Lithium-Induced Phase Transitions in Lead-Free Bi0.5Na

Article pubs.acs.org/JPCC

Lithium-Induced Phase Transitions in Lead-Free Bi0.5Na0.5TiO3 Based Ceramics Giuseppe Viola,†,§ Ruth McKinnon,† Vladimir Koval,‡ Arturas Adomkevicius,† Steve Dunn,† and Haixue Yan*,†,§ †

School of Engineering and Materials Science, Queen Mary University of London, 380 Mile End Road, London E1 4NS, United Kingdom ‡ Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Kosice, Slovak Republic § Nanoforce Technology Ltd, 380 Mile End Road, London E1 4NS, United Kingdom ABSTRACT: Lithium-substituted 0.95[0.94(Bi0.5Na (0.5−x)Lix)TiO3−0.06BaTiO3]− 0.05CaTiO3 materials include the polar rhombohedral R3c and the weakly polar tetragonal P4bm phases. On increasing lithium content, the (R3c/P4bm) phase ratio decreased, while the rhombohedral and tetragonal lattice distortions remained the same. The temperature corresponding to the shoulder in the dielectric permittivity shows no clear shift with respect to lithium substitution because of the rhombohedral distortion remaining constant. Electrical poling produced an increase of the rhombohedral phase fraction together with a rise of the rhombohedral and tetragonal distortion. This confirmed the occurrence of a phase transition from the weakly polar to the polar phase during electrical poling. Four peaks found in the current−electric field (I−E) loops are related to reversible electric field induced transitions. By studying the temperature dependence of the current peaks in the I−E loops, it was found that the minimum temperature where these electric field induced transitions take place decreases with increasing lithium substitution. local and average structures are still under debate.20−22 In BNT-based systems, temperature variations and the application of an electric field induce modification of crystal structures and properties. Characteristic temperatures can be identified from the temperature dependence of dielectric permittivity and loss. The temperature Tm indicates the temperature corresponding to the maximum dielectric constant, and Ts corresponds to a visible shoulder in the dielectric permittivity above which the frequency dispersion significantly reduces. On the basis of early transmission electron microscopy (TEM) investigations, Ts does not correspond to any structural transition,17 although more recent studies suggested that Ts could be related to the appearance of a nonferroelectric phase with antiphase tilting.20 As described earlier, the structure of BNT-based materials is sensitive to the application of an electric field. According to insitu TEM studies on 0.91BNT−0.06BT−0.03KNN, the reversible appearance of ferroelectric domains can be observed by applying an alternating electric field,23 which is linked to the observed antiferroelectric-like P−E loops. The temperature range within which these reversible electric field induced transitions take place is dependent on composition. The widening of this temperature range could be beneficial for energy storage applications.5 For this purpose, in this paper, lithium (Li) was used to substitute sodium (Na) in

1. INTRODUCTION Bismuth-based perovskites have attracted worldwide research interest over recent years for the development of lead-free ferroelectric materials1−3 in a range of technological applications, including piezoelectric actuators and capacitors, because of their large electric field induced strain4 and antiferroelectriclike polarization-electric field (P−E) loops.5,6 Many different compositional modifications of the basic Bi0.5Na0.5TiO3 (BNT) have been studied, and significant progress on how to obtain desired properties by compositional variation has been made. For example, several solid solutions based on two end members have been investigated including Bi 0.5Na 0.5TiO3−BaTiO3 (BNT-BT),7 Bi0.5Na0.5TiO3−Bi0.5K0.5TiO3 (BNT-BKT),8 and Bi0.5Na0.5TiO3−K0.5Na0.5NbO3 (BNT-KNN),9 where the presence of morphotropic phase boundaries have been found, which generally determine an enhancement of dielectric and piezoelectric properties at the corresponding compositions. Further elemental additions led to the development of more complex solid solutions with Bi 0.5 Na 0.5 TiO 3 −BaTiO 3 − K 0.5 Na 0.5 NbO 3 (BNT-BT-KNN), 10−12 Bi 0.5 Na 0.5 TiO 3 − BaTiO3−Bi0.5K0.5TiO3 (BNT-BT-BKT),13,14 and Bi0.5Na0.5TiO3−BaTiO3−CaTiO3 (BNT-BT-CT)3 as indicative examples of three end member systems. The latter are becoming increasingly interesting because they offer more degrees of freedom to obtain desired properties that cannot be achieved from two end member systems. Bismuth sodium titanate was considered to be rhombohedral at room temperature,15−17 although more recent structural refinements suggest monoclinic symmetry Cc.18,19 However, the © 2014 American Chemical Society

