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

Mishra, Anupam and Khatua, Dipak Kumar and De, Arnab and Majumdar, Bhaskar and Frömling, Till and Ranjan, Rajeev :
Structural mechanism behind piezoelectric enhancement in offstoichiometric Na0.5Bi0.5TiO3 based lead-free piezoceramics.
In: Acta Materialia, 2019 (164) pp. 761-775. ISSN 13596454
[Article] , (2018)

Abstract

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 Na0.5þxBi0.5TiO3 (Na-excess Na-series), Na0.5-xBi0.5TiO3 (Na-deficient Naseries),Na0.5Bi0.5þxTiO3 (Bi-excess Bi-series), and Na0.5Bi0.5-xTiO3 (Bi-deficient Bi-series), the best piezoelectric response (d33 ~ 100pC/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 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (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.

Item Type: Article
Erschienen: 2018
Creators: Mishra, Anupam and Khatua, Dipak Kumar and De, Arnab and Majumdar, Bhaskar and Frömling, Till and Ranjan, Rajeev
Title: Structural mechanism behind piezoelectric enhancement in offstoichiometric Na0.5Bi0.5TiO3 based lead-free piezoceramics
Language: English
Abstract:

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 Na0.5þxBi0.5TiO3 (Na-excess Na-series), Na0.5-xBi0.5TiO3 (Na-deficient Naseries),Na0.5Bi0.5þxTiO3 (Bi-excess Bi-series), and Na0.5Bi0.5-xTiO3 (Bi-deficient Bi-series), the best piezoelectric response (d33 ~ 100pC/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 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (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.

Journal or Publication Title: Acta Materialia
Volume: 2019
Number: 164
Publisher: Elsevier
Uncontrolled Keywords: Lead-free piezoceramics Na0.5Bi0.5TiO3 Off-stoichiometry Size effect Structure-property correlation
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Nonmetallic-Inorganic Materials
Date Deposited: 28 Nov 2018 09:53
DOI: 10.1016/j.actamat.2018.11.015
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