Chem. Mater. 2006, 18, 4987-4989
Bi2ZnTiO6: A Lead-Free Closed-Shell Polar Perovskite with a Calculated Ionic Polarization of 150 µC cm-2 Matthew R. Suchomel, Andrew M. Fogg, Mathieu Allix, Hongjun Niu, John B. Claridge, and Matthew J. Rosseinsky* Department of Chemistry, The UniVersity of LiVerpool, LiVerpool L69 7ZD, United Kingdom ReceiVed May 9, 2006 ReVised Manuscript ReceiVed August 4, 2006
Commercially important ferroelectric and piezoelectric materials, especially those based on Pb-containing perovskite oxides, are dominated by derivatives of the tetragonal perovskite PbTiO3. However, increasing regulatory pressures to remove lead from such commercial products, combined with the search for superior performance, has recently spurred considerable research into low Pb and Pb-free replacement materials. Bi based materials, which are quite environmentally benign in their oxide form, are a promising alternative to Pb due to the similar “inert-pair” 6s2 electronic configuration of the Pb2+ and Bi3+ cations. Recent theoretical calculations have predicted exceptional ferroelectric behavior for several Bi-based perovskites,1 and experimental results have demonstrated their potential as high transition temperature (TC) piezoelectrics with low Pb content.2 Unfortunately, it seems that bulk Bi perovskite chemistries possess less stability than their Pb based equivalents. While numerous perovskite compounds based on the latter have been reported, only two Bi based perovskites, BiFeO33 and the recently reported Bi2Mn4/3Ni2/3O6,4 are known to form under ambient conditions. Alternative synthetic routes have been pursued to affect the synthesis of these otherwise unstable Bi compounds. High pressure techniques have been successfully employed to synthesize bulk perovskites of BiB′O3 for B′ ) Al, Sc, Cr, Mn, Co, Ni, Ga, Y, and In.5-9 The mixed B site Bi-based bulk perovskites Bi2NiTiO6 and Bi2MnNiO6 have also been reported in high-pressure studies.10,11 Most of these com* To whom corresondence should be addressed. E-mail: m.j.rosseinsky@ liv.ac.uk.
(1) Baettig, P.; Schelle, C. F.; LeSar, R.; Waghmare, U. V.; Spaldin, N. A. Chem. Mater. 2005, 17, 1376. (2) Eitel, R. E.; Randall, C. A.; Shrout, T. R.; Rehrig, P. W.; Hackenberger, W.; Park, S.-E. Jpn. J. Appl. Phys., Part 1 2001, 40, 5999. (3) Kumar, M. M.; Palkar, V. R.; Srinivas, K.; Suryanarayana, S. V. Appl. Phys. Lett. 2000, 76, 2764. (4) Hughes, H.; Allix, M. M. B.; Bridges, C. A.; Claridge, J. B.; Kuang, X.; Niu, H.; Taylor, S.; Song, W.; Rosseinsky, M. J. J. Am. Chem. Soc. 2005, 127, 13790. (5) Tomashpol’skii, Y. Y.; Venevtsev, Y. N. Kristallografiya 1971, 16, 1037. (6) Ishiwata, S.; Azuma, M.; Takano, M.; Nishibori, E.; Takata, M.; Sakata, M.; Kato, K. J. Mater. Chem. 2002, 12, 3733. (7) Belik, A. A.; Wuernisha, T.; Kamiyama, T.; Mori, K.; Maie, M.; Nagai, T.; Matsui, Y.; Takayama-Muromachi, E. Chem. Mater. 2006, 18, 133. (8) Belik, A. A.; Stefanovich, S. Y.; Lazoryak, B. I.; TakayamaMuromachi, E. Chem. Mater. 2006, 18, 1964. (9) Belik, A. A.; Iikubo, S.; Kodama, K.; Igawa, N.; Shamoto, S.; Niitaka, S.; Azuma, M.; Shimakawa, Y.; Takano, M.; Izumi, F.; TakayamaMuromachi, E Chem. Mater. 2006, 18, 798.
