BiFeO3-xPbTiO3 Solid Solutions

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Single Crystal Growth and Hierarchical Ferroelectric Domain Structure of (1-x)BiFeO3-xPbTiO3 Solid Solutions Jian Zhuang, Alexei A. Bokov, Nan Zhang, Jie Zhang, Jinyan Zhao, Shuming Yang, Wei Ren, and Zuo-Guang Ye Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00484 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Crystal Growth & Design

Single Crystal Growth and Hierarchical Ferroelectric Domain Structure of (1-x)BiFeO3xPbTiO3 Solid Solutions Jian Zhuang,*,† Alexei A. Bokov,‡ Nan Zhang,† Jie Zhang,† Jinyan Zhao,† Shuming Yang,# Wei Ren,*,† and Zuo-Guang Ye*,‡, † †

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education &

International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, P. R. China ‡

Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada

#

State Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China

KEYWORDS: Bismuth ferrite, Crystal growth, Ferroelectric domain structure, Charged domain walls, Domain wall nanoelectronics.

ABSTRACT: Besides outstanding multiferroic performance, BiFeO3 has recently demonstrated

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a potential for novel promising applications in domain wall nanoelectronics employing domain walls with different functional properties. Rarely observed in ferroelectrics charged domain walls are of special interest for such applications as they possess enhanced electric conductivity. In this work, single crystals of the multiferroic (1-x)BiFeO3-xPbTiO3 (BFPT) solid solution were successfully grown using flux method. Structural characterization by X-ray diffraction confirmed perovskite rhombohedral R3c and tetragonal P4mm phases in crystals with x ≈ 0.2 and x ≈ 0.6, respectively. The domain structure of crystals was established with the help of polarized light microscopy, scanning electron microscopy and piezoresponse force microscopy. In tetragonal crystals a complex hierarchical structure of 90o lamella domains is observed with the thickness of lamellae varying from dozens of nanometers to dozens of micrometers. In the rhombohedral composition, twin 109o domains are realized with the domain width of about several micrometers. Besides, the nano- and submicrometer sized rounded 180o domains are embedded randomly among twin domains in both tetragonal and rhombohedral crystals. Mechanical and electrical compatibility conditions are satisfied for all observed 90o and 109o domain walls. However, the 180o walls in tetragonal crystals are proved to be charged. Successful fabrication of high-quality single crystals with the desirable structures of charged and uncharged domain walls can enhance the multiferroic performance and also open a door to explore promising domain boundaries related phenomena.

1. Introduction Bismuth ferrite (BFO) is one of the most promising single-phase multiferroic materials showing the coexistence of magnetic and ferroelectric orders at room temperature. It has drawn a lot of attention due to fascinating fundamental physical phenomena and potential applications in


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advanced technologies [1-5]. Besides having outstanding multiferroic properties, BFO is also a potential high temperature piezoelectric/ferroelectric material due to its giant ferroelectric polarization and high Curie temperature (TC) at 1123 K. Recently BFO has demonstrated novel promising applications in so-called domain wall nanoelectronics employing domain walls with novel electronic, magnetic, and conductive functionalities [6-10]. Specially, the above-bandgap voltages realized in BFO films and the remarkable photoconductivity of domain walls led to a revival of the entire field of photoferroelectrics [11, 12]. The open circuit voltage (Voc) is proportional to the intrinsic resistivity [12]. Unfortunately, the high conductivity in BFO is a well-know problem, which is detrimental to the achievement of large Voc. Also, as the Voc is limited in micrometer or submicrometer scale of the films, it is potentially possible to obtain higher Voc in the bulk BFO materials, i.e. single crystals. However, in contrast to the intensively studied BFO films where the domain structure can be engineered by controlling the structure of substrate [13], the reported as-grown BFO single crystals [14] generally show monodomain structures free of domain walls, making it difficult, if not impossible, to realize those novel physical effects originating from domain boundaries. Of particular interest for domain wall nanoelectronic applications are the charged domain walls because they are known to possess enhanced electrical conductivity [15, 16]. However, the electrical compatibility conditions for ferroelectric domain arrangement [17] suggest that their walls should be neutral to reduce the associated electrostatic energy. Therefore, the head-to-head and tail-to-tail configurations of spontaneous polarization in the domains adjacent to the wall, which are required for the wall to be charged, are observed very rarely. Special technological efforts are typically needed to prepare charged walls [15, 16, 18].


