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Driven by environmental concerns and governmental directives, a sustained research effort in the last decade and half has led to the development of lead-free alternatives which can potentially replace the commercial lead-based piezoceramics in niche applications. Na 0.5 Bi 0.5 TiO 3 (NBT)-based lead-free piezoceramics have found acceptance as promising lead-free transducers in high power ultrasonic devices. An issue of concern however is the low depolarization temperature which limits the device's tolerance for temperature rise during operation. While several strategies have been reported to improve thermal depolarization in NBT-based piezoceramics, there is a lack of consensus regarding the most fundamental factor/mechanism which enhances the depolarization temperature. In this paper we unravel a coupled microstructural-structural mechanism which controls the thermal depolarization in NBT-based piezoceramics. First, we demonstrate the phenomenon of a considerable increase in the depolarization temperature, without significantly losing the piezoelectric property in unmodified NBT by increasing the grain size. We then establish a grain size controlled structural mechanism and demonstrate that the rise in depolarization temperature is primarily associated with the bigger grains allowing relatively large lattice distortion to develop in the poling stabilized long range ferroelectric phase. We reconfirmed the validity of this mechanism in the model morphotropic phase boundary (MPB) composition 0.94Na 0.5 Bi 0.5 TiO 3 -0.06BaTiO 3. For the sake of generalization, we demonstrate that the same mechanism is operative in another lead-based relaxor-ferroelectric system 0.62PbTiO 3 -0.38Bi(Ni 0.5 Hf 0.5)O 3. Our study provides the fundamental structural basis for understanding thermal depolarization delay in relaxor ferroelectric based piezoceramics. Image 1 [ABSTRACT FROM AUTHOR]

Abstract While studies in the past have shown that certain kinds of off-stoichiometry enhance the piezoelectric response of the lead-free piezoceramic Na 0.5 Bi 0.5 TiO 3 (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 0.5 Bi 0.5 TiO 3 (NBT). Of the four different types of off-stoichiometric samples synthesized as per nominal formulae namely Na 0.5+x Bi 0.5 TiO 3 (Na-excess Na-series), Na 0.5-x Bi 0.5 TiO 3 (Na-deficient Na-series), Na 0.5 Bi 0.5+x TiO 3 (Bi-excess Bi-series), and Na 0.5 Bi 0.5-x TiO 3 (Bi-deficient Bi-series), the best piezoelectric response (d 33 ∼ 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. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]