Tuning the Ferroelectric and Piezoelectric Properties of 0.91Pb(Zn1

May 12, 2010 - UniVersity of Science and Technology Beijing, Beijing 100083, China, Shanghai Institute of Ceramics, Chinese. Academy of Sciences ...
1 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 9955–9960

9955

Tuning the Ferroelectric and Piezoelectric Properties of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 Single Crystals and Lead Zirconate Titanate Ceramics by Doping Hydrogen Ming Wu,† Haiyou Huang,‡ Wuyang Chu,† Liqiu Guo,† Lijie Qiao,*,† Jiayue Xu,§ and Tong-Yi Zhang| Corrosion and Protection Center, Key Laboratory for EnVironmental Fracture (MOE), UniVersity of Science and Technology Beijing, Beijing 100083, China, Key Laboratory for AdVanced Materials Processing (MOE), UniVersity of Science and Technology Beijing, Beijing 100083, China, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China, and Department of Mechanical Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed: December 16, 2009; ReVised Manuscript ReceiVed: April 10, 2010

Systematically experimental investigations were conducted on H-doped 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 (PZNT) single crystals and lead zirconate titanate (PZT) ceramics. The experimental results show that doping hydrogen with appropriate content advances the ferroelectric and piezoelectric properties effectively. For example, when trapped hydrogen concentration was Ct ) 1.7 wppm in the H-doped PZNT single crystals, the remanent polarization Pr and piezoelectric constant d33 increased by 80% and 41.7% in comparison with the H-free samples, respectively, to 28.8 µC/cm2 and 391 pC/N. However, high hydrogen concentration damages the ferroelectric and piezoelectric properties. The remanent polarization Pr and piezoelectric constant d33 of the H-doped PZNT single crystals decreased to 0 µC/cm2 and 24 pC/N, respectively, when Ct ) 3.1 wppm. The H-doped PZT ceramics exhibited the same behavior, where the values of Pr and d33 increased respectively by 45.3% and 27.3% to 48.1 µC/cm2 and 420 pC/N, when Ct ) 1.1 wppm, and decreased to 0 µC/cm2 and 84 pC/N, respectively, as Ct increased to 2.1 wppm. Hydrogen also increases the dielectric constant and dielectric loss for both ferroelectric materials. The experimental results indicate that doping hydrogen into ferroelectric materials is a feasible technique to tune the ferroelectric and piezoelectric properties effectively, furthermore as a subsequent treated technology for the ferroelectric product. Introduction Ferroelectric (1 - x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZNT) single crystals show exceptional properties such as high electromechanical coupling coefficients, high piezoelectric constants, and high dielectric constants at compositions near the morphotropic phase boundary (MPB).1-6 Because of these exceptional properties, they have wide applications in modern technologies, such as in voltage generators, transducers, sensors, filters, etc.7 Similarly, ferroelectric Pb(Zr,Ti)O3 (PZT) materials possess excellent material properties and are extensively used in the ferroelectric random access memory (FRAM).8-13 In cuttingedge technology applications of ferroelectric materials, remanent polarization and piezoelectric constants are the most important ferroelectric and piezoelectric properties, which are crucial in the stored charge density of FRAM and in the conversion efficiency of transducers. That is why various technologies such as doping noble metallic elements, increasing the orientation, etc., have been developed to improve the polarization and piezoelectric constant of ferroelectric materials.14,15 These methods, however, are limited in their usefulness. In academic and industrial practice, microelectronic devices are often annealed in forming gas (4%H2 + N2) or H2 gas to * Corresponding author. E-mail: [email protected]. Tel.: +86-10-62334499. Fax: +86-10-6233-2345. † Key Laboratory for Environmental Fracture (MOE), University of Science and Technology Beijing. ‡ Key Laboratory for Advanced Materials Processing (MOE), University of Science and Technology Beijing. § Shanghai Institute of Ceramics. | Hong Kong University of Science and Technology.