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As an important derivate of BFO, (1-x)BiFeO3-xPbTiO3 (BFPT) solid solution system has been intensively studied due to the complicated composition-induced crystal and magnetic structural evolution including the existence of morphotropic phase boundary [19-30]. With the increasing concentrations of PbTiO3 (PT), BFPT shows a phase transformation from the rhombohedral R3c symmetry in pure BFO to a morphotropic phase boundary (MPB) with a mixture of rhombohedral or monoclinic and tetragonal phases around x=0.3, and then to the tetragonal P4mm phase. Interestingly, in our previous work the promising coexistence of ferroelectric and magnetic morphotropic phase boundaries was established in Dy-doped BFPT [31, 32], indicating a potential way to enhance the magnetoelectric coupling in single phase multiferroics. Up to now, most studies of BFPT based materials were carried out on polycrystalline samples, where the high dielectric loss inherited from its parent BFO component hinders the realization of expected outstanding multiferroic properties. It is also inconvenient or impossible to study the functionality of domain walls in polycrystalline samples. Therefore, the BFPT single crystals are highly desirable because they are able not only to improve electric performance due to low concentration of defects, but also are very suitable to investigate the domain walls related phenomena. In this work, BFPT single crystals with two typical crystal structures, i.e. rhombohedral and tetragonal, are grown using flux method, the desirable ferroelectric nano/micro-domain structures which include charged and uncharged domain walls are successfully realized and systematically analyzed by various techniques. 2. Experimental Section Crystal growth The growth of (1-x)BiFeO3-xPbTiO3 single crystals with several different concentrations of PT using high temperature solution method has been reported [33-37]. In this work similar flux


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method is used to grow BFPT single crystals with nominal composition of x = 0.2 and 0.5. Considering the fact that the MPB of the BF-PT system is located around x = 0.3, these compositions were chosen to investigate different structures across the MPB. A mixture of Bi2O3 and PbO, which is effective in dissolving the refractory oxides due to the high polarizability of Bi3+ and Pb2+ ions, was used as flux in the crystal growth. The constituent oxides of the solute, Bi2O3 and PbO form a self-flux which can prevent introduction of impurities in the grown crystals of BFPT. To avoid or diminish the composition segregation observed in crystals grown by other groups [33, 34, 37], the ratio of oxides in flux and the weight percentage of flux in the solution were carefully adjusted depending on the intended compositions. The oxides of the solute and the flux were thoroughly mixed and then placed into a Pt crucible with a volume of 100 mL. The high temperature solution was obtained at 1453 K and soaked for 10 hrs. Single crystals were gradually grown from saturated solution during the subsequent slow cooling process down to 1173 K at a rate of 1 K/h. Before naturally cooling down to room temperature with furnace, a slow cooling process with a rate of 1 K/h was added in the temperature range from (TC+50) K to (TC-50) K in order to diminish the internal stress related to lattice distortion around TC. Finally, the grown crystals were separated from the solidified flux by leaching in diluted nitric acid. Characterization Powder X-ray diffraction (XRD) study was performed on crushed single crystal powders using a Bruker D8 Advance Diffractometer. The Rietveld structural refinements were carried out using TOPAS Academic Software. The platelets with (001) orientation were polished for further characterizations. The temperature dependence of the dielectric constant was measured using a precision impedance analyzer (4294A, Agilent) from room temperature to 1100 K. The polar