Received: January 18, 2014 Revised: April 3, 2014 Published: April 3, 2014 8564

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Bi0.5Na0.5TiO3−BaTiO3−CaTiO3. Lithium is known to partially substitute Na in BNT to form a solid solution,24 with a promotion of a weakly polar tetragonal phase,25,26 which could result in a reduction of the temperature where the electric field induced transitions from weakly polar to polar phase start to take place with the presence of antiferroelectric-like loops. In BNT-based materials, the antiferroelectric-like loops were initially attributed to phase transitions between antiferroelectric and ferroelectric structures. However, there is no direct evidence to support antiferroelectric order using diffractions methods including X-ray diffraction (XRD) and TEM.15−18 The main objective of this work is to clarify the relationship between current peaks, antiferroelectric-like P−E loops, and electric field induced phase transitions in BNT-based materials.

3. RESULTS AND DISCUSSION The relative density of the ceramics was estimated as ρ = 97.2, 97.7, and 96.8% for x = 0, 0.05, and 0.15, respectively. The room-temperature XRD patterns of powders (Figure 1) and

2. EXPERIMENTAL SECTION Ceramic powders were prepared by a solid-state reaction process. Oxides and carbonates of the raw materials Bi2O3 (99.9% Sigma-Aldrich), TiO2 (99.8% Sigma-Aldrich), Na2CO3 (99.5% Alfa Aesar), BaCO3 (99.8% Alfa Aesar), CaCO3 (99.5% Alfa Aesar), and Li2CO3 (99.0% Alfa Aesar) were weighed according to the stoichiometric formula 0.95[0.94(Bi0.5Na(0.5−x)Lix)TiO3−0.06BaTiO3]−0.05CaTiO3 (x = 0, 0.05, and 0.15). The mixture was ball milled for 4 h in nylon pots using ethanol and zirconia balls. The slurry was then dried in air, was calcined at 850 °C for 4 h, and was ball milled again for 24 h to homogenize the particle size. After drying, the obtained powder was sieved through a 250 μm mesh. A coldpress was used to compact pellets which were then sintered at the optimized temperature of 1150 °C for 4 h in a conventional furnace. The density of the pellets was measured using the Archimedes’ method in deionized water. Room-temperature X-ray diffraction (XRD) was performed on calcined powders and ceramics using a Siemens D5000 diffractometer (Siemens AG, Karlsruhe, Germany) operating at 40 kV and 30 mA with Cu Kα radiation. Data from the X-ray diffractometer of powders were analyzed by the Rietveld method27 using the FullProf program.28 Silver paste (Gwent Electronic Materials Ltd., C2011004D5, Pontypool, U.K.) was homogeneously brushed and fired at 600 °C for 10 min to obtain smooth electrodes for electrical characterization. Impedance spectroscopy (frequency range 100 Hz to 1 MHz) was performed at room temperature using an impedance analyzer (Agilent 4294A, Hyogo, Japan). The temperature dependence of the dielectric constant and loss was measured from room temperature up to 600 °C by applying an alternating voltage of 1 V at five different frequencies in the range 1 kHz to 1 MHz, using an LCR meter (Agilent, 4284A, Hyogo, Japan) connected to a tube furnace with controlled temperature. The relative dielectric permittivity εr was calculated as εr = Cd/(Aε0), the dielectric loss tan δ was obtained as tan δ = R/Xc, and the capacitance C was calculated as C = 1/(2πf Xc), where d, A, ε0, f, R, and Xc are the sample thickness, the electroded area of the sample, the vacuum permittivity, the measuring frequency, and the real and imaginary part of the electrical impedance, respectively. Current−polarization−electric field (I−P−E) hysteresis loops were measured using a hysteresis tester (NPL, Teddington, U.K.) in a silicone oil bath at different temperatures in the range 25−150 °C using triangular voltage waveforms29 at a frequency of 10 Hz.

