Ferroelectric domain structures in strained BiFeO₃ ceramics synthesized by spark plasma sintering

Highlights

• The high strain changes the contents and morphologies of domains in BiFeO₃ ceramics significantly.

• Domain topographies heavily rely on the grain orientations.

• The deformed 71° domain walls emerged to compensate the excess thermal sintering strains.

Abstract

The ferroelectric domain structures were analyzed in strained multiferroic BiFeO₃ ceramics via X-ray diffraction (XRD), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). XRD refinements indicate that the SPS method introduced the large strain into the ceramic samples even though ~50% strain reduction could be conducted through annealing process. The large strain increased the contents of unusual 71° ± 1° grain boundaries and deformed 71° domain walls (~89° twin boundaries) both in the as-sintered sample and the annealed sample, which could accommodate the grain boundary energy and the large thermal strain. The large strain was more conducive to form typical curved and bridge-like 180° domain morphologies. And the domain topographies heavily relied on the grain orientations.

Introduction

Domain structures play important roles in technological exploitations of ferroelectric properties of multiferroic materials which have been researched extensively due to their potential applications in the designs of electronic devices such as transducers, information storages and energy fields [[1], [2], [3], [4], [5]]. It has been well known that domain structures tend to be formed to reduce the depolarization effects and the macroscopic strains [6]. The strain induced domain switching is implicated in the tailoring of ferroelastic and ferroelectric properties [7]. As a typical multiferroic material, bismuth ferrite (BiFeO₃) equipped with anti-ferromagnetism and strong ferroelectricity simultaneously at room temperature [[8], [9], [10]] is popularly studied. The huge spontaneous polarization (P_s ~ 100 μC/cm²) in BiFeO₃ is along the 〈111〉_pc pseudo-cubic directions [[11], [12], [13]], which are confirmed to have strong interactions with strain [14,15]. Therefore, exploring the domain structures is imperative in the strained BiFeO₃ ceramics with switchings of large polarizations.

The contents and morphologies of domains are significantly affected by the strain. The ferroelectric domains in rhombohedral BiFeO₃ [16] include 180° and non-180° (71° or 109°) domain walls, which have been demonstrated both in BiFeO₃ films [17,18] and BiFeO₃ ceramics [19,20]. The formation of 180° domain walls is subject to reduce the depolarization energy, while the non-180° (71° or 109°) domain walls can release interior strains in crystals [21]. It's worth noting that changes of the strain and the depolarization field, caused by introducing a certain dielectric layer between the epitaxial BiFeO₃ film and the SrRuO₃ substrate, would drive 71° domain walls to 109° domain walls in BiFeO₃ films [22]. When the BiFeO₃ ceramic is restricted by the strain, more ferroelectric twinnings, in form of irregular stripe patterns at the sub-micrometer scale, will form within the elastic twin domains [23,24]. Nevertheless, non-180° domain switching cannot change the macroscopic strain of the equiaxed polycrystal materials [14].

With regard to the domain morphologies, the elastic strain energy induces striped domain patterns in the epitaxial BiFeO₃ thin films [25]. Interestingly, well-ordered BiFeO₃ epitaxial nanodots could be obtained from the BiFeO₃ film using the Ar ion beam etching. Compared with BiFeO₃ films, strain conditions and the depolarization field have changed in a large degree, which make striped domain patterns in BiFeO₃ films evolve to novel flux-closure vortex or antivortex topologic domains in BiFeO₃ epitaxial nanodots [26,27]. Other researchers also indicated that the large strain could result in the curved domain walls in BaTiO₃ ceramics [28]. In BiFeO₃ ceramics, the large strain shows significant pinning effects on complexly striped domain structures when clarifying the influences of defects on the domain-wall mobilities [29]. However, at the present stage, these studies commonly focused on the observation of microscopical domain structures using transmission electron microscope (TEM) or piezoresponse force microscopy (PFM). The relationship between domain morphologies and crystal orientations with strain tailoring in large scale are still needed to be revealed in BiFeO₃ ceramics. The electron backscatter diffraction (EBSD) is considered to be an ideal supplement tool to observe such relationship [30,31].

In this paper, the ferroelectric domain structures were investigated in the as-sintered BiFeO₃ ceramic with a large strain by spark plasma sintering (SPS) technique. The annealed BiFeO₃ ceramic with the relative smaller strain was implemented to observe possible changes of domain structures under the surface strain relaxation. Effects of strains on the domain switching were investigated using X-ray diffraction (XRD). The subtle differences and the intrinsic topographies of the ferroelectric domains were exhibited by chemical etching methods. And the EBSD was also conducted to analyze the domain morphologies and crystal orientations in strained BiFeO₃ ceramics.