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domain structures of the single crystal platelets were studied by three kinds of techniques: Polarized Light Microscopy (PLM, Olympus), HNO3 chemical etched patterns observed by Scanning Electron Microscop (SEM, FEI, Qunta FEG) and Piezoresponse Force Microscopy (PFM) on a modified commercial Atomic Force Microscope system in piezoresponse mode (AFM, Dimension Icon, NanoScope V, Bruker). 3. Results and Discussion 3.1 Crystal structure For the sake of convenience, the crystal samples with nominal compositions of x=0.2 and 0.5 are named as R and T, respectively. As shown in insets of Fig. 1(a), the R single crystals exhibit cubic-like morphology and a size of 2-3 mm. The T single crystals have a size of 2-5 mm with shinning and crack-free surfaces. All samples show dark color characteristic of BFO-based compounds. Fig. 1(a) shows the crushed crystal powders XRD patterns of different compositions. The R crystal XRD pattern corresponds to a main rhombohedral phase. As indicated by the arrows in Fig 1(a), two small shoulder-peaks are observed on both sides of the (110) peak, suggesting the admixture of the tetragonal phase. This means that the R crystals have composition close to MPB. The refinement results indicate the coexistence of dominant rhombohedral R3c phase and a trace amount of tetragonal P4mm phase. In T crystals the (100) and (110) peaks split into two distinct peaks and no observable splitting of (111) peak can be found. The refinement results confirm the formation of pure tetragonal phase with P4mm symmetry. The orientation of polished crystal (100) platelets investigated below is checked by XRD. As shown in Fig. 1(b) for both samples, only (100), (200) and (300) peaks are clearly present in the XRD patterns, verifying (100) orientation.


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Figure 1. XRD patterns of (a) powders and (b) (100) PC-orientated platelets of BFPT single crystals. R and T represent rhombohedral and tetragonal compositions, respectively. The insets in (a) and (b) show photographs of as-grown single crystals and (100) polished crystals, respectively. 3.2 Phase transitions and actual compositions In order to determine the ferroelectric Curie temperature, the variation of dielectric constant as a function of temperature was measured in the crystals, and the results are shown in Fig. 2. A dielectric peak is observed around 950 K in R sample, which can be related to phase transition from the ferroelectric rhombohedral phase to the paraelectric cubic phase upon heating. As indicated by arrow in Fig. 2(a), additional dielectric anomaly is found to appear around 750 K upon heating and disappears during the subsequent measurements upon cooling, as shown in the inset of Fig. 2(a). This indicates that it is a dielectric relaxation process probably related to the existence of mobile charges in samples and has nothing to do with phase transitions. As shown in Fig. 2(b), a sharp dielectric peak appears around 800 K for T single crystals, indicating the phase transition from the tetragonal to the cubic phase. The temperature TC shows a significant


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decrease when the structure changes from rhombohedral phase for R sample to pure tetragonal phase for T sample. An addition bump is also revealed in T sample around 700 K, which is probably related to similar dielectric relaxation process as observed in R sample.

Figure 2. Temperature dependences of dielectric constant of (a) rhombohedral and (b) tetragonal compositions. To determine the actual compositions of as-grown single crystals, the lattice parameters calculated using XRD refinement method and measured TC are compared with those of ceramic samples reported in the literature. For the R crystals the lattice parameters of the R3c phase a = 5.59 Å and c = 13.85 Å are found, which are consistent with the parameters reported for 0.8BF0.2PT ceramics based on XRD [38] and neutron diffraction [39] investigations. In addition, the TC 950 K of R crystals agrees with the reported value of 933 K [39] in 0.8BF-0.2PT ceramics, confirming that the actual composition of R samples is x  0.2. For the T crystals, the lattice parameters of the P4mm phase are a = 3.870 Å and c = 4.305 Å. These parameters and c/a ratio of 1.11 are located between corresponding parameters of 0.4BF-0.6PT and 0.3BF-0.7PT ceramics reported by Zhu et al [19]. Also, the TC  800 K of the T crystals is close to the value of TC = 797-810 K reported for 0.3BF-0.7PT ceramics [39]. The comparison indicates that the


