Elsevier

Ceramics International

Volume 46, Issue 4, March 2020, Pages 4180-4190
Ceramics International

(Ba,Sr)TiO3 solid solutions sintered from sol-gel derived powders: An insight into the composition and temperature dependent dielectric behavior

https://doi.org/10.1016/j.ceramint.2019.10.136Get rights and content

Abstract

Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 ≤ x ≤ 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 °C the BST ceramics are ferroelectric for 0.3 ≤ x ≤ 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7–0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

Introduction

Ferroelectric materials are very interesting not only for fundamental research, but for due to their specific features and multi-functionality, they are promising candidates for a large number of applications in microelectronics [1]. The ability of being nonlinear and hysteretic with the dielectric permittivity and polarization dependent on the applied field is a main feature that makes these materials interesting and suitable for wireless communications and for microwave devices, data storage, random access memories, ferroelectric tunneling barriers and ferroelectric gate transistors [[2], [3], [4], [5]]. In addition, ferroelectric materials show pyroelectricity, piezoelectricity and specific electro–optic properties that lead to a broad spectrum of applications as transducers, medical imaging, ultrasonic devices, modulator switches, etc. [[1], [2], [3], [4], [5]].

Barium strontium titanate (BST) is one of the most common and studied ferroelectric oxide alongside the solid solutions of barium titanate (BT) [[6], [7], [8]] since the discovery of ferroelectricity in BT [9]. With their promising composition-dependent nonlinear dielectric and hysteretic properties, the BST materials have been the subject of many previous studies. Therefore, thorough research works were dedicated to dielectric properties, frequency- and electric field-dependences, as well as to dielectric relaxation phenomena related to the ferroelectric - paraelectric phase transition in a large range of temperatures [7,[10], [11], [12]]. Nevertheless, the dielectric properties of BST materials still present aspects that deserve a closer insight. In the light of the critical role played by temperature in the ferroelectric state or close to the Curie point, where relative permittivity and dielectric losses reaches their maximum values, a complex analysis on the low field properties via broadband impedance spectroscopy is proposed.

To investigate ferroelectrics at temperatures below, close to and above their phase transitions is an appealing task for a better understanding of the low field functional properties in relationship with specific structural and microstructural characteristics. The present study involves a detailed investigation of several Ba1-xSrxTiO3 compositions having the ferroelectric-paraelectric phase transition located at temperatures near the room temperature (RT = 20 °C). They are: (i) compositions that exhibit a paraelectric behavior at RT (x = 0.4 and x = 0.375), (ii) a composition facing the ferroelectric-paraelectric phase transition at RT (x = 0.35), and (iii) compositions that are in the ferroelectric state at RT (x = 0.325 and x = 0.3).

Irrespective of heterogeneities type in the ferroelectric materials (structural defects, solutes/dopants, charged vacancies, localized polar species) the short and long-range interactions are affected when increasing the temperature. In addition, the dielectric relaxation mechanisms may change according to the amount of free charge carriers in the oxide ceramic material [13]. Several thermally activated physical processes (e.g. charges migration, reorientation and disorder of dipoles) are influencing the electrical polarization and have impact on the electrical conductivity. Considering these aspects, our experimental study on the complex low-field properties aims to elucidate in which way the charged species at the boundaries of ferroelectric phases affect the functional properties of the BST materials.

Section snippets

Experimental methods

The Ba1-xSrxTiO3 ceramics, with x = 0.3, 0.325, 0.35, 0.375, 0.4, and abbreviated in the following as BST30, BST32.5, BST35, BST37.5 and BST40, respectively have been prepared in a parallel plate-capacitor configuration from powders synthesized via sol-gel route. The preparation process details were previously described elsewhere [14].

X-ray diffraction (XRD) measurements were performed at room temperature (RT = 20 °C) in order to investigate the phase purity and crystal structure of the sol-gel

Phase purity, structure and microstructure

The X-ray diffraction patterns shown in Fig. 1 were recorded at ambient temperature and they show that all the ceramics are single phase BST. The presence of the well-crystallized perovskite solid solution as unique phase was revealed by the main reflections, which are shifted toward higher diffraction angles as the Sr addition increases, proving the complete homovalent incorporation of Sr2+ on Ba sites (Fig. 1(a)).

Even if for the samples with a lower solute content (0.30 ≤ x ≤ 0.35) no clear

Conclusions

The dielectric properties at low electric fields of Ba1-xSrxTiO3 ceramics, with x = 0.3, 0.325, 0.35, 0.375, 0.4, were investigated on a wide range of temperatures and frequencies and correlated with the structural features. It was found that:

  • -

    Increased addition of Sr produces structural changes such as reduction of tetragonality c/a, reduction of the unit cell volume and reduction of the grain size accompanied by an increased weight of grain boundaries.

  • -

    Increased Sr addition leads to a reduction

Acknowledgements

Lucian Pintilie, Ioana Pintilie and Roxana Patru acknowledge the financial support of the Romanian Ministry of Research and Innovation (MCI) and of the Executive Unit for Funding High Education, Research, Development and Innovation, Romania (UEFISCDI) through the grant PCCF 16/2018. Catalina-Andreea Stanciu, Vasile-Adrian Surdu, Roxana Trusca, Adelina-Carmen Ianculescu acknowledge the financial support of MCI and UEFISCDI through the project PN–III–P4-ID-PCE-2016-0072. Constantin Paul Ganea

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