Hydrothermal Synthesis of SrTiO3 Mesocrystals: Single Crystal to

Article pubs.acs.org/crystal

Hydrothermal Synthesis of SrTiO3 Mesocrystals: Single Crystal to Mesocrystal Transformation Induced by Topochemical Reactions Vishwanath Kalyani,†,‡ Bogdan S. Vasile,§ Adelina Ianculescu,§ Maria Teresa Buscaglia,‡ Vincenzo Buscaglia,*,‡ and Paolo Nanni†,‡ †

Department of Process and Chemical Engineering, University of Genoa, Fiera del Mare, P.le Kennedy, I-16129 Genoa, Italy Institute for Energetics and Interphases, National Research Council, Via De Marini 6, I-16149 Genoa, Italy § Department of Science and Engineering of Oxide Materials, Polytechnics University of Bucharest, 1-7 Gh. Polizu, P.O. Box 12-134, 011061 Bucharest, Romania ‡

S Supporting Information *

ABSTRACT: The mechanisms driving the mutual crystallographic alignment of nanocrystals in mesocrystals are various and not yet fully understood. As discussed in this paper, formation of mesocrystals can result from a topochemical reaction between single crystal particles or templates suspended in a liquid phase and ionic/molecular species in solution. In such a case, the initial particle morphology is retained in the final mesocrystal. If the transformation is not topotactic, the final product will maintain no memory of the precursor morphology. The magnitude of the lattice mismatch as well as the defective state of the precursor surface probably determine the degree of mutual crystallographic alignment of the nanocrystals which nucleate and grow on the substrate. Although identified by studying the hydrothermal crystallization of SrTiO3 from different single crystal titania precursors, this mechanism is very general and applicable to a variety of compounds and experimental situations, including solvothermal and molten salt syntheses.

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INTRODUCTION The study of mesocrystals is a very recent research topic which is attracting the attention of scientists working in different fields of physics, chemistry, and materials science.1−8 A mesocrystal can be defined as a 3D ordered superstructure composed of crystalline nanoparticles which are aligned along well-defined crystallographic directions. Mesocrystals can be considered as a special kind of colloidal crystals, systems corresponding to the ordering of monodispersed spherical particles in a superlattice.9 However, the high degree of crystallographic order typical of mesocrystals is not generally exhibited by colloidal crystals, which can also be obtained using amorphous particles. Mesocrystals have recently been investigated for their novel or improved properties for different applications, including Li batteries, terahertz radiation sources, photocatalysis, and cancer therapy.10 Because of the high mutual order of the nanocrystalline building units, it is difficult to distinguish a mesocrystal from a single crystal. For example, the electron diffraction pattern of a mesocrystal shows spots corresponding to the reciprocal lattice points as a single crystal. The border between mesocrystals and single crystals can only be defined on the basis of the coherence length in scattering experiments, which is typically 2%, but edge dislocations and vertical grain boundaries are formed to relieve the high tensile strain, leading to films with mosaic structure or columnar grains rather than single crystal films.19,27 In a recent study,28 epitaxial films of anatase (100 nm) were grown by reactive electron beam evaporation on SrTiO3 substrates with (100) and (110) orientation. Anatase has a tetragonal structure with space group I41/amd and lattice parameters a = b = 0.3785 nm and c = 0.9514 nm (JCPDS 211272). In the case of (100) oriented substrates, the orientation relationship is (001) anatase//(100) SrTiO3; [100] anatase// [001] SrTiO3. This situation corresponds to a similar orientation of the TiO6 octahedra at either side of the interface. The mismatch between (100) anatase (d = 0. 378 nm) and (100) SrTiO3 (d = 0.390 nm) is 3.2%. The morphology of the anatase film, corresponding to columnar grains with a high degree of crystallographic alignment, is determined by the need to release the high tensile strain. A complementary yet similar situation is encountered during the hydrothermal growth of SrTiO3 on the anatase wires. Since the (004) anatase planes are parallel to the longest dimension of the NWs (Figure 4a), the elongation direction can be either

Figure 3. Morphology (TEM) of HT-SrTiO3 heterostructures obtained after 15 min of hydrothermal reaction at 200 °C. (a, b) W/EtOH solvent. (c, d) Water. The insets in parts a and c show the ED pattern. The lattice images (HRTEM) reported in part d correspond to the three different particles of the heterostructure shown in the right lower corner. Note the different orientations of the lattice planes.