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concentration of PT in as-grown T crystals is x  0.6, i.e. slightly larger than the nominal one. 3.3 Domain Structure Tetragonal BFPT crystals The as-grown T crystals show dark appearance characteristic of perovskites containing iron ions. In order to investigate the ferroelastic domains using PLM, the T sample was polished along (100)PC plane to a thickness of 65 μm. It appeared to be semi-transparent for the light after that. The PLM images are shown in Fig. 3. The whole crystal becomes brightest when the angle between PC directions and polarizers orientation is 45o (shown in Fig. 3(a)) and appears to be in extinction when the angle changes to 0o (shown in Fig. 3(b)), which is consistent with P4mm symmetry determined by XRD. Clear ferroelastic domain bands are revealed with the width of 10-65 μm as shown in Fig. 3(a). The crystal can be roughly divided into three regions, marked as Ⅰ, Ⅱ and Ⅲ. Region III consists of alternating 90o a-a domains with the walls along direction. In Fig. 3(c) the sketch of domain configuration in this region is presented and directions of optical indicatrices and spontaneous polarizations in individual domains are indicated. In regions I and II the domain bands with the boundaries along directions are observed. The nature of these bands will be discussed below. It is also found in Fig. 3(a) that many comparatively small dark areas exist in all regions which remain dark at any position of the polarizes. These regions can be related to c-domains propagating through the whole crystal plate from top to bottom. In addition, in region III, two extinction angles (θe) with difference of Δθe ~11o are observed in neighboring domain bands as seen in Figs. 3(d, e). This behavior can be ascribed to the following two reasons: ⅰ), the 90o twining in tetragonal crystals can give rise to the appearance of Δθe due to the tetragonal lattice distortion, which can be calculated by Δθe = 90-2×tan-1(a/c) ≈ 6o.[36] ⅱ), the surface canting formed during polishing process could further


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increase Δθe, contributing the left Δθe ≈ 5o. Actually, such slightly inclined surface is even helpful: it helps the observation in region III of the homogeneous 90o domains with different direction of indicatrix. Interestingly, in most areas of regions Ⅰ and II no complete extinction is observed though the contrast becomes weak as shown in Fig. 3(b). This is related to the complex hierarchical domain structures and will be discussed below.

Figure 3. The polarized light microscopy images at 45o position (a) and 0o position (b) and the sketch of domain structure (c) of T single crystal; ((d) and (e) the extinction positions of different domains in region Ⅲ. The ellipse marked in (a), (c), (d) and (e) represents the same area for location purpose. The domains are numbered in panels (d) and (e) to help distinguishing them. As the PLM technique is only sensitive to non-180o (ferroelastic) domain walls, the etching method is applied to T sample in order to further explore the type of domain structure observed in regions Ⅰ and II of Fig. 3. The SEM images of etched (001)PC surface are shown in Fig. 4. The domain bands similar to those observed with PLM are clearly demonstrated in Fig. 4(a) with alternating dark and bright contrast, marked as A and B, respectively. The band width is in the range of several dozens of micrometers, which is well consistent with the domains observed by PLM in Fig. 3(a). Moreover, under magnification as high as 40,000 times (shown in Fig. 4(b)), fine domain structure is clearly revealed within the domain band B, i.e. the band which looks


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brighter at lower magnification. The dark and white fine lamellar domains with the width of dozens of nanometers alternate, while the neighboring dark band A shows a uniform contrast. The overall domain structure is sketched in Fig. 4c. Band B formed of 90o a-c domains demonstrates a different etching rate and resulting difference in contrast. The fine dark lamellar domains within band B generally show the same contrast, i.e. the same etching rate as that of the neighboring a domains in region A, indicating that they are a domains. Therefore, alternating fine lamellar a-c domain structure is established in band B. The fine domain structure in band A shown in Fig. 4c is constructed based on the domain pattern in neighboring region B and on the mechanical and electrostatic compatibility conditions. The walls between the fine 90o domains of band B as well as boundaries of the bands are expected to be inclined by the angle of 45o with respect to the plane of drawing, i.e. (001) plane, while the walls of the fine 90o domains of band A should be perpendicular to this plane. As explained previously, extinction angles vary slightly in such neighboring fine 90o twining domain bands. Those fine domains cannot be distinguished separately and only act as a whole due to their small size far below the visible light wave length. As a result, no complete extinction can be seen in PLM images, as observed in Fig. 3(b). It is also worth noting that in band B numerous rounded domains of size about dozens to hundreds nanometers are embedded within fine lamellar domains. Those domains with darkest contrast are presumably c domains with polarizations opposite to the fine white lamellar c domains and form 180o c domains, as sketched in Fig. 4(c).


