Communication pubs.acs.org/IC
Giant Polarization and High Temperature Monoclinic Phase in a Lead-Free Perovskite of Bi(Zn0.5Ti0.5)O3‑BiFeO3 Zhao Pan,† Jun Chen,*,†,‡ Runze Yu,‡ Hajime Yamamoto,‡ Yangchun Rong,† Lei Hu,† Qiang Li,† Kun Lin,† Li You,§ Kun Zhao,† Longlong Fan,† Yang Ren,∥ Kenichi Kato,⊥ Masaki Azuma,‡ and Xianran Xing† †
Department of Physical Chemistry and §State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China ‡ Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan ∥ X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan S Supporting Information *
distorted perovskite structure, there are some candidates that exhibit polar structure, such as tetragonal structures of BZT, BiCoO3, and Bi(Zn0.5V0.5)O3 and rhombohedral structures of BiFeO3 and BiAlO3.6−8 BZT, exhibiting an isostructural property with PbTiO3 (PT; space group P4mm), could be a new lead-free tetragonal polar component for the construction of MPB.2 The first study of BZT was performed to enhance c/a of PT by chemical substitution of BZT.9 Furthermore, BZT consists of ions with d10 (Zn2+) and d0 (Ti4+) electron configurations, which could provide an insulating property to withstand high electric field loading. Here, the present study designs a new ternary system of (1 − x)Bi(Zn0.5Ti0.5)O3-xBiFeO3 that is constructed by tetragonal BZT and rhombohedral BiFeO3. It exhibits intriguing properties of giant spontaneous polarization, unusually enhanced tetragonality, and high TC. The MPB consists of tetragonal and monoclinic phases near the composition of x = 0.5. Particularly important is that a single monoclinic phase (x = 0.6) has been observed over a wide temperature range. The present study indicates that new lead-free piezoelectric candidates could be found in bismuth-based perovskites. Details on sample preparation, synchrotron X-ray diffraction (SXRD), and transmission electron microscopy are provided in the Supporting Information. The crystal structures for all investigated compositions have been determined by the room temperature SXRD patterns (Figure S1). With chemical substitution of BiFeO3, (1 − x)BZT-xBF transforms from the tetragonal phase (Figure 1a) to mixed phases of tetragonal and monoclinic and finally to a single monoclinic phase (Figure 1b). For compositions of x = 0.1−0.4, the tetragonal phase can be observed without any trace of a second phase (Figure S1). It is interesting to observe that the tetragonality (c/a) is unusually enhanced by chemical substitution of BiFeO3. c/a increases from 1.21 of BZT to 1.228 of 0.6BZT-0.4BF (Figure 1c). c/a of 0.6BZT-0.4BF is much larger than that of PT (c/a = 1.06) and most lead-based compounds. It is comparable with PbVO3 (c/a = 1.229) and BiCoO3 (c/a = 1.267), which have been reported to have giant c/a.10,11 The large tetragonal distortion leads to a
ABSTRACT: Lead-free piezoelectrics have attracted increasing attention because of the awareness of lead toxicity to the environment. Here, a new bismuth-based lead-free perovskite, (1 − x)Bi(Zn0.5Ti0.5)O3-xBiFeO3, has been synthesized via a high-pressure and high-temperature method. It exhibits interesting properties of giant polarization, morphotropic phase boundary (MPB), and monoclinic phase. In particular, large tetragonality (c/a = 1.228) and giant spontaneous polarization of 110 μC/cm2 has been obtained in 0.6 Bi(Zn0.5Ti0.5)O3-0.4BiFeO3, which is much higher than most available lead-free materials and conventional Pb(Zr,Ti)O3. MPB is clearly identified to be constituted of tetragonal and monoclinic phases at x = 0.5. Notably, a single monoclinic phase has been observed at x = 0.6, which exhibits an intriguing hightemperature property. The present results are helpful to explore new lead-free MPB systems in bismuth-based compounds.
