Structural mechanism behind piezoelectric enhancement in off-stoichiometric Na0.5Bi0.5TiO3 based lead-free piezoceramics

While studies in the past have shown that certain kinds of off-stoichiometry enhance the piezoelectric response of the lead-free piezoceramic Na0.5Bi0.5TiO3 (NBT), there is a lack of clarity regarding the mechanism associated with this interesting phenomenon from the fundamental structural perspective. In this paper, we have investigated this issue comprehensively and succeeded in establishing a mutual correspondence between off-stoichiometry, grain size, crystal structure, dielectric and piezoelectric properties in Na0.5Bi0.5TiO3 (NBT). Of the four different types of off-stoichiometric samples synthesized as per nominal formulae namely Na0.5+xBi0.5TiO3 (Na-excess Na-series), Na0.5-xBi0.5TiO3 (Na-deficient Na-series), Na0.5Bi0.5+xTiO3 (Bi-excess Bi-series), and Na0.5Bi0.5+xTiO3 (Bi-deficient Bi-series), the best piezoelectric response (d(33) similar to 100 pC/N) was obtained in the Na-deficient series with x = 0.04. We succeeded in establishing the structural link between off-stoichiometry and piezoelectricity of this series by examining the structural state of the specimens in their poled state. We show that the off-stoichiometric compositions exhibiting higher piezoelectric response contain a higher fraction of the disordered ferroelectric phase coexisting with the field stabilized long-range ferroelectric (R3c) order. Beyond the critical off-stoichiometry (x > 0.04), the dominance of the structural disorder collapses the piezoelectric response of the system. We also show that what can be achieved by off-stoichiometry can as well be achieved by reducing the grain size of stoichiometric NBT. Our results suggest that the enhanced piezoelectric response of the off-stoichiometric compositions is due to their reduced grain size as compared to the stoichiometric composition, and that the nature of the defect species has a secondary role, if any. We found the same phenomenon/mechanism to be operative in the off-stoichiometric morphotropic phase boundary composition 0.94Na(0.5)Bi(0.5)TiO(3)-0.06BaTiO(3) (NBT-6BT). While our experiments confirm the role of the surviving structural heterogeneity (after poling) as an important contributing factor which enhances the piezoelectric response of NBT-based lead-free piezoceramics, we also use dielectric dispersion as a tool to show that the off-stoichiometric composition exhibiting highest piezoelectric response is characterized by maximum suppression of the disordered phase by the poling field. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.