grown at a certain distance from the HT surface (Figures 1d,f and 3). Differently from the anatase nanowires which are completely coated by the SrTiO3 nanocubes after 15 min of reaction (Figures 1a and 2f), many HT wires show significant portions of their surface not decorated by the product particles (Figures 1d,f and 3b,c). Overall, the heterostructures obtained from the HT wires after a short reaction time are rather disordered and with a nonuniform morphology. A very similar morphological evolution is observed independently of the solvent. The ED patterns (insets of Figure 3a and c) show circles and indicate that the SrTiO3 particles which have grown on a single HT wire have random orientation, as also confirmed by the different orientations of the lattice planes of neighboring SrTiO3 particles (Figure 3d). In conclusion, the hydrothermal reaction of HT nanowires does not result in the formation of SrTiO3 mesocrystals. This becomes more evident with increasing reaction time: the wirelike shape is progressively

Figure 4. (a) Lattice image of a single crystal anatase NW with zone axis [010]. The white arrow indicates the elongation direction of the NW. (b) Sketch illustrating the epitaxial growth of a SrTiO3 nanocube on an anatase NW. D

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growth of the SrTiO3 nanocubes should proceed by addition of Ti(OH)n4−n (n = 1−4) or HTiO3− species (originated by the dissolution of titania in the strongly alkaline solution) and Sr2+ or Sr(OH)+ ions at the perovskite surface.29 Once the anatase surface is completely coated, a strong decrease of the reaction rate is expected, as apparent by comparing the XRD patterns of the powders obtained after 15 min and 48 h of reaction, respectively (see Figure S2, Supporting Information). Indeed, dissolution of titania can only occur at a few locations where the anatase surface is still in contact with the alkaline solution. It can be concluded that anatase nanowires are appropriate reactive templates for the synthesis of 1D SrTiO3 nanostructures in that the overall transformation is mediated by a topochemical reaction (Figure 5) . The product largely maintains the dimensions and shape of the precursor. The same approach is likely to apply to the crystallization of other perovskites of broad technological importance, such as CaTiO3 and BaTiO3. In the case of hydrogen titanate, the situation is entirely different. Hydrogen titanates as well as sodium and potassium titanates have a stepped layered structure composed of layers of edge-sharing TiO6 octahedra. These layers are intercalated with cations H3O+, Na+, and K+.26,30 The topotactic transformation of layered sodium and potassium titanate nanowire ions in BaTiO3 nanowires by ion exchange in alkaline solution containing Ba2+ has been reported.31 However, topotactic conversion of HT in SrTiO3 is apparently inhibited in the present experimental conditions and the very simple picture of a transformation driven by ionic exchange between H3O+ and Sr2+ ions does not apply. If this were the case, the morphology of the HT nanowires would have been retained in the final product. Since the concentration of Na+ ions in the solution is very high ([Na+] = 0.88 mol/L; [Sr2+] = 0.2 mol/L), one could assume that Na+ ions are more readily exchanged with H3O+ ions than Sr2+ ions, thus preventing strontium intercalation. The existence of a series of intermediate hydrogen sodium titanates would support this hypothesis.30 However, specific experiments carried out keeping the same concentration of Sr(OH)2 without addition of NaOH did not result in the formation of mesocrystals (Figure S3, Supporting Information), although the final morphology is more homogeneous and ordered. Therefore, it can be concluded that the topochemical transformation of hydrogen titanate in strontium titanate mediated by ion exchange does not easily occur, at least in the present experimental conditions. Moreover, the random orientation of the perovskite nanoparticles suggests that epitaxial growth of SrTiO3 on the HT surface does not take place. The formation of an incoherent interface makes nucleation on HT more difficult than on anatase. Indeed, the size of the SrTiO3 particles which have grown on HT is larger (150−200 nm against 50 nm) and a significant fraction of the HT surface is not coated by the SrTiO3 particles which remain mostly isolated. This indicates that nucleation of the perovskite only proceeds at some special sites on the HT surface. Moreover, many particles have grown not in direct contact with the HT surface (see Figure 3), suggesting competition between different nucleation mechanisms. At a later stage of the reaction, when the HT wires are almost consumed, the SrTiO3 particles are no longer held together by the central core and no memory of the initial wire morphology is retained in the final product (Figure 1e). The formation of the SrTiO3 particles from the HT nanowires is schematically summarized in Figure 5b. It turns out that HT nanowires cannot be used as templates

