Unveiling the high-temperature dielectric response of text {Bi}_{0.5}text {Na}_{0.5}text {TiO}_{3}

Julio Cesar Camilo Albornoz Diaz,Paulo Sergio Da Silva,Michel Venet,Jean-Claude M’Peko

doi:10.1038/s41598-020-75859-z

Julio Cesar Camilo Albornoz Diaz, Paulo Sergio Da Silva + Show 2 more

Open Access

https://doi.org/10.1038/s41598-020-75859-z

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Abstract

Understanding the physics behind changes in dielectric permittivity and mechanical response with temperature and frequency in lead-free ferroic materials is a fundamental key to achieve optimal properties and to guarantee good performance in the technological applications envisaged. In this work, dense text {Bi}_{0.5}text {Na}_{0.5}text {TiO}_{3} (BNT) electroceramics were prepared through solid-state reaction of high-grade oxide reagents, followed by sintering at high temperature (1393 K for 3 h). In good agreement with previous reports in the literature, the thermal behaviour of dielectric response from these BNT materials showed the occurrence of a high-temperature diffuse-like permittivity peak, whose origin has been so far controversial. Thermally stimulated depolarization current, impedance and mechanical spectroscopies measurements were here conducted, over a wide range of temperature and frequency, to get a deep insight into the mechanism behind of this event. The approach included considering both as-sintered and reduced BNT samples, from which it is demonstrated that the broad high-temperature dielectric peak originates from interfacial polarization involving oxygen vacancies-related space-charge effects that develop at the grain-to-grain contacts. This mechanism, that contributes to the anomalous behavior observed in the mechanical response at low frequencies, could also be responsible for the presence of ferroelastic domains up to high temperatures.

Highlights

Understanding the physics behind changes in dielectric permittivity and mechanical response with temperature and frequency in lead-free ferroic materials is a fundamental key to achieve optimal properties and to guarantee good performance in the technological applications envisaged
By using transmission electron microscopy, on the other hand, Dorcet et al observed a local modulated phase between 470 and 550 K, constituted of Pnma orthorhombic sheets between two R3c ferroelectric d omains[19,20]. This rhombohedral/orthorhombic coexistence was observed at room temperature by Jones et al.[21] when pressure was applied to BNT crystals
We present a detailed study based on thermally stimulated depolarization current (TSDC), impedance spectroscopy and mechanical spectroscopy approaches that allow demonstrating that the origin of the dielectric behaviour observed in BNT ceramics around Tm is closely related with oxygen vacancies-associated space-charge effects developing at the grain-to-grain contacts with a clear contribution to the broadening of mechanical losses for temperatures between 600 and 800 K, in low frequencies

Summary

Introduction

Understanding the physics behind changes in dielectric permittivity and mechanical response with temperature and frequency in lead-free ferroic materials is a fundamental key to achieve optimal properties and to guarantee good performance in the technological applications envisaged. The approach included considering both as-sintered and reduced BNT samples, from which it is demonstrated that the broad high-temperature dielectric peak originates from interfacial polarization involving oxygen vacancies-related spacecharge effects that develop at the grain-to-grain contacts This mechanism, that contributes to the anomalous behavior observed in the mechanical response at low frequencies, could be responsible for the presence of ferroelastic domains up to high temperatures. The temperature dependence of dielectric permittivity of BNT has been shown to exhibit a dielectric peak usually found to occur around 600 ≤ Tm ≤ 650 K , where there exists no evidence of structural transformation taking place, albeit report of some phases coexistence[18], as commented above This peak normally is extremely broad, extending to temperatures that may reach 800 K22,24–28, that is, approaching the temperature region of tetragonal to cubic phase transition[18,29]. Liu et al have deconvoluted this broad high-temperature dielectric peak in BNT-based materials by using two Gaussian functions, that were associated with the rhombohedral R3c to tetragonal P4bm phase transition and with the clamping of domain walls into the polar regions, induced by oxygen v acancies[36]

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