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pounds are reported to possess a distorted perovskite structure similar to that of BiFeO3 or GdFeO3, a notable exception being the large tetragonal distortion displayed by BiCoO3.9 High-pressure phases have also been stabilized by epitaxial strain during thin film deposition by judicious selection of substrates and growth conditions, as demonstrated for BiFeO312 and BiMnO3.13 Given their lack of ambient pressure solid-state synthetic stability, many potential Bi-based perovskite compounds have also been explored as end members in pseudo-binary PbTiO3 solid solutions.2,14,15 Substitution of Bi-based perovskites into PbTiO3 solid solutions typically results in a reduced tetragonal distortion and lowered TC. However, it has recently been shown that the substitution of select Bibased chemistries results in a significant increase in the tetragonal distortion and TC of these solid solutions.16 This is most dramatically realized in the (1 - x)PbTiO3-(x)Bi2ZnTiO6 solid solution, where a tetragonal c/a ratio of ∼1.11 (compared to 1.06 for PbTiO3) and TC in excess of 700 °C is realized at the ambient pressure solid solution limit (x ) 0.40). Inspired by these bulk solid solution results and by the possibility of new tetragonal perovskites with the A site solely occupied by Bi, in the current study we have used high pressure synthesis methods to extend this aforementioned solid solution to its Bi-based end member. In this communication we report the synthesis and characterization of the highly tetragonal Bi2ZnTiO6 perovskite; a new Pb-free polar compound with a calculated ionic polarization of over 150 µC cm-2, the largest point-charge calculated polarization of any previously reported Pb or Bi-based perovskite. The tetragonal distortion of Bi2ZnTiO6, quantified by a c/a ratio of 1.211 is the highest reported for any d0 B site Pb or Bi based perovskite. Reaction of the component oxides for the Bi2ZnTiO6 composition at 900 °C and 6 GPa for 1 h in a Pt-lined Al2O3 crucible within a cylindrical graphite furnace in a Walkertype multianvil press affords a single phase perovskite product which is stable under ambient pressures. Full experimental details are found online as Supporting Information. Figure 1 displays the observed and calculated powder X-ray diffraction (XRD) patterns for Bi2ZnTiO6 at 23 °C. The diffraction data could be fitted with a P4mm unit cell similar to, but significantly more tetragonally distorted than, PbTiO3.17 The refined lattice parameters are a ) 3.82190(10) Inaguma, Y.; Katsumata, T. Ferroelectrics 2003, 286, 833. (11) Azuma, M.; Takata, K.; Saito, T.; Ishiwata, S.; Shimakawa, Y.; Takano, M. J. Am. Chem. Soc. 2005, 127, 8889. (12) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299, 1719. (13) dos Santos, A. F. M.; Cheetham, A. K.; Tian, W.; Pan, X.; Jia, Y.; Murphy, N. J.; Lettieri, J.; Schlom, D. G. Appl. Phys. Lett. 2004, 84, 91. (14) Alberta, E. F.; Bhalla, A. S.; Takenaka, T. Ferroelectr., Lett. Sect. 1999, 25, 45. (15) Suchomel, M. R.; Davies, P. K. J. Appl. Phys. 2004, 96, 4405. (16) Suchomel, M. R.; Davies, P. K. Appl. Phys. Lett. 2005, 86, 262905. (17) Glazer, A. M.; Mabud, S. A. Acta Crystallogr. 1978, 1065.
10.1021/cm061085r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/15/2006
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Figure 1. Rietveld refinement of powder XRD data from Bi2ZnTiO6 at room temperature. Space group P4mm. a ) 3.82190(3) Å, c ) 4.62803(7) Å, and V ) 67.601(1) Å3. The origin along z is fixed at the Bi site; Bi at 1a 0,0,0, U11 ) 0.0595(3) Å2, U33 ) 0.0369(5) Å2, Zn (50%), Ti (50%) at 1b 1/2,1/2,0.5662(5), U ) 0.0239(7) Å2, O1 at 1b 1/2,1/2,0.184(2), U ) 0.075(4) Å2, O2 at 2c 1/2,0,0.692(1), U ) 0.036(2) Å2, Rwp ) 7.78%, R(F2) ) 4.84%, R(F) ) 2.88% for 35 reflections and 8 structural parameters. χ2 ) 1.14.