improve the device quality.16,17 Experimental results already show that doping hydrogen into ferroelectric materials varies the material properties. For instance, the maximum polarization of Bi4-xLaxTi3O12 film capacitors was increased after annealing in the forming gas (4%H2 + N2) at 150 °C, but a significant reduction in the remanent polarization was observed when (Pb,La)(Zr,Ti)O3(PLZT) capacitors were baked in a hydrogen atmosphere at 150 °C.18,19 It is often reported in the literature that high content hydrogen causes severe degradation of resistivity and ferroelectricity.20,21 For example, the insulation resistivity and the dielectric constant of PMN-based relaxed ferroelectric ceramics declined nonlinearly with the increase of hydrogen concentration, whereas the dielectric loss increased.22 However, there are fewer reports in the literature on systematic investigations of ferroelectric materials doped with different H concentrations, which motivates us to carry on such a comprehensive study on the effect of hydrogen on the ferroelectric and piezoelectric properties. The goal of this study is to show that doping hydrogen with controllable content will effectively change the ferroelectric and piezoelectric properties of PZNT single crystals and PZT ceramics, which establishes a facile technique to tune, within a large range, the ferroelectric and piezoelectric properties of ferroelectric materials. Experimental Methods PZNT single crystals at the MPB were provided by the Shanghai Institute of Ceramics, Chinese Academy of Sciences. The PZNT single crystals were cut into samples with the dimensions of 1 mm × 5 mm × 5 mm. Commercial soft lead zirconate titanate ceramics Pb(Zr0.52Ti0.48)O3 (PZT) were also

10.1021/jp101463e  2010 American Chemical Society Published on Web 05/12/2010

9956

J. Phys. Chem. C, Vol. 114, No. 21, 2010

Wu et al.

TABLE 1: H Concentrations of the H-Doped PZNT Single Crystals electrochemically charged (i, mA/cm2)

C0, wppm Ct, wppm C, wppm

0.5

5

50

400

1.3 0.4 1.7

5.2 0.8 6.0

8.5 1.7 10.2

10.7 2.5 13.2

TABLE 2: H Concentrations of the H-Doped PZT Samples electrochemically charged (i, mA/cm2)

charged in H2 gas 100 °C, 400 °C, 800 °C, 1 atm 4 atm 1 atm 6.6 1.2 7.8

12.1 2.6 14.7

14.0 3.1 17.1

used in this study, whose bulk plates of 8 mm × 40 mm × 100 mm were sintered at 1260 °C for 1.5 h and had an average grain size of 2.5 µm. The samples of 1 mm × 25 mm × 25 mm were cut from the bulk plates. The surfaces of both ferroelectric materials samples were coated with silver layers as electrode. In fact, hydrogen could pass into ferroelectric ceramics when they are annealed in the forming gas or during an electroplating procedure in solutions. Some PZNT single crystals samples were charged in 0.2 mol/L NaOH + 0.25 g/L As2O3 solution at 20 °C for 24 h at a current density of 0.5, 5, 50, or 400 mA/cm2 and some samples were charged in H2 gas of 4 atm at 400 °C for 20 h or 1 atm at 800 °C for 20 h. Similarly, some PZT ceramics samples were charged in the 0.2 mol/L NaOH + 0.25 g/L As2O3 solution at 20 °C for 24 h with a current densities of 5, 100, 325, or 400 mA/cm2 and some samples were charged in H2 gas of 1 atm at 100 °C for 20 h or 4 atm at 400 °C for 20 h. After charging, the total Hconcentration in the samples, CT, is composed by two parts, i.e., CT ) C0 + Ct, where C0 denotes the diffusible hydrogen concentration, which is measured by using the gas-replacingoil method, and Ct is the trapped hydrogen concentration measured by using the vacuum thermal extraction method. The charging process and the measurement of hydrogen concentration were reported in our previous publications in detail.20,23,24 Before measuring piezoelectric constant d33, PZNT samples and PZT samples were polarized by applying 1.5 kV/mm at 180 °C for 10 min and by applying 3.0 kV/mm for 15 min at room temperature, respectively. Then, piezoelectric constant d33 was measured using a quasi-static piezoelectric d33 meter (Model ZJ-6A). The ferroelectric properties of PZNT single crystals and PZT ceramics, such as polarization-field (P-E) hysteresis loops, were evaluated by using the TF Analyzer 2000 System with a high voltage amplifier at frequency f ) 1 Hz. The dynamic leakage current compensation (DLCC) method was applied to subtract the leakage current from the measured current response.25 The electrical resistivity was measured by using a HP4140B semiconductor measurement system. The frequency spectra of dielectric constant ε and dielectric loss tgδ were measured on an impedance analyzer (Hewlett Parckard 4194A). X-ray Diffraction (XRD) was conducted to measure lattice constants of H-free and H-doped PZNT single crystals and PZT ceramics.