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Figure 4. SEM images of T crystal demonstrating the topography of etched (001) surface with different magnification: (a) 1,300 times, (b) 40,000 times and (c) sketch of domain structure derived from the analysis of images. The labels “⊙” and “⊕” in panel c indicate the polarization of c domains pointing outward and inward, respectively. As both PLM and etching methods are not sensitive to the 180o a domain structures, the PFM technique is then applied to the (001) surface of T crystal. Fig. 5 shows the obtained results. Again, clear domain bands along direction are observed in both in-plane and out-of-plane images. The bands with strong out-of plane (OOP) amplitude always reveal weak in-plane (IP) amplitude and vice versa. The domain size distributions are consistent with the results obtained by PLM and etching technique. As shown in the sketch of Fig. 5(g), the domain structure established with PFM exactly corresponds to the structure in Fig. 4(c) found with etching. In particular, the OOP amplitude component of a domains should be zero and, accordingly, no OOP response is observed in bands marked by A consisting of a domains. In band bands marked by B the OOP amplitude demonstrates fine lamellar patterns of sub-domains with the walls perpendicular to [100], i.e. to the band boundary as it is shown in enlarged Fig. 5(d). In IP images c domains are expected to possess zero amplitude, while the amplitude and phase of a domains can vary depending on the angle between their polarization and the cantilever direction.


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Therefore, IP amplitude images of A bands consisting only of a domains look brighter than B bands in which a and c domains alternate. Moreover, numerous small rounded regions distributed inside A bands in IP images should be classified as 180o a domains due to their irregular shape. The largest of such domains have the size of ~ 1 m, but many domains with the size of ~ 100 nm and smaller are visible. Their 180o direction with respect to host lamellar a domains is evidently prescribed by the condition of mechanical compatibility. However, the electrostatic compatibility condition cannot be satisfied: the walls nonparallel to the polarization must be charged due to head-to-head or tail -to-tail polarization arrangement. These fine rounded 180o domains are evidently similar to those observed in B bands with SEM technique (see Fig. 4b). The existence of energetically unfavorable charged walls can be explained by comparatively large conductivity characteristic of BFO and other Fe-containing perovskites: the charge of domain walls can be effectively neutralized by carriers responsible for electric conduction. On the other hand, a single-domain state observed in pure BFO [14] is not formed in BFPT because of smaller conductivity which appears to be not large enough to screen the depolarizing field in the course of cooling the crystal through the Curie temperature. A multi-domain state is developed in BFPT as a result.


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Figure 5. The piezoresponse force microscopy images of T single crystal: (a) topography; (b) out-of-plane amplitude; (c) out-of-plane phase; (d) enlarged out-of-plane amplitude and phase images of selected rectangular area marked in panels (b, c); (e) in-plane amplitude and (f) inplane phase; (g) the sketch of domain structure derived from the analysis of images. The labels “⊙”in panel g indicates the polarization perpendicular to the crystal surface, OOP and IP represent out-of-plane and in-plane, respectively. Rhombohedral BFPT single crystals The rhombohedra R single crystals containing 80% of iron ions on B sites are still of weak transparency even for the sample as thin as 30 μm. Besides, for (001)pc crystal plate the etching rate is expected to be the same for all domains of rhombohedral symmetry and they should not be distinguishable in SEM images. Therefore, the PLM and etching techniques are not applicable to this composition and the PFM method is chosen for investigation. As shown in Fig. 6(a, i), measurements are carried out on the (100)PC surface of as-grown R crystal. The PFM tip scans horizontally as indicated by thick arrow in Fig. 6(a, i). The crystallographic PC direction marked by double-headed arrow is along the scanning direction. A bundle of stripes is observed in the topography image with peak-to-valley roughness around 10 nanometers, as shown in Fig. 6(a, ⅰ). The stripe structures with alternating dark and bright domains along PC are clearly revealed by phase image in Fig. 6 (a, ⅲ). Similar domain structures have been also observed in rhombohedral BFO films as well as ceramics [10, 40-43]. However, in contrast to the stripe dominated OOP piezoresponse, the IP phase image in Fig. 6 (a, v) shows uniform bright contrast, regardless of many small dark spots. Actually, those spots are also observed in OOP images, making the strip domain boundaries very ragged. Generally, the domain boundaries revealed in phase image are well consistent with the dark lines with zero piezoresponse in amplitude image