[100] or [010]. However, these two directions are equivalent because a = b and the discussion can be restricted to the first case. The major surfaces of the NWs will correspond to the (001) and (010) lattice planes, and accordingly, epitaxial growth of the SrTiO3 nanocubes can occur as shown in the sketch of Figure 4 with orientation relationships similar to those observed in the case of thin films.28 When the diameter of the titanate NW is small enough, two rows of perovskite nanocubes will nucleate on opposite surfaces of the wire, leading to the simple morphology shown in the inset of Figure 2f. For wider wires, formation of multiple rows of nanocubes will occur. The overall mesocrystal formation mechanism is schematically illustrated in Figure 5a.

Figure 5. Schematic view of two crystallization pathways of SrTiO3 from single crystal precursors. (a) Formation of SrTiO3 mesocrystals from single crystal anatase driven by a topochemical reaction. Nucleation and epitaxial growth of the perovkite occurs on opposite surfaces of the anatase wire determining the formation of a mesocrystal composed of columns of aligned and well welded nanocubes (see text). (b) Crystallization of SrTiO3 from single crystal hydrogen titanate does not involve a topochemical reaction and results in the formation of loose SrTiO3 particles. Nucleation of the perovskite occurs on or close to the HT surface, but particle growth proceeds with random orientation. The SrTiO3 particles which decorate the NW are held together by the residual core. When the core is completely consumed, no memory of the original wirelike morphology is retained in the final product.

As shown in Figures 1 and 2, the surface of the anatase nanowires is already coated by a continuous layer of SrTiO3 nanocubes with uniform size after 15 min of reaction. This means that the heterogeneous nucleation of the perovskite is very fast and mostly occurs at the very early stages of reaction. According to the classic nucleation theory, the formation of a coherent or semicoherent interface significantly lowers the interfacial energy and decreases the free energy barrier for nucleation, resulting in a high nucleation rate. Subsequent E

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is driven by a topochemical reaction. Multiple perovskite nuclei form on the precursor surface and grow epitaxially by addition of strontium and titanium ionic species. The existence of irregularities and defects on the precursor surfaces as well as the formation of dislocations needed to release the elastic stresses determined by the lattice mismatch will result in a nonperfect matching of the lattices of contiguous SrTiO3 nanocubes. Indeed, small misalignments, dislocations, and phase boundaries are commonly observed in the boundary region between cubic subunits. This mechanism is very general and can determine the formation of mesocrystals and, more generally, of ordered superstructures, when a topochemical reaction involves a suitable single crystal precursor dispersed in a liquid and ionic/molecular species in solution. This situation is often encountered in precipitation processes, hydrothermal and solvothermal reactions, molten salt synthesis, and liquid-phase sintering. The liquid phase guarantees rapid mass transport, and consequently, the proposed mechanism can operate even at relatively low temperatures. The degree of crystallographic alignment of the primary nanocrystals will be determined by the quality of the precursor surfaces and the magnitude of lattice mismatch. The larger the mismatch and the higher the density of surface defects, the lower the level of ordering. When the lattice mismatch is very large or crystallographic incompatibility exists between precursor and product, a topochemical transformation will not occur and the reaction will result in loose particles or nonoriented polycrystalline aggregates. This happens, for instance, during the crystallization of SrTiO3 from single crystal HT nanowires. The importance of topochemical reactions in the transformation of mesocrystals was already pointed out by Zhou et al.36 In their study, NH4TiOF3 mesocrystals were converted to anatase mesocrystals either by calcination or washing with aqueous H3BO3, retaining a high degree of alignment among the primary nanocrystals. In the present case, we have shown that a topotactic reaction can also determine the transformation of a single crystal in a mesocrystal. Overall, these observations suggest that topochemical transformation of suitable precursors, either mesocrystals or single crystals, can be used to generate a wide variety of structures characterized by the desired morphology and different levels of ordering and crystallographic alignment of the primary nano building blocks.