C0, wppm Ct, wppm CT, wppm

charged in H2 gas

5

100

325

400

100 °C, 1 atm

4.8 0.4 5.2

8.1 1.1 9.2

9.7 1.5 11.2

11.7 1.7 13.4

6.2 0.7 6.9

400 °C, 4 atm 13.7 2.1 15.8

Figure 1a shows polarization-electric field hysteresis loops of PZNT samples, where the electric field of 1 Hz-frequency with 2100 V/mm amplitude is applied to all samples except the Ct ) 3.1 wppm H-doped sample. The variation of remanent polarization Pr with increasing trapped hydrogen concentrations Ct is obtained from the hysteresis loops and plotted in Figure 2a. Figure 2a shows that both Pr and d33 of the H-doped PZNT single crystals increase with increasing trapped hydrogen concentration within the range of 0 e Ct e 1.7 wppm and increase by 80% to 28.8 µC/cm2 and by 41.7% from 276 pC/N to 391 pC/N, respectively, when Ct ) 1.7 wppm. When Ct g 2.5 wppm, Pr of the H-doped PZNT single crystals decreases with increasing Ct, which indicates the degradation of ferroelectricity. Ferroelectricity in highly-H-doped (Ct g 3.1 wppm) PZNT single crystals disappear completely as evidenced by the linear P-E behavior shown in Figure 1a and become electrical semiconductors under higher applied electric fields. That is why only the P-E curves under lower electric fields are plotted in Figure 1a. When Ct e 1.7 wppm, both d33 and Pr increase almost linearly with Ct. The d33 value of the H-doped PZNT single

Results Tables 1 and 2 list hydrogen concentrations, C0, Ct, and CT, under different H-charging conditions for H-charged PZNT and PZT samples, respectively. The C0, Ct, and CT increased with the increasing current density for electrochemically charged samples and with the increasing H-gas pressure and temperature for gas-charged samples. Diffusible hydrogen atoms are able to escape from the samples during aging at room temperature. We, therefore, pay more attention to trapped hydrogen atoms. The effects of hydrogen, which will be discussed below, are focused on the effect of trapped hydrogen atoms.

Figure 1. P-E hysteresis loops of the (a) PZNT single crystals and (b) PZT ceramics with different trapped hydrogen concentrations, Ct.

Properties of PZNT Single Crystals

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9957

Figure 3. P-E hysteresis loops of the PZNT sample before H-doping, immediately after H-doping, H-doped and then aged in air at room temperature for 10 months, and degassing in air at 130 °C for 20 h (0.8 wppm H-doped sample).

Figure 2. Piezoelectric constant d33 and remanent polarization Pr versus trapped hydrogen concentration Ct in the (a) H-doped PZNT single crystals and (b) H-doped PZT ceramics, and (c) shows the lattice constant c/a of the H-free and H-charged PZNT single crystals and PZT ceramics.

crystals reaches 391 pC/N when Ct ) 1.7 wppm and then decreased to 24 pC/N when Ct increased to 3.1 wppm, as shown in Figure 2a. The sample H-charged at current density of 5 mA/ cm2 was aged in air at room temperature for 10 months. The P-E hysteresis loop after 10 month aging is almost the same as that before aging, as shown in Figure 3, thereby showing the stability of the ferroelectric properties in the H-doped samples. The stable material properties are essential for industrial applications of the H-doped ferroelectric materials. If we annealed the H-doped sample at 130 °C in air for 20 h, the P-E loop would recover to that before H-doping, as shown in Figure 3. The measuring results of dielectric constant and dielectric loss for hydrogen-free sample and Ct ) 0.8 wppm H-doping sample are showed in Figure 4(a). When Ct ) 0.8 wppm, hydrogen increases the dielectric constant and dielectric loss. XRD results of the H-free sample and the Ct ) 0.8 wppm H-doped sample were shown in Figure 5a.

