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

Abstract-Submicrometer-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics (?G?<720nm) 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)pCN-1, d15=(158.3-8.31i)pCN-1, kt=40.4%, kp=26.8%, and k15=40.2%] and d33(148 pCN-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 Ti=230°C on heating and Ti=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 ?? 33?(T) curves or, as the degree of poling decreases, as a soft change of slope of the curves at TFELTNPZF=Td=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 Td is a consequence of the coexistence of nanoregions of the FE phase in the interval between TFE-LTNPZF and Ti