Anomalous influence of grain size on the global structure, ferroelectric and piezoelectric response of Na_0.5Bi_0.5TiO₃

Abstract

Na_0.5Bi_0.5TiO₃ (NBT) based lead-free piezoelectrics have received significant attention in the last few years for their anomalous high-field electrostrain response. The positional disorder on the nano/meso scale in these systems have a profound effect on their perceived global structure and dielectric properties. Here we have carried out a systematic analysis of the grain size dependent structural, ferroelectric, piezoelectric and high temperature electrical conduction behaviour of NBT. We show that the global monoclinic lattice distortion collapses even while the grains are unusually large (∼2.5 μm). We also found that the polarization switching ability and piezoelectric response of bulk NBT ceramics reduce drastically when grains are submicron in size. We use a powder poling technique to prove that the significant reduction of piezoelectricity in the submicron grain ceramic is not due to suppression of ferroelectricity within the grains itself, but rather due to inability of the grains to transform collectively to a long range rhombohedral ferroelectric state, either because of the clamping effect and/or enhanced incoherence of the grain boundaries. We also noted a qualitative change in the electrical conduction behaviour as a function of grain size and comment on a possible correlation between the grain size, global structure, piezoelectric response and oxygen ion conductivity of this lead-free compound.