Figure 4. Dielectric constant (solid square scatter) and dielectric loss (open circle scatter) of (a) PZNT single crystals and (b) PZT ceramics versus frequency.

Similarly, Figures 1b, 2b, and 4b show the experimental results on PZT samples. When Ct e 1.1 wppm, the Pr value of the H-doped PZT ceramics increases as Ct increases and reached the maximum value of 48.1 µC/cm2 when Ct ) 1.1 wppm. The experimental results demonstrate again that doping hydrogen with an appropriate content into ferroelectric ceramics will enhance the remanent polarization. When Ct g1.1 wppm, however, the Pr value decreases with the rise of Ct, which was reported before, and meanwhile the resistivity of PZT ceramics is less than 109 Ω · m and becomes semiconductors.26,27 In this case, a high electric field could not be applied to the samples

9958

J. Phys. Chem. C, Vol. 114, No. 21, 2010

Figure 5. XRD results of the H-free and H-charged (a) PZNT single crystals and (b) PZT ceramics; the inset shows the XRD measured at 42° ∼ 47° and 42° ∼ 48°, respectively.

because the leakage current is so large. That is why the hysteresis loop is unsaturated when Ct > 1.1 wppm. When Ct ) 2.1 wppm, the hysteresis loop is almost a straight line with an approximate zero Pr value. Figure 2b indicates d33 and Pr properties of PZT ceramics behave correspondingly the same as these of PZNT single crystals. When Ct e 1.1 wppm, both d33 and Pr values increase almost linearly with the increase of Ct. The Pr and d33 values increase by 45.3% to 48.1 µC/cm2 and by 27.3% to 420 pC/N, respectively, when Ct ) 1.1 wppm and then decreased to approximate 0 µC/cm2 and 84 pC/N, respectively, when Ct increased to 2.1 wppm. For PZT ceramics, dielectric constant and dielectric loss also increase with the rise of Ct, as shown in Figure 4b. XRD results of the H-free sample and H-doped samples were shown in Figure 5b. Discussion Previous researchers paid attentions to the degradation mechanism in the ferroelectric polarization of ferroelectric films. For instance, Fujisaki and his co-workers contributed the degradation of ferroelectricity of Pt/PZT/Pt capacitor to the loss of oxygen.28-30 Aggarwal and his co-workers proposed that the degradation of ferroelectric properties during forming gas annealing was induced both by the oxygen loss and by the formation of polar hydroxyl bonds.27,31,32 Cao et al. suggested that the valence number of some metallic atoms in the PMNbased ferroelectrics might be reduced by hydrogen atoms, which resulted in oxygen vacancies and free electrons and the change in electric and dielectric properties.33 X-ray photoelectron spectroscopy (XPS) analysis showed that Nb5+ and Pb2+ in PMN-based relaxor ferroelectrics were partially reduced to Nb4+ and metallic Pb by hydrogen doping.22 In the present research, hydrogen also decreased the ferroelectric and piezoelectric

Wu et al.