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shown in Figs. 6(a, ⅱ and ⅳ). The domain structure is further investigated by rotating the sample. After rotation by 90o, the stripe OOP structures in Fig. 6 (b, ii and iii) are generally the same, regardless of the intensity variation between different measurements. However, the IP phase image changes significantly from uniform white contrast to alternating white and dark stripes with domain walls along PC direction, as shown in Fig. 6(b, v). Such strips are also very obvious in IP amplitude image in Fig. 6(b, iv), the IP strip structure being consistent with the OOP strips. In addition, the small size spots and regions extended along [100] direction are widely distributed in all piezoresponse images, resulting in complex domain structure. For the sake of convenience, two types of such small domains are chosen for discussion and marked by circles and squares, respectively. Rotation by 90o leads to no change in the OOP piezoresponse of those spot domains. However, the dark IP spots turn to bright spots after rotation within dark strip as marked in the figure by squares, while keeping dark contrast within bright strip as marked by circles.

Figure 6. The piezoresponse force microscopy images of (001) surface in R single crystal: (ⅰ) topography, (ⅱ) out-of-plane amplitude and (ⅲ) phase images, (ⅳ) in-plane amplitude and (ⅴ) inplane phase images; (a) before and (b) after rotating the crystal by 90o. The same numbers indicate in panels (ⅴ a,b ) the same domains.


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The domain structure in rhombohedral single crystal is constructed based on the IP/OOP piezoresponse and domain wall orientation in Fig. 7(a). Four polarization pairs are marked by P1± , P2± , P3± , P4± , where the “+” represents an upward polarization, while “–” represents a downward polarization. Any two of eight polarizations can form twin domains. The domain boundary between twin domains can be deduced from mechanical and electrostatic compatibility conditions [17, 44]. The strip alternating dark/bright OOP piezoresponse, uniform bright IP piezoresponse and (100)PC orientated domain boundaries observed in Fig. 6(a) can arise from the following four possible polarization pairs: P3+/ P2−, P3−/ P2+, P1+/ P4− and P1−/ P4+. One pair, P3−/ P2+ , is sketched as an example in Fig. 7(b). The resultant 109o twin domains form (100)PC domain walls, as marked by the shadow plane. As the PFM measurements are carried out on (001)PC surface, the pair polarizations and domain walls are projected on (001)PC surface in order to interpret the PFM results conveniently, as shown in Fig. 7(c). The projections of such polarization pairs on (001)PC surface are indicated by inclined arrows pointing to top-left or topright, while the projection of corresponding domain walls are marked by dash lines along PC direction. Their corresponding OOP/IP components are also given in Fig. 7(c), which can provide the piezoresponse observed in Fig. 6(a). After rotating the sample by 90o and keeping the coordinate system unchanged, the original polarization pairs P3− / P2+ in Fig. 7(b) change to P2− / P1+ in Fig. 7(d). As sketched in Figs. 7(c, e), the alternating OOP components remain unchanged, while the original uniform IP image transforms to alternating strips. This is exactly consistent with the PFM results in Fig. 6. Therefore, alternating dark and white strips with the width of about 1-3 m are the domains separated by uncharged 109o walls. They are theoretically expected to be flat and parallel to {100}pc planes but actually appear to be slightly curved in the images, evidently due to the crosstalk with the topography features. Finally, the


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small size spots widely distributed in the whole investigated area can be identified as 180o domains existing inside strip domains. Before rotation, in Fig. 7(c), the 180o domains in each strip lead to the reverse of IP component and resultant dark domains in contrast to bright domains of matrix, as observed in Fig. 6a(ⅴ). After rotating the sample by 90o, the same reverse happens in 180o domains. As a result, the 180o dark domains are distributed in white strip domains and vice versa. This is in good agreement with experiment results. All the dark areas seen in Fig. 6(a, ⅴ) i.e. in the IP image corresponding to the scan in the direction of 109o domain walls are 180o domains. Most of them have the size of ~0.5-1 m.

