Ferroelectric domain structures in strained BiFeO3 ceramics synthesized by spark plasma sintering
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 (Ps ~ 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.
Section snippets
Experimental procedure
The polycrystalline BiFeO3 ceramics with large micro strains were obtained by SPS technique. The raw BiFeO3 powders were synthesized by the chemical co-precipitation process published in our previous work [32] and calcined at 550 °C for 60 min in air atmosphere. Then the powders were compacted in a cylindrical graphite die (~20 mm in diameter) at ~18.7 MPa pressure and further sintered at 750 °C for 10 min in nitrogen atmosphere under a uniaxial pressure of 40 MPa by SPS. The high
Analyses of XRD patterns and strains
Fig. 1a, b and c show the XRD Rietveld refinements for BiFeO3 powders, the as-sintered and annealed BiFeO3 ceramics, respectively, using the GSAS-EXPGUI program [35,36]. The refined profiles fit well with the experimental datas, with reliability factors Rwp values of 5.82%, 6.0% and 6.08%, respectively. The powders and ceramics show typical R3c space group without any impurities. In BiFeO3 ceramics with rhombohedral R3c symmetry, the 180° domain switching is independent of the reflection
Conclusions
The XRD refinements revealed that the large strain increased the 71° domain contents, and the surface residual strain relaxation by annealing processes below the Curie temperature had no obvious effects on the switching between 71° and 109° domains. The detailed domain patterns depicted by SEM showed the typical 180° and non-180° domain walls. The domain topographies were very relevant to the grain orientations, and the non-180° domain walls could be classified as different domain topographies
Declaration of competing interest
All authors have approved this manuscript, and no interest conflict exists in its submission.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51772065, 51472063 and 51621091).
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2022, Progress in Materials ScienceCitation Excerpt :In polycrystalline ceramics, intergranular interaction is a possible way to impose stress/strain, in particular in those sintered with pressure. Zhang et al. [110] reported that, in spark plasma sintered BFO ceramics, residual strain is detected by X-ray diffraction, regardless of the post annealing process. 71° domains are deformed to be near 90°-like due to the constraints of surrounding grains.