Field-induced phase transition and relaxor character in submicrometer-structured lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoceramics at the morphotropic phase boundary

Submicrometer-structured (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics ((G) <; 720 nm) from nanopowders were studied. The real part of the optimum room temperature set of piezoelectric coefficients obtained from resonances of the BNBT6 dense ceramic disks and shear plates [d₃₁ = (-37 + 1.33i) pC·N^-1, d₁₅ = (158.3 - 8.31i) pC·N^-1, k_t = 40.4%, k_p = 26.8%, and k₁₅ = 40.2%] and d₃₃ (148 pC·N^-1) can be compared with the reported properties for coarse-grained ceramics. Shear resonance of thickness-poled plates is observed at T = 140°C. Permittivity versus temperature curves of poled samples show relaxor character up to T_i = 230°C on heating and T_i = 210°C on cooling of the depoled samples. The phase transition from the room-temperature ferroelectric (FE) to a low-temperature non-polar at zero field (LTNPZF) phase can be observed as a sharp jump in ε^σ₃₃´ (T) curves or, as the degree of poling decreases, as a soft change of slope of the curves at T_FELTNPZF = T_d = 100°C. This dielectric anomaly is not observed on cooling of depoled samples, because the FE phase is field-induced. The observed macroscopic piezoelectric activity above T_d is a consequence of the coexistence of nanoregions of the FE phase in the interval between T_FELTNPZF and T_i .