Figure 7. (a) Schematics of eight equivalent polarization directions in the rhombohedral phase of R crystal, (b, d) 71o pair domains and domain walls and (c, e) their projections on (001)PC plane before (b, d) and after (d, e) rotation by 90o. As mentioned at the beginning, many promising novel functionalities have recently emerged


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for BFO and are found to be closely related to the existence of domain walls. In contrast to the reported monodomain BFO single crystals [14], the introduction of ~20% per cent PT into BFO results in the formation of multiple 109o domains in as-grown single crystals of the same rhombohedral symmetry, providing a valuable chance to further explore the domain boundaries related phenomena in BFO-based bulk materials. 4. Conclusions Single crystals of the multiferroic (1-x)BiFeO3-xPbTiO3 solid solution were grown using the high temperature solution method. Structural characterization by X-ray diffraction confirmed perovskite rhombohedral R3c phase or tetragonal P4mm phase in different samples depending on the PbTiO3 concentration in solid solution. Real compositions have been established as x ≈ 0.2 for rhombohedral crystals and x ≈ 0.6 for tetragonal crystals, based on the comparison of the lattice parameters and TC for as-grown crystals and BF-PT ceramic samples. The domain structure of crystals was investigated by various techniques including polarized light microscopy, SEM of etched surfaces and piezoresponse force microscopy. It was analyzed based on mechanical and electrostatic compatibility conditions. In tetragonal BFPT crystal (001) plates, a complex hierarchical domain structure is revealed (as in Regions Ⅰ and Ⅱ in Fig. 3) where thin (10 – 300 nm) lamellar domains separated by planar 90o walls are arranged into alternating a-a and a-c domain bands having the width of 10 – 100 m. Fine rounded 180o domains of size below several hundred nanometers are distributed among larger lamella 90o domains. Segments of domain walls of these 180o domains not parallel to spontaneous polarization are charged. In some other regions of tetragonal plates (as in Regions Ⅲ in Fig. 3) much thicker 90o domains (dozens of micrometers) are observed. In the rhombohedral composition, major stripe-type 109o twin domains with domain walls parallel to {100}PC crystallographic planes are realized with


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domain width around several micrometers. Rounded 180o domains are also distributed widely in all twin domains. Successful fabrication of high-quality single crystals with the desirable structures of charged and uncharged domain walls can enhance the multiferroic performance and also open a door to explore the promising domain boundaries related phenomena.

AUTHOR INFORMATION Corresponding Author *Email:

[email protected]

(J.Z.),

*E-mail:

[email protected]

(Z.-G.Y.).

*E-mail:

[email protected] (W.R.). Author Contributions All authors contributed to the discussion and writing of the manuscript. The final version was approved by all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Grant No. 51602243), China Postdoctoral Science Foundation (Grant No. 2016M592786), Natural Science Foundation of Shaanxi Province (Grant No. 2017JQ5073), Shaanxi Province Postdoctoral Science Foundation (Grant No. 2017BSHEDZZ01), the Fundamental Research Funds for the Central Universities (Grant No. xjj2017061), the “111 Project” of China (Grant No. B14040), the U.S. Office of Naval Research (Grants No.N00014-12-1-1045 and N00014-16-1-3106) and the


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For Table of Contents Use Only Single Crystal Growth and Hierarchical Ferroelectric Domain Structure of (1-x)BiFeO3-xPbTiO3 Solid Solutions Jian Zhuang,*,† Alexei A. Bokov,‡ Nan Zhang,† Jie Zhang,† Jinyan Zhao,† Shuming Yang,# Wei Ren,*,† and Zuo-Guang Ye*,‡, † Table of Contents Graphic:

Synopsis: Single crystals of (1-x)BiFeO3-xPbTiO3 solid solution with rhombohedral and tetragonal phase were grown by the self-flux method and characterized for their hierarchical ferroelectric domain structures, suggesting the potential applications on domain wall nanoelectronics.


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BiFeO3-xPbTiO3 Solid Solutions

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