Dependence of dielectric behavior in BiFeO3 ceramics on intrinsic defects

doi:10.1016/j.jallcom.2012.06.110

Journal of Alloys and Compounds

Volume 541, 15 November 2012, Pages 94-98

https://doi.org/10.1016/j.jallcom.2012.06.110 Get rights and content

Abstract

Multiferroic BiFeO₃ (bismuth ferrite, BFO) ceramics were sintered using a rapid heating and quenching technique in an atmosphere of argon or oxygen. The resulting ceramics were phase pure with the space group R3c. BFO ceramics sintered under an argon atmosphere contained more oxygen vacancies and Fe²⁺ ions compared with those sintered under an oxygen atmosphere, which resulted in high dielectric loss and large dielectric dispersion. It confirms that the unavoidable Fe²⁺ ions may play a fundamental role in these special materials, such as in the evaporation of the Bi component and valence change of the Fe component. This work also discusses the dielectric response mechanism in BFO ceramics.

Graphical abstract

Highlights

► BiFeO₃ ceramics were sintered using a rapid heating and quenching technique. ► The XRD and XPS analyses showed the changeable valence of Fe component. ► The unavoidable Fe²⁺ ions may play a fundamental role in BFO ceramics. ► The high Fe²⁺ ions led to high dielectric loss and large dielectric dispersion.

Introduction

Multiferroic materials, which possess both ferroelectric and magnetic properties, have attracted great attention due to their applicability in spintronics and information storage, among others, and as transducers [1], [2], [3]. Single-phase bismuth ferrite (BFO) is a potential candidate for multiferroic application because its ferroelectric Curie temperature (1073 K) and antiferromagnetic Néel temperature (643 K) are higher than room temperature [4]. However, the multiferroelectric properties of BFO often suffer from impurities. From the viewpoint of thermodynamics, the narrow synthesis area of single-phase BiFeO₃ in the phase diagram of Bi₂O₃–Fe₂O₃ would result in two kinds of impurities: Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ [5], [6], [7]. The poor ferroelectric properties of BFO, including its low remnant polarization and high leakage current, are often connected to these impurities [8]. In addition, the remnant Bi₂O₃ component is another impurity during the fabrication of BFO [9]. Many studies to date have focused on the synthesis of phase-pure BFO [10], [11], [12].

Except for macro-impurities, however, the multiferroelectric properties of BFO, including ferroelectric and dielectric properties, are also very sensitive to its intrinsic defects, such as vacancy and polyvalence of Fe ions, even if the crystal structure is on its equivalent state. Few studies have investigated the electric polarization properties that may be affected by those intrinsic defects. Significant differences in dielectric constants reported by different studies hinder our understanding of the intrinsic electric polarization of BFO because its dielectric behavior is more sensitive to microstructures and intrinsic defects [13]. This clearly warrants investigation into the dielectric abnormal behavior of different samples to help us understand the dependence of intrinsic defects on electric polarization behavior.

In this study, we first prepared phase-pure BFO ceramics under different atmospheres and then studied the dependence of dielectric behavior on intrinsic defects. We focused on two kinds of intrinsic defects: polyvalence of Fe ion and oxygen vacancy. The dielectric response mechanism in BFO ceramics is also discussed herein.

Section snippets

Fabrication of BFO ceramics

Phase-pure BFO ceramics are necessary for investigating the dependence of dielectric properties on intrinsic defects. First, phase-pure BFO powders were prepared using a chemical co-precipitation method. The reaction precursors were Bi(NO₃)₃·5H₂O and Fe(NO₃)₃·9H₂O powders dissolved in nitric acid. The pH level was fixed at 9.3 through synchronized dropping of ammonia (2.5 M) and nitrate solution (2 M) to precisely control the chemical precipitation process. The preparation is comparable with the

Morphologies and EDS analysis

Fig. 1 illustrates the SEM morphologies of BFO ceramics sintered at 700 °C in an atmosphere of argon or oxygen. The microstructures of both kinds of BFO ceramics were uniform and dense. Their grain sizes ranged from 500 to 1500 nm. More small holes were observed in the surface of the argon-sintered BFO ceramics compared with the oxygen-sintered sample due to the deficiency of oxygen atoms in the former group. Excessive oxygen deficiency or sintering temperature would result in impurities because

Conclusions

We successfully synthesized and compared two groups of phase-pure multiferroic BiFeO₃ ceramics with an R3c structure using a rapid heating and quenching technique in either an argon atmosphere or an oxygen atmosphere. The higher oxygen vacancies and Fe²⁺ ions in the argon-sintered sample led to high dielectric loss and large dielectric dispersion. The results show the Fe²⁺ ions play key roles not only in the evaporation of the Bi component but also in the dielectric response of BFO ceramics.

Acknowledgment

This project was financially supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 50902033 and 51021002)

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Dependence of dielectric behavior in BiFeO3 ceramics on intrinsic defects

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Fabrication of BFO ceramics

Morphologies and EDS analysis

Conclusions

Acknowledgment

J. Cryst. Growth

Mater. Lett.

Mater. Lett.

J. Alloys Compd.

J. Alloys Compd.

Science

Science

Nat. Mater.

Soviet Physics Uspekhi

Phys. Rev. B

Chem. Mater.

Appl. Phys. Lett.

Appl. Phys. Lett.

Dependence of dielectric behavior in BiFeO₃ ceramics on intrinsic defects