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n the past decade, great progress has been made in lead-free piezoelectric materials because of increasing environmental concerns. Most of the available lead-free materials are focused on perovskite-type compounds of BaTiO3, Bi0.5Na0.5TiO3, and K0.5Na0.5NbO3 and their related solid solutions.1 New lead-free candidates are demanded in order to eliminate the unsatisfied properties such as low Curie temperature (TC) for BaTiO3 or low depolarization temperature (Td) for Bi0.5Na0.5TiO3. Bismuth has a 6s2 stereochemically active lone-pair electron configuration similar to that of lead, which could bring strong hybridization with oxygen and thus produce large polarization and high TC. Novel properties could be expected in bismuth-based compounds, such as the strong polarity in Bi(Zn0.5Ti0.5)O3 (BZT), multiferroic in BiFeO3, and the good photovoltaic effect in PbTiO3-Bi(Ni2/3Nb1/3)O3.2−4 For the design of new lead-free candidates, it is very important to construct a morphotropic phase boundary (MPB), which mainly consists of tetragonal and rhombohedral phases like the conventional Pb(Zr,Ti)O3.5 For the bismuth-based perovskites, even though most of them exhibit nonpolar GdFeO3-type © XXXX American Chemical Society
Received: July 16, 2016
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DOI: 10.1021/acs.inorgchem.6b01661 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
structural information for those MPBs with strongly overlapped peaks. In the present BZT-BF, the phase construction of MPB can be well distinguished. It reveals direct experimental evidence for linkage of the monoclinic phase to the tetragonal and rhombohedral phases. With further increasing content of BiFeO3, the fraction of the monoclinic phase increases, while the tetragonal one decreases. Finally, a single monoclinic phase emerges at x = 0.6. The monoclinic phase can be well determined by both SXRD structure refinement and electron diffraction. The structure of 0.4BZT-0.6BF can be well refined according to the 2a × √2a × √2a unit cell with the space group of Cc (Figure 2a). The
Figure 1. Crystal structures of the (a) tetragonal (x = 0.1−0.4) and (b) monoclinic (x = 0.6) phases. (c) c/a ratio. (d) Calculated spontaneous polarization of (1 − x)Bi(Zn0.5Ti0.5)O3-xBiFeO3.
pyramidal rather than an octahedral coordination in the tetragonal phase (Figure 1a). Generally, chemical substitutions lead to reduced c/a for both PT and BZT, such as in PTBi(Mg0.5Ti0.5)O3, PT-Bi(Ni0.5Ti0.5)O3, BZT-BaTiO3, BZT(Bi1/2Sr1/2)(Zn1/2Nb1/2)O3, and Bi(Zn0.5Ti0.3Mn0.2)O3; thus, other properties of T C and polarization are generally reduced.12−16 The abnormal enhancement of c/a in the present (1 − x)BZT-xBF ternary system could be attributed to the enhanced hybridization between the A/B site and oxygen with the addition of BiFeO3. A similar phenomenon has also been observed in the PT-BF system, where c/a increases with the introduction of BiFeO3.17 It is known that large c/a has a close relationship with the ferroelectric spontaneous polarization (PS). Large c/a means an enhanced PS. Herein, the PS value of BZT-BF can be estimated from the refined atomic positions assuming a point charge model. It increases from 104 μC/cm2 for BZT (x = 0) to as large as 110 μC/cm2 for x = 0.4 (Figure 1d). The present compound of BZT-BF exhibits a giant ferroelectric polarization property, which is much higher than those of most representative piezoelectrics, such as simple perovskites of PT (59 μC/cm2)10 and BiFeO3 (60 μC/cm2),3 or MPB systems of PbZr0.52Ti0.48O3 (54 μC/cm2) and PT-BiScO3 (40 μC/cm2),18,19 lead-free systems of Bi 0.5 Na 0.5 TiO 3 -BaTiO 3 (41 μC/cm 2 ) and K0.5Na0.5NbO3 (20 μC/cm2).20,21 It is comparable with the strong polar compounds of PbVO3 (101 μC/cm2)7 and BiCoO3 (120 μC/cm2).6 Furthermore, the present lead-free composition of BZT-BF could have high TC, according to the Landau theory,10 in which TC can be well quantitatively correlated to PS with the relationship of TC = αPS2. If taking PT as the reference for the calculation (PS = 59 μC/cm2 and TC = 763 K),10 the TC of 0.6BZT-0.4BF could be approximately 2000 K. The present BZT-BF is expected to be a promising high-TC candidate of leadfree piezoelectric materials. Particularly, a monoclinic phase starts to appear with the further substitution of BiFeO3 (x > 0.4). The determination of the monoclinic phase will be discussed in the following paragraph. The MPB composition can be clearly identified at x = 0.5 in which the monoclinic phase coexists with the tetragonal phase (Figures S1 and S3). It has been known that most MPBs in lead- or lead-free-based systems have extremely overlapped peaks of two phases, such as in the MPB of Pb(Zr,Ti)O3 and Bi0.5Na0.5TiO3-BaTiO3.18,20 It is difficult to reveal the actual
Figure 2. (a) Rietveld full profile refinement of SXRD patterns of 0.4BZT-0.6BF at room temperature. Observed (red, + ), calculated (blue line), and their difference (bottom line) are shown. Bragg reflection positions are indicated by the green ticks. (b-d) The SAED patterns of 0.4BZT-0.6BF viewed along the [001], [010], and [1̅01] zone axes, respectively.
