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 (Bi0.5Na0.5)(0.94)Ba0.06TiO3 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 [d(31) = (-37 + 1.33i) pC.N-1, d(15) = (158.3 -8.31i) pC.N-1, k(t) = 40.4%, k(p) = 26.8%, and k(15) = 40.2%] and d(33) (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 degrees C. Permittivity versus temperature curves of poled samples show relaxor character up to T-i = 230 degrees C on heating and T-i = 210 degrees 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 epsilon(sigma)(33)'(T) curves or, as the degree of poling decreases, as a soft change of slope of the curves at TFE-LTNPZF = T-d = 100 degrees 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 TFE-LTNPZF and T-i.