Figure 6. Possible positions of hydrogen in the tetragonal ferroelectric phase ABO3 crystal.

properties if the hydrogen concentration was higher than the critical value. Our previous work showed that hydrogen doping caused the color change of ferroelectric materials.34 In the following, we discussed possible mechanisms about hydrogeninduced change in the ferroelectric and piezoelectric properties of the H-doped PZNT single crystals and PZT ceramics. To explore the mechanism for the hydrogen doping-induced improvement, we take with the hydrogen doped ABO3 instead of PZNT single crystals and PZT ceramics in the following discussion for the sake of simplicity. In the crystal structure of ABO3, O(c) atoms in the dimerized B-O-B chains along the polarization c axis and O(ab) atoms in nondimerized B-O-B chains along the a and b axes, as marked by O(c) and O(ab) in Figure 6. In the cubic paraelectric phase of ABO3 crystals, O(c) atoms are equivalent to O(ab) atoms due to the crystal symmetry. Therefore, a hydrogen atom locates at (0.5, 0.25, 0.05) (R-site in Figure 6a) in the cubic cell and has the same possibility to form a strong bond with each of the oxygen atoms.35 In the tetragonal ferroelectric phase of ABO3 crystals, O(c) and O(ab) become distinguished. There might be four kinds of R-sites for hydrogen atoms: an H-O(c)-type bonding at RC and three kinds of H-O(ab) bonding sites, RD, RS, and RU, as shown in Figure 6b. First principles calculations on PbTiO3 show that the RD H-O(ab) site is the most stable site.36 If a hydrogen atom occupies the RC, RU, or RS site, the system energy will correspondingly increase 0.31, 0.54 or 0.08 eV in comparison with the energy for a hydrogen atom occupying the RD site. The results indicate that hydrogen prefers to bond to an oxygen atom along an undimerized B-O(ab) chain and that the direction of the H-O dipole is favorably aligned with the host polarization. This explains the fact that doping H-atoms with an appropriate concentration will improve the ferroelectric and piezoelectric properties.

Properties of PZNT Single Crystals

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9959

TABLE 3: Lattice Constants of H-Free and H-Charged PZT Ceramics Ct, wppm

a, Å

c, Å

c/a

hydrogen-free 0.4 1.1 1.7 2.1

4.036 4.037 4.039 4.045 4.057

4.092 4.114 4.152 4.053 4.058

1.014 1.019 1.028 1.002 1.000

The experimental results of Raman spectroscopy exhibit the presence of the OH- group in H-doped PZT ceramics.27,32 The OH- group might push the Ti4+ ion to move along the c upward direction slightly and the movement of Ti4+ ion along the c upward direction will enhance the electric dipole moment, meaning that the polarization increases.37 When H-atoms are doped into the crystals, the crystal constants will be changed. Recently, Zeches and his coworkers contribute that the epitaxial strain imposed by the substrate causes the BiFeO3 system morph into allotropic and ferroelectric properties change, which should motivate a search for similar control in other related perovskite systems.38 In present work, both materials are near their MPB and H-charging introduce an external stress to the samples. The XRD results show that the lattice constants of hydrogenfree PZNT single crystals are a ) b ) 4.034 Å and c ) 4.075 Å, and that of H-doped PZNT single crystals (Ct ) 0.8 wppm) are a ) b ) 4.045 Å and c ) 4.101 Å. The ratio c/a is a strong indicator of spontaneous polarization in a ferroelectric material, which increases from 1.010 to 1.014 when H-concentration increases from zero to Ct ) 0.8 wppm, thereby confirming the H-doping-improved ferroelectricity. The XRD results are consistent with the previous results and the first principles calculations.39-41 For PZT ceramics, the crystal constants of hydrogen-free and H-doped samples are shown in Table 3. Results show that the lattice constants of hydrogen-free PZT ceramics are a ) b ) 4.034 Å and c ) 4.075 Å, which are consistent with the previous results and the first principles calculations.42,43 Lattice constants of H-doped PZT ceramics (Ct ) 1.1 wppm) are a ) b ) 4.039 Å and c ) 4.152 Å. The ratio c/a increases from 1.014 to 1.028 when H-concentration increases from zero to Ct ) 1.1wppm, thereby confirming the H-doping-improved ferroelectricity. Further increasing the H-concentration, the c/a decreases to 1.002 (Ct ) 1.7 wppm) and 1.000 (Ct ) 2.1 wppm), which means the decrease of the ferroelectricity. The c/a versus the trapped hydrogen concentration is shown in Figure 2c, which has the same trend with the remanent polarization and piezoelectric constants. On the other hand, H-element is a shallow donor impurity in the ferroelectric crystals.36 When H-concentration is too high, the ferroelectric crystals will behave more like a conductor and the ferroelectric and piezoelectric properties will be degraded. Therefore, controlling H-concentration is crucial for the property improvement of ferroelectric materials. Conclusions In summary, doping hydrogen with appropriate content into PZNT single crystals and PZT ceramics improves greatly the ferroelectric and piezoelectric properties. This improvement seems to be permanent as evidenced by the experimental results on the samples aged at room temperature for 10 months after H-doped. Although further research is needed to explore the mechanism, the reported experimental results may already pave a way to improve the material properties

