Structural mechanism behind piezoelectric enhancement in off-stoichiometric Na_0.5Bi_0.5TiO₃ based lead-free piezoceramics

Abstract

While studies in the past have shown that certain kinds of off-stoichiometry enhance the piezoelectric response of the lead-free piezoceramic Na_0.5Bi_0.5TiO₃ (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 Na_0.5Bi_0.5TiO₃ (NBT). Of the four different types of off-stoichiometric samples synthesized as per nominal formulae namely Na_0.5+xBi_0.5TiO₃ (Na-excess Na-series), Na_0.5-xBi_0.5TiO₃ (Na-deficient Na-series), Na_0.5Bi_0.5+xTiO₃ (Bi-excess Bi-series), and Na_0.5Bi_0.5-xTiO₃ (Bi-deficient Bi-series), the best piezoelectric response (d33 ∼ 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_0.5Bi_0.5TiO₃-0.06BaTiO3 (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.