for the hydrothermal preparation of strontium titanate 1D nanostructures. Preliminary results indicate that the same conclusion applies to the crystallization of barium titanate from the same precursor. In conclusion, we have shown that formation of mesocrystals can be the result of a topochemical reaction which determines the epitaxial growth of the product on a single crystal precursor. The lattice mismatch as well as the growth of the reaction product on a more or less uneven and defective surface will determine the degree of mutual crystallographic alignment of the nanocrystals which have nucleated on the precursor surface. If the mismatch is small (2%), the strain is relieved by the formation of dislocations and grain boundaries, but epitaxial growth will still occur at the level of each grain or domain. This is a very general mechanism which can explain the formation of mesocrystals and superstructures in many different conditions, providing that a liquid phase is available. In particular, it can operate even when synthesis and processing of materials is performed at high temperature, such as in molten salt synthesis and liquid phase sintering. Poterala et al.32 have described the high-temperature (950−1050 °C) conversion of plateletshaped single crystal Na3.5Bi2.5Nb5O18, PbBi4Ti4O15, and BaBi4Ti4O15 Aurivillius phases to NaNbO3, PbTiO3, and BaTiO3 perovskites in a molten salt. Observations from TEM showed that perovskite crystallites grow from multiple nucleation sites but become slightly misaligned during growth. According to the authors, this misalignment is caused by a loss of epitaxy with the parent Aurivillius phase and subsequent exfoliation of the particles, likely caused by the expulsion of byproduct Bi2O3 liquid on phase boundaries. Recrystallization then determines the formation of oriented, polycrystalline perovskite platelets. Barium titanate mesocrystals with rodlike morphology were obtained by reaction between single crystal titania rods and BaCO3 in molten NaCl-KCl eutectic at 700 °C.33 Although this feature was not discussed by the authors, the TEM images clearly show rods with a diameter of ≈1 μm composed of smaller aligned crystals. These mesocrystals can be interpreted as the result of a topochemical reaction. Peculiar grain growth phenomena during the sintering of (Na, K)NbO3 based piezoceramics have been recently reported.34,35 Large recrystallized grains, with a size up to some tenths of micrometers, are composed of highly parallel subgrains with small misorientations and can be considered as mesocrystals. The formation of these microstructures was described as a selfassembly process induced by either (i) liquid-phase mediated interactions and capillary forces or (ii) intrinsic electric fields originated by oppositely charged counter faces of crystals.34 However, a topochemical reaction mediated by a dissolution− precipitation process more likely explains the final microstructure. As shown by Rubio-Marcos et al.,35 there are significant differences in chemical compositions between recrystallized grains and normal grains and probable coexistence of two polymorphs.

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ASSOCIATED CONTENT

* Supporting Information S

Morphology and XRD patterns of anatase and hydrogen titanate nanowires; XRD patterns of reaction products with indication of identified phases; and microstructure of a strontium titanate wire obtained in slightly different experimental conditions (see manuscript text for a more detailed description). This material is available free of charge via the Internet at http://pubs.acs.org.

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SUMMARY AND CONCLUSIONS Hydrothermal crystallization of strontium titanate using single crystal anatase nanowires as precursor results in the formation of mesocrystals retaining the overall wirelike morphology and characterized by the ordered alignment of nanocubes in a crystallographic register. The formation of SrTiO3 mesocrystals

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39-010-6475708. Fax: +39-010-6475700. Notes