of ferroelectric ceramics by doping them with appropriate concentration of hydrogen. This method also has good feasibility in industry application. Acknowledgment. This work was supported by the NNSF of China under Grant Nos., 50632010, 50572006, IRT0509. H.Y.H. and T.-Y.Z. thank the support from Hong Kong University of Science and Technology through a Research Project Competition Grant, RPC07/08.EG17. References and Notes (1) Scott, J. F.; Paz de Araujo, C. A. Science 1989, 246, 1400. (2) Park, B. H.; Kang, B. S.; Bu, S. D.; Noh, T. W.; Lee, J.; Jo, W. Nature 1999, 401, 682. (3) Service, R. F. Science 1997, 275, 1878. (4) Frantti, J. J. Phys. Chem. B 2008, 112, 6521. (5) Yu, S. H.; Yao, K.; Tay, F. E. H. Chem. Mater. 2004, 16, 346. (6) Yu, S. H.; Yao, K.; Tay, F. E. H. Chem. Mater. 2007, 19, 4373. (7) Weitzing, H.; Schneider, G. A.; Steffens, J.; Hammer, M.; Hoffmann, M. J. J. Eur. Ceram. Soc. 1999, 19, 1333. (8) Lee, E. S.; Li, D. H.; Chung, H. W.; Lee, S. Y. J. Appl. Phys. 2006, 100, 4107. (9) Cross, J. S.; Kurihara, K.; Sakaguchi, I.; Haneda, H. J. Appl. Phys. 2006, 99, 4105. (10) Wu, Q.; Li, D.; Wu, L.; Wang, J.; Fu, X.; Wang, X. J. Mater. Chem. 2006, 16, 1116. (11) Huang, H. J.; Li, D. Z.; Lin, Q.; Shao, Y.; Chen, W.; Hu, Y.; Chen, Y. B.; Fu, X. Z. J. Phys. Chem. C 2009, 113, 14264. (12) Martin, C. R.; Aksay, I. A. J. Phys. Chem. B 2003, 107, 4261. (13) Longo, E.; Figueiredo, A. T. D.; Silva, M. S.; Longo, V. M.; Mastelaro, V. R.; Vieria, N. D.; Cilense, M.; Franco, R. W. A.; Varela, J. A. J. Phys. Chem. A 2008, 112, 8953. (14) Koduri, R.; Lopez, M. Eur. Phys. J. Appl. Phys. 2007, 37, 93. (15) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84. (16) Armelao, L.; Pascolini, M.; Bottaro, G.; Bruno, G.; Giangregerio, M. M.; Losurdo, M.; Malandrino, G.; Lo Nigro, R.; Fragala, M. E.; Tondello, E. J. Phys. Chem. C 2009, 113, 2911. (17) Heitsch, A. T.; Lee, D. C.; Korgel, B. A. J. Phys. Chem. C 2010, 114, 2512. (18) Tamura, T.; Matsuura, K.; Ashida, H.; Kondo, K.; Otani, S. Appl. Phy. Lett. 1999, 74, 3395. (19) Chon, U.; Kim, K. B.; Jang, H. M. Appl. Phy. Lett. 2001, 79, 2450. (20) Huang, H. Y.; Chu, W. Y.; Su, Y. J.; Qiao, L. J. J. Am. Ceram. Soc. 2007, 90, 2062. (21) Chen, W. P.; Li, L. T.; Wang, Y.; Gui, Z. L. J. Mater. Res. 1998, 13, 