Communication pubs.acs.org/cm
RbBiNb2O7: A New Lead-Free High‑Tc Ferroelectric Bao-Wen Li,† Minoru Osada,*,†,‡ Tadashi C. Ozawa,†,‡ and Takayoshi Sasaki†,‡ †
International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, Japan CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
‡
S Supporting Information *
KEYWORDS: layered perovskites, lead-free ferroelectric, high Curie temperature
F
characterization of the dielectric, ferroelectric, and piezoelectric properties of RbBiNb2O7. It is demonstrated that RbBiNb2O7 is a high-TC ferroelectric with TC = 945 °C, room-temperature remnant polarization Pr = ∼10 μC/cm2, and piezoelectric coefficient d33 = 12 pC/N. RbBiNb2O7 ceramic was synthesized by a conventional solidstate reaction. Figure 1a depicts the X-ray diffraction data measured at room temperature for RbBiNb2O7. A small trace of BiNbO4 (∼4 mass%) was detected as an impurity phase. BiNbO4 is antiferroelectric at room temperature but becomes ferroelectric at 360 °C and paraelectric at 570 °C.13 All the
erroelectric materials, which have a reversible spontaneous polarization (Ps) and generate an electric potential when subjected to a mechanical stress (piezoelectricity), have applications in nonvolatile random access memory devices and microelectromechanical systems. Within this area, the development of high piezoelectric sensitivity in lead-free ferroelectric materials remains to date as a primary scientific challenge, driven by the toxicity of lead oxide and nowadays directives for environmental protection.1 Despite extensive research efforts, there are only a limited number of lead-free ferroelectric materials with a Curie temperature TC sufficiently high and the performance good enough for piezoelectric applications. The most promising of these materials are the KNbO3−NaNbO3 ceramics2 and modified versions.3 For memory applications, where good piezoelectric properties are not required, there are more suitable lead-free options such as BiMO3 (M = Fe, Al, Ga),4 SrBi2Ta2O9,5 Bi4−xLaxTi3O12,6 and a few others. Now, a key challenge in this field is the development of high-TC ferroelectric/piezoelectric materials applicable for high-temperature sensing devices (preferably operated at T > 500 °C). In the search for such high-temperature ferroelectrics, recent studies have focused on layered perovskites with a high-TC value.7 Layered perovskites display an amazing variety of both structural and physical properties by changing the A and B cations as well as the number of perovskite units (n). Ferroelectricity is ubiquitous in the ABO3 perovskite titanates, niobates, and tantalates. However, the only layered perovskites to display a ferroelectric transition have been the Aurivillius compounds,8 while a predicted ferroelectricity has yet to be confirmed experimentally in other layered perovskites with Ruddlesden−Popper9 and Dion−Jacobson structures.10 Our principal concern is that the solution can be in the Dion−Jacobson-type layered perovskite RbBiNb2O7, which has been neither characterized nor documented in the ferroelectric database. The presently known candidate for Dion−Jacobsonbased ferroelectric is the double-layered compound CsBiNb2O7. Recent theoretical and structural studies have revealed that CsBiNb2O7 crystallizes in a polar space group P21am at room temperature.11,12 Nevertheless, from electrical measurements, CsBiNb2O7 did not display ferroelectricity.12 Therefore, we then set RbBiNb2O7 isostructural with CsBiNb2O7 as the target in the search for a new lead-free high-TC ferroelectric since smaller interlayer metal ions in layered perovskites often cause new ordered structures. Here, we present the first © 2012 American Chemical Society
Figure 1. (a) Diffraction pattern for RbBiNb2O7 at room temperature. Impurity BiNbO4 (∼4 mass %) are represented with the asterisk. (b) Plausible crystal structure of RbBiNb2O7 phase with b−a and c−a projections. Dashed lines perpendicular to the b- and a-axes represent glide and mirror planes, respectively. Received: April 27, 2012 Revised: August 1, 2012 Published: August 2, 2012 3111
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Communication
phase transition points are far below the TC observed in RbBiNb2O7. Furthermore, there are no detectable transition points corresponding to BiNbO4 phase in the εr−T profiles (Figure 2a). Therefore, we may conclude that the BiNbO4
Such lattice instabilities lead naturally to a polar orthorhombic ground state, which allows a ferroelectric polarization to develop along the c-axis. However, the ordinary fired ceramic RbBiNb2O7 is highly textured along the a-axis. Such a textured feature strongly suppresses the realization of saturated polarization because of the high coercive filed, above which the dielectric breakdown occurs. In fact, from Berry-phase calculations of the polarization, we find that RbBiNb2O7 should have a polarization Ps = 25 μC/cm2, which is comparable to the polarization of Aurivillius-type ferroelectric SrBi2Ta2O9.14 Figure 2a shows the temperature dependence of the dielectric constant εr in RbBiNb2O7. It exhibited a ferroelectric transition with TC of 945 °C, a behavior being in contrast to that of CsBiNb2O7. This value is comparable to that (943 °C) of Aurivillius-type CaBi2Nb2O9,14 which has been reported as the highest TC material among layered perovskites. The broadening of εr−T profiles near TC is possibly caused by cation order−disorder, which has been observed