The authors declare no competing financial interest. F

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(25) (a) Kato, H.; Kudo, A. J. Phys. Chem. B. 2002, 19, 5029. (b) Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992. (c) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2000, 12, 3. (26) (a) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 2286. (b) Yoshida, R.; Suzuki, Y.; Yoshikawa, S. J. Solid State Chem. 2005, 178, 2179. (c) Peng, X.; Chen, A. Adv. Func. Mater. 2006, 16, 1355. (d) Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2006, 128, 8217. (27) (a) Kalyanaraman, R.; Vispute, R. D.; Oktyabrsky, S.; Dovidenko, K.; Jagannadham, K.; Narayan, J.; Budai, J. D.; Parikh, N.; Suvkhanov, A. Appl. Phys. Lett. 1997, 71, 1709. (b) Shin, H. J.; Kim, S. H.; Yang, H. J.; Kuk, Y. Nanotechnology 2007, 18, 175304. (28) Lotnyk, A.; Senz, S.; Hesse, D. Thin Solid Films 2007, 515, 3439. (29) (a) Lencka, M. M.; Riman, R. E. Chem. Mater. 1993, 5, 61. (b) Testino, A.; Buscaglia, V.; Buscaglia, M. T.; Viviani, M.; Nanni, P. Chem. Mater. 2005, 17, 5346. (30) (a) Pradhan, S. K.; Mao, Y.; Wong, S. S.; Chupas, P.; Petkov, V. Chem. Mater. 2007, 19, 6180. (b) Tsai, C.-C.; Teng, H. Chem. Mater. 2006, 18, 367. (31) (a) Feng, Q.; Hirasawa, M.; Yanagisawa, K. Chem. Mater. 2001, 13, 290. (b) Bao, N.; Shen, L.; Srinivasan, G.; Yanagisawa, K.; Gupta, A. J. Phys. Chem. C 2008, 112, 8634. (c) Bao, N.; Shen, L.; Gupta, A.; Tatarenko, A.; Srinivasan, G.; Yanagisawa, K. App. Phys. Lett. 2009, 94, 253109. (d) Kang, S.-O.; Park, B. H.; Kim, Y.-I. Cryst. Growth. Des. 2008, 8, 3180. (32) Poterala, S. F.; Chang, Y.; Clark, T.; Meyer, R. J., Jr.; Messing, G. L. Chem. Mater. 2010, 22, 2061. (33) Huang, K.-C.; Huang, T.-C.; Hsieh, W.-F. Inorg. Chem. 2009, 48, 9180. (34) (a) Zhen, Y.; Li, J.-F. J. Am. Ceram. Soc. 2007, 90, 3496. (b) Li, Z.; Li, Y.; Zhai, J. W. Curr. Appl. Phys. 2011, 11, S2. (35) Rubio-Marcos, F.; Romero, J. J.; Navarro-Rojero, M. G.; Fernandez, J. F. J. Europ. Ceram. Soc. 2009, 29, 3045. (36) (a) Zhou, L.; Smyth Boyle, D.; O’Brien, P. Chem. Commun. 2007, 144. (b) Zhou, L.; Smyth-Boyle, D.; O’Brien, P. J. Am. Chem. Soc. 2008, 130, 1309.