1110. (22) Cao, J. L.; Li, L. T.; Wang, Y. L.; Zhao, J. Q.; Gui, Z. L. Mater. Res. Bul. 2001, 36, 2103. (23) Wang, Y.; Chu, W. Y.; Qiao, L. J.; Su, Y. J. Mater. Sci. Eng. B 2002, 98, 1. (24) Peng, X.; Su, Y. J.; Gao, K. W.; Qiao, L. J.; Chu, W. Y. Mater. Lett. 2004, 58, 2073. (25) Meyer, R.; Waser, R.; Prume, K.; Schmitz, T.; Tiedke, S. Appl. Phys. Lett. 2005, 86, 142907. (26) Joo, H. J.; Lee, S. H.; Kim, J. P.; Ryu, M. K.; Jang, M. S. Ferroelectrics 2002, 272, 149. (27) Aggarwal, S.; Perusse, S. R.; Tipton, C. W.; Ramesh, R.; Drew, H. D.; Venkatesan, T.; Romero, D. B.; Podobedov, V. B.; Weber, A. Appl. Phys. Lett. 1998, 73, 1973. (28) Abdelghafar, K. K.; Miki, H.; Torii, K.; Fujisaki, Y. Appl. Phys. Lett. 1996, 69, 3188. (29) Shimamoto, Y.; Abdelghafar, K. K.; Miki, H.; Fujisaki, Y. Appl. Phys. Lett. 1997, 70, 3096. (30) Miki, H.; Abdelghafar, K. K.; Torii, K.; Fujisaki, Y. Jpn. J. Appl. Phys. 1997, 36, 1132. (31) Aggarwal, S.; Perusse, S. R.; Nagaraj, B.; Ramesh, R. Appl. Phys. Lett. 1999, 74, 3023. (32) Aggarwal, S.; Perusse, S. R.; Kerr, C. J.; Ramesh, R.; Romero, D. B.; Evans, J. T.; Boyer, L., Jr.; Velasquez, G. Appl. Phys. Lett. 2000, 76, 918. (33) Cao, J. L.; Li, L. T.; Zhao, J. C.; Wang, Y. L.; Gui, Z. L. Ceram. Int. 2001, 27, 895. (34) Wu, M.; Huang, H. Y.; Jiang, B.; Chu, W. Y.; Su, Y. J.; Li, J. X.; Qiao, L. J. J. Mater. Sci. 2009, 44, 5768. (35) Huang, H. Y.; Chu, W. Y.; Su, Y. J.; Li, J. X.; Qiao, L. J. Appl. Phys. Lett. 2006, 89, 142904. (36) Park, C. H.; Chadi, D. J. Phys. ReV. Lett. 2000, 84, 4717. (37) Xiong, K.; Robertson, J. Appl. Phys. Lett. 2004, 85, 2577.

9960

J. Phys. Chem. C, Vol. 114, No. 21, 2010

(38) Zeches, R. J.; Rossell, M. D.; Zhang, J. X.; Hatt, A. J.; He, Q.; Yang, C.-H.; Kumar, A.; Wang, C. H.; Melville, A.; Adamo, C.; Sheng, G.; Chu, Y.-H.; Ihlefeld, J. F.; Erni, R.; Ederer, C.; Gopalan, V.; Chen, L. Q.; Schlom, D. G.; Spaldin, N. A.; Martin, L. W.; Ramesh, R. Science 2009, 326, 977. (39) Gao, F.; Wang, C. J.; Liu, X. C.; Tian, C. S. Ceram. Int. 2007, 33, 1019.

Wu et al. (40) 96. (41) (42) (43)

Zhang, L.; Dong, M.; Ye, Z. G. Mater. Sci. Eng., B 2000, 78, Wensell, M.; Krakauer, H. J. Phys. Chem. Solids 2000, 61, 309. Boutarfaia, A. Ceram. Int. 2000, 26, 583. Freire, J. D.; Katiyar, R. S. Phys. ReV. B 1988, 37, 2074.

JP101463E