(3) Å and c ) 4.62803(7) Å. No additional reflections stemming from impurities are observed. Refinement of anisotropic peak broadening was required, and the final goodness of fit parameter χ2 at room temperature is 1.14 compared with the structure-independent Le Bail fit of 1.12, indicating the correctness of the proposed structural model. The caption of Figure 1 gives further details of the Rietveld refinement. The accuracy with which oxygen positions can be defined is more limited in X-ray than in neutron diffraction, but the change in χ2 to 1.85 when all oxygen anions are removed from the model indicates that the X-ray data are sufficiently sensitive to the oxygen positions to draw conclusions regarding bonding. Electron diffraction confirms this structural model. Reconstruction of the reciprocal space by selected area electron diffraction confirms the tetragonal symmetry and reveals a primitive Bravais lattice with 4/mmm Laue symmetry, consistent with the expected P4mm polar symmetry of PbTiO3. No superstructure reflections or streaks of diffuse intensity associated with ordering of the Zn and Ti cations on the B site are observed. (Supporting Information, Figure 1) Elemental analysis of Bi2ZnTiO6 samples by energydispersive X-ray spectroscopy (EDS) demonstrates that the cation composition is Bi2ZnTi as targeted. It is noted that BZT samples examined initially by XRD after synthesis display an unusual asymmetrically broadened diffraction peak shape, similar to that previously observed for compositions in the (1 - x)PbTiO3-(x)Bi2ZnTiO6 solid solution,16 which is consistent with the existence of strained regions at pyroelectric domain boundaries. However, it is found that ambient-pressure annealing of the sample at 400 °C after high pressure synthesis greatly reduces peak broadening and permits acceptable single-phase Rietveld refinement. It is important to note that there is no change in composition (as judged by EDS) or other microstructure (by scanning electron microscopy) as a result of this post-synthesis treatment. Powder and pellet samples of Bi2ZnTiO6 were found to be structurally stable at ambient pressure from room temperature up to temperatures e 550 °C, with variable temperature synchrotron XRD experiments revealing that the polar distortion, as quantified by c/a, decreases only slightly
Communications
Figure 2. Dielectric permittivity and tetragonal c/a parameter (top) and dielectric loss (bottom) of Bi2ZnTiO6 plotted vs temperature for selected frequencies. Table 1. Summary of Refined Unit Cell Parameters and C/A Ratios for Bi2ZnTiO6 at Various Temperatures (P4mm) temp (°C)
a (Å)
c (Å)
V (Å3)
c/a ratio
23 250 500
3.82190(3) 3.83353(8) 3.85019(8)
4.62803(7) 4.61682(16) 4.59973(14)
67.601(1) 67.8484(32) 68.1862(29)
1.211 1.204 1.195
over this temperature range. Above 550 °C, samples under ambient pressure were found to decompose to Bi4Ti3O12 and other minor non-perovskite impurity phases. A summary of refined structural parameters and c/a ratios for selected temperatures is given in Table 1. The evolution of c/a with temperature for Bi2ZnTiO6 (Figure 2) and a comparison with correlations between c/a values and measured TC’s in the ferroelectric (1 - x)PbTiO3-(x)Bi2ZnTiO6 system16 suggest that Bi2ZnTiO6 itself may also be ferroelectric, possessing an extremely high TC lying above both the decomposition and the synthesis temperature (∼900 °C), given the intactness and sintered density of the samples obtained after quenching from temperature during the high-pressure synthesis. However, it is important to note that no direct evidence of ferroelectric behavior in Bi2ZnTiO6 exists at present. This is similar to other high-pressure polar perovskites, such as BiCoO3 and PbVO3,9,18 which also exhibit very large polar distortions (c/a ratios of 1.27 and 1.23, respectively). In materials possessing such significant structural distortion, the exceptionally large electric field required for switching makes it very difficult, if not impossible, to obtain a ferroelectric P-E hysteresis loop, particularly in the case of bulk samples. Therefore, like the aforementioned polar perovskites, Bi2(ZnTi)O6 should be considered a pyroelectric compound. However, variable temperature measurements of dielectric permittivity and loss, shown in Figure 2, are consistent with possible ferroelectric behavior in Bi2ZnTiO6. The increase in relative permittivity on heating to 500 °C is consistent with the approach to a ferroelectric transition at a significantly higher TC. The magnitude and temperature dependence of the dielectric loss are typical for a Pb based dielectric and are lower than those found for other Bi based perovskites.3,10 The reduced loss can be associated with the reduced (18) Shpanchenko, R. V.; Chernaya, V. V.; Tsirlin, A. A.; Chizhov, P. S.; Sklovsky, D. E.; Antipov, E. V.; Khlybov, E. P.; Pomjakushin, V.; Balagurov, A. M.; Medvedeva, J. E.; Kaul, E. E.; Geibel, C. Chem. Mater. 2004, 16, 3267.
Communications
Figure 3. (a) Oxygen coordination environment at the B site, occupied equally in a site-disordered manner by Zn and Ti, in Bi2ZnTiO6. The polar c axis is vertical. The Zn/Ti cations are displaced away from the centroid of the six neighboring oxide anions to form one short (1.77(1) Å) and one long (2.86(1) Å) Zn/Ti-O1 bond. There are four 1.997(2) Å Zn/Ti-O2 distances. (b) The A site in the ABO3 perovskite structure is described by a cuboctahedron of 12 oxide anions. The Bi cation in Bi2ZnTiO6 is strongly displaced toward one square face of this cuboctahedron, making four short 2.385(3) Å bonds represented as solid lines. There are two sets of four longer bonds at 2.833(3) Å (shown as dashed lines) and 3.728(4) Å (no bonds indicated). (c) Refined crystal structure of Bi2ZnTiO6. The Ti/Zn coordination polyhedron is represented as a square based pyramid because of the elongation of the bond to O1 along c. The coordination around Bi is represented by the shortest four bonds.