REFERENCES

(1) Cölfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (2) Cölfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (3) Niederberger, M.; Cölfen, H. Phys. Chem. Chem. Phys. 2006, 8, 3271. (4) Cö lfen, H.; Antonietti, M. Mesocrystals and Nonclassical Crystallization, John Wiley & Sons, New York, 2008. (5) Zhou, L; O’Brien, P. Small 2008, 4, 1566−1574. (6) Zhang, Q.; Liu, S.-J.; Yu, S.-H. J. Mater. Chem. 2009, 19, 191. (7) Song, R.-Q.; Cölfen, H. Adv. Mater. 2010, 22, 1301. (8) Banfield, J. F.; Welch, S. A.; Zhang, H.; Thomsen Ebert, T.; Penn, R. L. Science 2000, 289, 751. (9) (a) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (b) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (10) (a) Popovic, J.; Demir-Cakan, R.; Tornow, J.; Morcrette, M.; Sheng, Su, D.; Schlögl, R.; Antonietti, M.; Titirici, M.-M. Small 2011, 7, 1127. (b) Bilecka, I.; Hintennach, A.; Djerdj, I.; Novak, P.; Niederberger, M. J. Mater. Chem. 2009, 19, 5125. (c) Wu, X. L.; Xiong, S. J.; Liu, Z.; Chen, J.; Shen, J. C.; Li, T. H.; Wu, P. H.; Chu, P. K. Nature Nanotechnol. 2011, 6, 103. (d) Tartaj, P.; Amarilla, J. M. Adv. Mater. 2011, 23, 4904. (e) Ye, J. F.; Liu, W.; Cai, J. G.; Chen, S. A.; Zhao, X. W.; Zhou, H. H.; Qi, L. M. J. Am. Chem. Soc. 2011, 133, 933. (f) Zhao, Y.; Lu, Y.; Hu, Y.; Li, J.-P.; Dong, L.; Lin, L.-N.; Yu, S.-H. Small 2010, 6, 2436. (11) (a) Jongen, N.; Bowen, P.; Lemaitre, J.; Valmalette, J. C.; Hofmann, H. J. Colloid Interface Sci. 2000, 226, 189. (b) Pujol, O.; Bowen, P.; Stadelmann, P. A.; Hofmann, H. J. Phys. Chem. B 2004, 108, 13128. (12) Calderone, V. R.; Testino, A.; Buscaglia, M. T.; Bassoli, M.; Bottino, C.; Viviani, M.; Buscaglia, V.; Nanni, P. Chem. Mater. 2006, 18, 1627. (13) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (b) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (14) (a) Busch, S.; Dolhaine, H.; DuChesne, A.; Heinz, S.; Hochrein, O.; Laeri, F.; Podebrad, O.; Vietze, U.; Weiland, T.; Kniep, R. Eur. J. Inorg. Chem. 1999, 1643. (b) Simon, P.; Zahn, D.; Lichte, H.; Kniep, R. Angew. Chem., Int. Ed. 2006, 45, 1911. (15) (a) Schaffer, T. E.; Ionescu Zanetti, C.; Proksch, R.; Fritz, M.; Walters, D. A.; Almqvist, N.; Zaremba, C. M.; Belcher, A. M.; Smith, B. L.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Chem. Mater. 1997, 9, 1731. (b) Oaki, Y.; Kotachi, A.; Miura, T.; Imai, H. Adv. Funct. Mater. 2006, 16, 1633. (c) Miura, T.; Kotachi, A.; Oaki, Y.; Imai, H. Cryst. Growth Des. 2006, 6, 612. (16) Ahmed, S.; Ryan, K. M. Nano Lett. 2007, 7, 2480. (17) Jiang, Y.; Gong, H. F.; Volkmer, D.; Gower, L.; Cölfen, H. Adv. Mater. 2011, 23, 3548. (18) (a) Bednorz, J. G.; Müller, K. A. Phys. Rev. Lett. 1984, 52, 2289. (b) Ang, C.; Yu, Z.; Vilarinho, P. M.; Baptista, J. L. Phys. Rev. B. 1998, 57, 7403. (c) Tkach, A.; Vilarinho, P. M.; Kholkin, A. L. Appl. Phys. Lett. 2005, 86, 172902. (d) Itoh, M.; Wang, R.; Inaguma, Y.; Yamaguchi, T.; Shan, Y.-J.; Nakamura, T. Phys. Rev. Lett. 1999, 82, 3540. (19) Haeni, J. H.; Irvin, P.; Chang, W.; Uecker, R.; Reiche, P.; Li, Y. L.; Choudhury, S.; Tian, W.; Hawley, M. E.; Craigo, B.; Tagantsev, A. K.; Pan, X. Q.; Streiffer, S. K.; Chen, L. Q.; Kirchoefer, S. W.; Levy, J.; Schlom, D. G. Nature 2004, 430, 758. (20) Tagantsev, A. K.; Sherman, V. O.; Astafiev, K. F.; Venkatesh, J.; Setter, N. J. Electroceram. 2003, 11, 5. (21) Müller, K. A.; Burkard, H. Phys. Rev. B 1979, 19, 3593. (22) Denk, I.; Munch, W.; Maier, J. J. Am. Ceram. Soc. 1995, 78, 3265. (23) Pellegrino, L.; Biasotti, M.; Bellingeri, E.; Bernini, C.; Siri, A. S.; Marrè, D. Adv. Mater. 2009, 21, 2377. (24) (a) Lee, S.; Yang, G.; Wilke, R. H. T.; Trolier-McKinstry, S.; Randall, C. A. Phys. Rev. B 2009, 79, 134110. (b) Ohta, H.; Kim, S.; Mune, Y.; Mizoguchi, T.; Nomura, K.; Ohta, S.; Nomura, T.; Nakanishi, Y.; Ikuhara, Y.; Hirano, M.; Hosono, H.; Koumoto, K. Nat. Mater. 2007, 6, 129. G

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