Preparation and functional characterization of BiFeO₃ ceramics: A comparative study of the dielectric properties

Highlights

• BiFeO₃ ceramics prepared by two thermal treatments are comparatively investigated.

• It was demonstrated that the dc-conductivity is a thermally-activated process.

• A conductivity anomaly takes place in the range of (189–244) K for both samples.

• This type of transitions is intrinsic in BiFeO₃ and is not due to secondary phases.

Abstract

In the present study, the electrical properties of BiFeO₃ ceramic specimens prepared by solid-state sintering method by using two thermal treatment strategies are comparatively investigated. The room temperature XRD pattern shows perovskite single-phase, in the limit of XRD accuracy, for BiFeO₃ ceramic prepared by single-step method. For two-step sintering method sample small amounts of secondary Bi₂Fe₄O₉ phases were identified. The ceramics show a non-homogeneous microstructure, consisting of ceramic grains with irregular morphology and interconnected porosity mainly in the grain boundary regions in the case of two-step sintering sample. The most interesting feature is the conduction anomaly observed on the conductivity in the low-frequency range close to dc-conductivity. The Arrhenius plot of the dc-conductivity determined at the lowest frequency vs. 1/T shows two distinct linear regions separated by the mentioned temperature range of (189–244) K, for which the dc conductivity could not be determined from the present impedance spectroscopy data only. It is clear that in the mentioned temperature range (for both samples), a conduction anomaly takes place.

Introduction

Some of the most interesting multifunctional materials which in the last years attracted the highest attention in the field of electroceramics are the magnetoelectric multiferroics [1], [2], [3], [4], [5], [6], which contain two types of order parameters (i.e. magnetic and dipolar, or ferroelectric). The conditions for the occurrence of ferroelectric and magnetic order in the same material require: (i) symmetry conditions; (ii) the presence of adequate structural building blocks permitting off-centre ion displacement associated to the ferroelectric spontaneous polarization or other alternative mechanism for ferroelectricity (as charge ordering: LuFe₂O₄ [7], [8], [9], lone pair: BiFeO₃, BiMnO₃ [10] or geometric frustration (e.g.YMnO₃ [11])) and (iii) magnetic-interaction pathways for the magnetic order, usually of super-exchange type.

Within this class of multiferroics, bismuth ferrite BiFeO₃ is by far the most studied system (the number of publications in the last five years related to BiFeO₃-based materials is above one thousand [3], [5], [12], [13], [14], [15], [16], [17]). Due to the high difficulties in preparing BiFeO₃ ceramics free of secondary phases, only a few reports showed relevant data concerning the properties of pure BiFeO₃ ceramics [18], [19], [20] and rarely their functional properties could be compared with the ones reported for single crystals or films, due to the major role of the extrinsic contributions (defects, secondary phases, porosity) [21]. BiFeO₃ presents at room temperature a distorted perovskite structure with rhombohedral R3c symmetry with lattice parameters: a = 3.958, and α = 89.30°, a high ferroelectric Curie temperature (T_C = 830 °C) which should be the sign of a large spontaneous polarization (∼100 μC/cm²) [22]. From the magnetic point of view, it has an antiferromagnetic order (Néel temperature T_N = 370 °C, magnetic Curie temperature ∼600 °C) and shows a weak ferro/ferrimagnetic characteristic in some temperature ranges [23], [24], [25], [26], [27]. The Fe magnetic moments are coupled ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between adjacent planes (so-called G-type antiferromagnetic order).

Although very promising for the expected multiferroic character, the bulk ceramics showed at room temperature poor dielectric and ferroelectric properties, with low values of polarization and of permittivity, lossy unsaturated P(E) loops, mainly due to the semiconducting character caused by charged defects and secondary phases [20]. The origin of the very scattered values of the ferroelectric characteristics in bulk ceramics and thin films [28], [29], [30], [31], [32], [33], [34], [35] produced in the last years by various groups (e.g. polarizations ranging from units, tenths and even over 100 μC/cm² and permittivity from tenths to thousands) are clearly due to the presence of secondary phases, grain boundary phenomena, charge defects and other extrinsic contributions. The data rarely could be compared between various groups, due to very different synthesis approaches resulting in various purities, microstructures and charged defects. Therefore, the debates related to the intrinsic/extrinsic nature of the functional properties of the BiFeO₃-based materials are still far to be exhausted.

In spite of the large number of publications related to the BiFeO₃ properties, dielectric relaxation in radiofrequency range were rarely reported, mainly due to the difficulty of obtaining pure phase and to very strong extrinsic effects manifesting at room temperature in this frequency range. Only a few reports the interpretation of such measurement results versus temperature: Krainik et al. [36] reported permittivities around 45 at room temperature and of ∼150 at 1150 K for microwave frequencies, while Kamba et al. [37] reported a giant low-frequency permittivity above 200 K explained by a combination of magnetoresistance and Maxwell–Wagner effect, with an intrinsic permittivity value below 40 at low temperature. An accurate dielectric study was performed for BiFeO₃ ceramics sintered from pure precipitation-synthesized BiFeO₃ powders [6]. More recently, magnetic, dielectric and thermodynamic properties in a broad temperature range were reported for single crystal and bulk BiFeO₃ ceramics [38], in which it was concluded that permittivity at ambient temperature is strongly influenced by Maxwell–Wagner contributions and an intrinsic room temperature permittivity of ∼50 is characteristic for the single crystal. Nevertheless, there is still an open question about the real values of the intrinsic permittivity of BiFeO₃ ceramics and on the dielectric relaxation-phenomena at cryogenic and high temperatures.

In a recent review on BiFeO₃ by Catalan and Scott [5] as well as in a series of papers of their groups [39], [40], [41], [42], [43], [44], a number of problems and open questions concerning the functional properties of BiFeO₃ were pointed out. Among them, it was mentioned the presence of a group of dielectric and/or conductivity anomalies in single-crystals and bulk ceramics at ∼50 K, ∼140 K, and ∼200 K, which are not related to any structural modification. These features were confirmed by anomalies found in the Raman spectra at the same temperatures and they were considered as an indirect proof of the magnetoelectric coupling. Only the above-mentioned publications reported such anomalies in bulk BiFeO₃ (single-crystal and ceramics). Therefore, such features were considered as being one topic to be further checked for other BiFeO₃ systems prepared by other groups and to be further investigated and understood. Recently, in one of our previous paper [14], it was reported the non-linear dielectric constant-field response in BiFeO₃ pure ceramics which was the first study of tunability properties (the static dielectric properties under high field) of BiFeO₃. The tunability property results from at least two contributions: one at low field (extrinsic contribution) that was correlated with the reversible orientation of the Bi lone pairs and one at moderate and high fields (intrinsic contribution) that represents the field-induced ferroelectric polarization response. However, this paper does not present a low-field dielectric behaviour analysis. Consequently, it is worthy to make a detailed dielectric study of the pure BiFeO₃ ceramics in order to analyze the eventually relaxation and conductivity mechanism vs. frequency at low and high temperature.

In this paper, the dielectric properties of BiFeO₃ ceramics prepared by solid-state method from the same powders, following two thermal treatment procedures were comparatively investigated in a range of temperatures from liquid nitrogen to a few hundreds of degrees Celsius. A detailed dielectric and conductivity analysis clearly demonstrated a range of temperatures of (189, 244) K for which the intrinsic dc-conductivity has uncommon smaller values than for the other temperatures.