electronic conductivity due to the absence of open-shell cations on the B site. Scanning electron microscope images (Supporting Information, Figure 2) show that the ceramic pellet on which the impedance data were recorded is dense and of high quality. Detailed analysis of the structure of Bi2ZnTiO6 (Figure 3) suggests that this material has a much larger polarization than PbTiO3. The extent of the distortion giving rise to pyroelectricity is revealed by the difference in size between the c (polar) and a axes of the tetragonal perovskite structure in PbTiO3 (c/a ) 1.06) and Bi2ZnTiO6 (c/a ) 1.21); the c axis of the latter is 0.5 Å larger than in PbTiO3. The large tetragonal distortion of the unit cell requires unusual coordination environments at both the A site (Bi) and B site (Zn, Ti) cations. It is important to note that electron diffraction does not give evidence for long- or short-range order of Zn and Ti cations over the B sites, requiring discussion of the structure in terms of the average B site environment. The unit-cell level disorder in the B site environment is confirmed by the local probe of Raman spectroscopy; the broadening of the (Ti, Zn)-O stretching and bending region (500 to 800 cm-1) compared with PbTiO3 (see Supporting Information, Figure 3) is particularly notable and can be associated with the cation disorder.19 The large c/a ratio is produced by a distortion of the B site coordination geometry from tetragonally elongated octahedral to square based pyramidal, coupled with an A site bismuth environment also considerably more distorted than that of lead in (19) Levin, I.; Cockayne, E.; Lufaso, M. W.; Woicik, J. C.; Maslar, J. E. Chem. Mater. 2006, 18, 854.
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PbTiO3, reflecting a more pronounced influence of the 6s2 stereochemically active electron pair. The displacement of Bi from the centroid of coordinating oxide anions is 0.88 Å compared with 0.48 Å in PbTiO3. The magnitude of this A site distortion is similar to that seen in BiCoO3 and PbVO3.9,18 The displacement of the B site cation from the centroid of the coordinating oxide positions in Bi2ZnTiO6 is 0.57 Å (cf. 0.32 Å in PbTiO3), comparable with 0.60 Å found for PbVO3 but smaller than 0.73 Å reported for BiCoO3, reflecting the mixed B site disorder and closed shell nature of both B site cations in Bi2ZnTiO6. Details of cation coordination environments and bond lengths are found in the caption of Figure 3. Calculation of the static polarization Ps using an ionic charge model gives 158 µC cm-2 for Bi2ZnTiO6, in comparison with 57 µC cm-2 for PbTiO3. This value is consistent with the considerably larger macroscopic c/a, although the refined root-mean-square displacements (see caption of Figure 1) are 2-3 times larger than in PbTiO317s these might reflect local noncorrelated displacements that may reduce Ps from the ionic value. X-ray, electron diffraction, and dielectric measurements thus confirm the synthesis of Bi2ZnTiO6, a lead-free analogue of PbTiO3. The structure of Bi2ZnTiO6 demonstrates the unusual coordination geometries accessible for s2/(d10/d0) cation combinations in the Bi2(B′B′′)O6 perovskite structure and the resulting impact on bulk properties such as polarization and permittivity. The successful synthesis of Bi2ZnTiO6 under high pressure conditions strongly suggests that epitaxial stabilization of Bi2ZnTiO6 in a thin film form might also be possible, as has recently been demonstrated for PbVO3.20 This should motivate both the growth of new materials in thin film form and the search, under a range of synthetic conditions, for new electroceramic oxides based on the electronic structures of the cations involved here. Acknowledgment. The authors thank EPSRC for funding (EP/C511794/1) and CCLRC for programme mode access to SRS. The authors thank the Royal Society for a University Research Fellowship (to A.M.F.) and a Wolfson Merit Award (to M.J.R.). Supporting Information Available: Electron diffraction pattern of Bi2ZnTiO6 and zone axis directions, SEM image of a dense Bi2ZnTiO6 polycrystalline pellet, Raman spectra of PbTiO3 and Bi2ZnTiO6 powders, and full experimental details (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM061085R (20) Martin, L. W.; Zhan, Q.; Jiang, W.; Chi, M.; Browning, N.; Suzuki, Y.; Ramesh, R. Growth and Properties of a New Correlated Electron Perovskite Thin Film PbVO3. Presented at APS March Meeting of the American Physical Society, Baltimore, MD, 2006.
