lattice vibrations which may facilitate the observed phase transition from a low-symmetry tetragonal to a high-symmetry cubic phase with a decrease in the lattice anisotropy of an undoped sample. This highly flexible (at a unit cell level) narrow compositional range triggers the enhancement of d₃₃ and P_r values.