detailed refined structural parameters are listed in Supplementary Table S2. The doubling of a axis can be observed through the crystal structure illustration (Figure 1b). The SAED patterns of 0.4BZT-0.6BF at room temperature have further confirmed the monoclinic character (Figure 2b-2d). The appearance of superlattice reflections indexed as (110) and (11̅0) viewing along the [001] zone axis indicates the doubled a axis of perovskite cell (Figure 2b). In perovskites, the monoclinic structure is attributed to the structural distortion in related to the BO6 octahedral tilting from any one of the following types: a−a−c0, a−a−c−, and a−b−c−, which result in the space groups of Cm, Cc, and P1, respectively.22 Here, the space group Cc gives the best refinement result,23 and the agreement Rwp factor is as low as 5.73% (Table S2). The other two possible models, such as Cm and P1, where the corresponding Rwp factor increases to 9.90% and 7.85%, respectively, indicating the worse refinement results.24 It needs to mention that the present monoclinic phase of 0.4BZT-0.6BF exhibits a giant spontaneous polarization PS of 92 μC/cm2 at room temperature. The value is much higher than those which exhibit the same monoclinic Cc phase, such as PbZr0.52Ti0.48O3 (PS = 39 μC/cm2) and lead-free Bi0.5Na0.5TiO3 (PS = 34 μC/cm2).25,26 It is well-known that the monoclinic phase of PbZr0.52Ti0.48O3 can only exist at low temperature or metastable after electrical poling.27,28 In order to study the temperature stability of the B
DOI: 10.1021/acs.inorgchem.6b01661 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
than typical lead-free materials, such as BaTiO3 (TC = 393 K), Bi0.5Na0.5TiO3 (TC = 610 K), K0.5Na0.5NbO3 (TC = 673 K) and their related solid solutions,1 and even higher than conventional Pb-based materials of Pb(Zr0.52Ti0.48)O3 (TC = 659 K)29 and the well-known high-temperature piezoelectric of 0.62PT0.38BiScO3 (TC = 723 K).19 The present results provide a new opportunity for exploring lead-free piezoelectric materials in bismuth-based compounds, especially for high-temperature applications. The present leadfree BZT-BF could be further studied via various chemical dopings or substitutions. Its perovskite can be stabilized via strain engineering by growing epitaxial thin films. Recently, epitaxial films of BZT-BF have been grown on (100)cSrRuO3// (100)SrTiO3 substrates.30 The present compounds of BZT-BF could be promising lead-free candidates. In conclusion, a new bismuth-based lead-free perovskite of BZT-BF has been synthesized by a high-pressure and hightemperature method. Its tetragonality is unusually enhanced by the chemical substitution of BiFeO3. BZT-BF exhibits a giant polarization not only for the tetragonal phase (110 μC/cm2 for x = 0.4) but also for the monoclinic one (92 μC/cm2 for x = 0.6). The MPB is well identified at x = 0.5, which consists of tetragonal and monoclinic phases. Particularly, a single monoclinic 2a × √2a × √2a structure with space group Cc has been observed for x = 0.6. The present BZT-BF could be a promising lead-free candidate for high-temperature applications.
present monoclinic phase in the 0.4BZT-0.6BF, temperature dependence of the SXRD patterns of monoclinic 0.4BZT-0.6BF was measured and shown in Figure 3a. It is interesting to observe
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01661. Room-temperature SXRD patterns of all investigated compositions and structure refinements of 0.6BZT-0.4BF, 0.5BZT-0.5BF, and 0.4BZT-0.6BF (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21322102, 21590793, 91422301, and 21231001), National Program for Support of Top-notch Young Professionals, the Program for Chang Jiang Young Scholars, and the Fundamental Research Funds for the Central Universities, China (Grant FRF-TP-14-012C1). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. The hightemperature synchrotron radiation experiments were performed at the BL44B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposals 2015B1127 and 2016A1060).
Figure 3. (a) The SXRD patterns, (b) lattice parameters of a, b, c and the monoclinic distortion angle β, and (c) the calculated spontaneous polarization of 0.4BZT-0.6BF as a function of temperature.
that the monoclinic phase can be maintained, and no phase transition occurs in the whole temperature range from 300 to 975 K. Further heating leads to the decomposition. The lattice parameters a, b, and c axes of 0.4BZT-0.6BF show slightly increasing tend with increasing temperature, while the monoclinic angle β is nearly temperature independent (Figure 3b). The calculated PS of 0.4BZT-0.6BF varies little with elevated temperature (Figure 3c). The present monoclinic phase shows a high temperature character. As a contrast, the monoclinic phase can be only observed at low temperature for PbZr0.52Ti0.48O3 (e.g., 20 K).27 For Bi(Zn0.5Ti0.3Mn0.2)O3, the monoclinic phase transforms to tetragonal one at 800 K.16 In addition, it suggests the present BZT-BF can be a promising high-TC lead-free piezoelectric material. As mentioned before, the TC for the monoclinic 0.4BZT-0.6BF could be about 1500 K if the TC is calculated with the reference of PT.10 The value is much higher
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DOI: 10.1021/acs.inorgchem.6b01661 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01661 Inorg. Chem. XXXX, XXX, XXX−XXX
