IMF 2023

Nonstoichiometry in NBT-based compositions

Andris Sternbergs Marija Dunce Eriks Birks
Institute of Solid State Physics, University of Latvia, Institute of Solid State Physics, University of Latvia, Riga, Latvia

Role of nonstoichiometry in Na0.5Bi0.5TiO3- (NBT-) based compositions has received wide attention recently. Unlike lead-containing perovskites, possible deviations of Bi concentration from the stoichiometric value in these materials are very limited. However, even the detected small deviations crucially influence ionic conductivity [1]. Change of depolarization temperature was also observed and interpreted as a result of changes in the grain size, strongly correlating with deviations of Bi content in the raw constituents used for preparing of the ceramics [2]. In present work, we have performed an extended study of Bi nonstoichiometry, covering impact of high-temperature treatment, addition of overstoichiometric amount of Bi during processing, differences in the limits of A-site and oxygen vacancies.

Sintering of NBT in a wide range of temperatures does not show any clear signs of Bi evaporation from the bulk part of the ceramics, neither in terms of Bi concentration in the matrix grains nor in terms of content of Bi-deficient inclusions. On the contrary, high-temperature treatment leads to severe increase of TiO2 inclusion concentration on the surface of the ceramics and is evidence of significant Bi evaporation.

Intentionally created Bi overstoichiometry in NBT leads to decreasing of the grain size and lowering of the depolarization temperature. Higher overstoichiometry leads to increased Na concentration in the matrix grains.

Investigation of NBT-(Sr0.7Bi0.2)TiO3 solid solutions reveal that A-site vacancies can exist in a wide concentration range depending on (Sr0.7Bi0.2)TiO3 content, until there is no necessity for oxygen vacancies due to ionic charge compensation. The latest condition seems to be relevant for deviations of Bi content also in other NBT-based solid solutions.

Role of Bi nonstoichiometry on the phase transition in NBT-BaTiO3 compositions in the range of the morphotropic phase boundary is also discussed.

References.

1. Li M., Pietrowski M. J., De Souza R. A., et al. Nat.Mater. (2014) vol.13, p.31-35.

2. Mishra A., Khatua D. K., De A., et al. Acta Materialia (2019) vol.164, p.761-775.

This work has been funded by the European Regional Development Fund in the framework of the postdoctoral research project No. 1.1.1.2/VIAA/3/19/558.





IMF 2023




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