in several Aurivillius compounds.8,16 However, the dielectric peak occurs at the same temperature for all three frequencies, suggesting the absence of relaxor-type behavior in RbBiNb2O7. From Raman spectroscopy, we also found that the lowest-frequency phonon exhibited a gradual softening toward the transition point and disappeared at ∼945 °C (Figure 2b). This is a signature for a typical displacive-type ferroelectric transition,17 which is in accordance with a linear εr−T relationship (T > TC) followed by the Curie−Weiss law. In addition, the extrapolated value of the squared frequency is still finite at TC. By comparison with the observations in SrBi2Ta2O9 and other Aurivillius compounds, such a relatively large finite value may be ascribed to the first order phase transition and the coupling between the soft mode and other physical parameters (e.g., strain).18 P−E measurements (the inset of Figure 2a) indicated ferroelectric polarization switching characteristics with a remanent polarization of 10 μC/cm2. This compound also showed a stable piezoelectric response up to high temperatures (Figure 2c). The room temperature piezoelectric coefficient (d33 =12 pC/ N) is rather modest compared to simple perovskites such as Pb(Zr, Ti)O3 and NaNbO3,2 but this piezoelectric response persisted up to 800 °C. These results suggest that RbBiNb2O7 is a promising candidate for use in lead-free high-T C ferroelectric/piezoelectric devices. Figure 3 shows the comparison of Ps and d33 values among developed RbBiNb2O7 and previously reported lead-free perovskites as a function of TC. As is well-documented in review literature,23 simple perovskites have rather high Ps values but with low TC (generally below 500 °C). In that view, bismuth-containing layered perovskites possess high-TC values and thus have an advantage for high temperature applications. We also find some correlation between Ps and TC values in double-layered (n = 2) perovskites; Ps linearly scales well with TC. A similar trend is also observed in layered perovskites with n = 3−5, but the correlation is different from the n = 2 system, possibly due to the difference in the polarization structure. Another unique correlation is found in piezoelectric responses. In simple perovskites, high-TC ferroelectrics often cause rather modest piezoelectric responses; the d33 value scales inversely with the TC value. In contrast, bismuth-containing layered perovskites show a different trend (Figure 3b); the d33 values tend to decrease from n = 5 to n = 3, but surprisingly increase with TC from n = 3 to n = 2. Thus, the best balance occurs in double-layered Dion−Jacobson-type perovskites where the simultaneous improvement of Ps and d33 values is achieved in
Figure 2. (a) Temperature dependence of the dielectric constant εr. The inset shows room-temperature P−E hysteresis loop. (b) Variation of the lowest-frequency mode as a function of temperature. The inset shows low-frequency Raman spectra above and below TC. (c) Temperature dependence of the piezoelectric constant d33.
impurity phase has no disturbance on the phase transition behavior of RbBiNb2O7, probably owing to its too small trace. Although precise structure parameters still await further determination, the crystal structure of the main phase can be indexed in orthorhombic structure with unit cell parameters a = 11.232(4) Å, b = 5.393(2) Å, c = 5.463(1) Å. These parameters are quite close to those of CsBiNb2O7,11,12 probably implying the isostructural feature. On the basis of structural data of CsBiNb2O7, the plausible crystal structure and the local dipole analysis of RbBiNb2O7 are presented in Figure 1b. An orthorhombic distortion arising from off-center displacements of the A-site Bi ions would be expected due to the Bi 6s lone pair, together with cooperative NbO6 octahedral tilting. This mechanism of distortion may be analogous to that of the predicted ferroelectric CsBiNb2O7.11,12 3112
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high-TC ferroelectrics. These results again suggest the importance of double-layered Dion−Jacobson-type perovskites for the target of lead-free high-TC ferroelectrics/piezoelectrics, and Figure 3 offers a useful guideline for optimizing the Ps, TC, and d33 values.
ASSOCIATED CONTENT
S Supporting Information *
Experimental procedure, J−V profile, and frequency-dependent dielectric response. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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Figure 3. Comparison of (a) spontaneous polarization Ps and (b) piezoelectric constant d33 as a function of Curie temperature TC for lead-free piezoelectric materials. Layered perovskites8,15,19−22: n = 1 (Bi2WO6), n = 2 (SrBi2Ta2O9, SrBi2Nb2O9, CaBi2Ta2O9, CaBi2Nb2O9, Bi3TiNbO9, and RbBiNb2O7), n = 3 (Bi4Ti3O12), n = 4 (PbBi4Ti4O15, BaBi4Ti4O15, SrBi4Ti4O15, Na0.5Bi4.5Ti4O15, and K0.5Bi4.5Ti4O15), and n = 5 (Pb2Bi4Ti5O18, Ba2Bi4Ti5O18, and Sr2Bi4Ti5O18). The fitted lines are a guide for the eyes.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +81-29-854-9061. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by World Premier International Research Center Initiative (WPI Initiative in Materials Nanoarchitectonics), MEXT, CREST, JST, the Industrial Technology Research Grant Program, NEDO, and a Grantin-Aid for Scientific Research on the Innovative Area “Fusion Materials” (2206), MEXT, Japan. 3113
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