Figure 1. Room-temperature XRD patterns for 0.95[0.94(Bi0.5Na(0.5−x)Lix)TiO3−0.06BaTiO3]−0.05CaTiO3 powders: (a) x = 0, (b) x = 0.05, and (c) x = 0.15. The black circles are experimental data, the red line is the calculated profile from Rietveld refinement, and the blue line is the difference profile between the observed and calculated diffraction patterns. The allowed Bragg reflections for the corresponding space groups in a proposed structural model (R3c + P4bm) are indicated by green ticks. The insets illustrate the enlarged views of diffractograms in selected 2θ ranges to demonstrate the deconvolution of Bragg peaks.

ceramics (Figure 2) show perovskite single-phase structure, within the limit of XRD accuracy, for all the investigated compositions. The effect of poling on the crystal structure was investigated by comparing the XRD patterns of the as-sintered and poled ceramics. A thorough analysis of the diffraction spectra revealed that the observed XRD profiles corresponded to a superposition of two main structural components. For x = 8565

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R3c phase into a weakly polar state upon lithium addition. The refined lattice parameters are listed in Table 1. Table 1. Calculated Lattice Parameters of 0.95[0.94(Bi0.5Na(0.5−x)Lix)TiO3−0.06BaTiO3]− 0.05CaTiO3 Compounds (x = 0, 0.05, and 0.15) by Refinement of Room-Temperature XRD Data cell sample x=0 x = 0.05 x = 0.15

phase fraction (weight %)

a (Å)

c (Å)

c/a

R3c 93 P4bm 7 R3c 86 P4bm 14 R3c 43 P4bm 57

5.491(0) 5.534(1) 5.497(0) 5.504(2) 5.494(0) 5.502(0)

13.489(1) 3.889(1) 13.492(2) 3.968(4) 13.504(1) 3.911(1)

2.46(1) 0.70(1) 2.45(1) 0.72(2) 2.46(1) 0.71(1)

αr (deg) 59.87 59.91 59.85

In general, the substitution of Na with Li in BNT-BT-CT solid solutions leads to shrinkage of the tetragonal unit cell. However, lattice distortions remained almost constant upon substitution: ct/at ≈ 0.71 for the tetragonal phase and ch/ah ≈ 2.46 and αr ≈ 59.87° for the rhombohedral phase. The XRD peaks of BNT-based ceramics near 2θ = 40° (Figure 2) show significant variations after poling. Both rhombohedral and tetragonal distortions increased as evidenced by the XRD peaks’ shift to the lower angle side. In addition, according to the fitting, the fraction of rhombohedral phase increased after poling. Figure 3 shows the frequency dependence of the dielectric permittivity and the loss of three unpoled ceramics in the range

Figure 2. XRD of unpoled and poled ceramics with x = 0, 0.05, and 0.15.

0 (Figure 1a), the dominant spectral contribution (∼93%) is attributed to the presence of a rhombohedral phase (space group R3c) with unit cell parameters ah ≈ 5.491 Å and ch ≈ 13.489 Å in the hexagonal representation. The reflections of the minor phase (∼7%) were indexed in the tetragonal P4bm system with the lattice constants at ≈ 5.534 Å and ct ≈ 3.889 Å. It is well-known that the R3c structure is polar, with parallel displacements of the A- and B-site cations along the [111] direction, while the tetragonal P4bm is weakly polar.15 The term weakly polar, used to describe the tetragonal phase, represents a small, nonzero dipole moment, typical of a weakly polar ferrielectric order, which is locally produced by the nearequal displacement of the A- and B-site cations in opposite direction along the polar axis.15 The Rietveld refinement of the XRD patterns for the compounds (x = 0, 0.05, 0.15) revealed that the P4bm structure transforms gradually from a minor phase into a dominating one with increasing Li amount, suggesting a composition driven polar to weakly polar phase transition in the Li-substituted BNT-BT-CT compounds. Similar substitution-driven structural transitions from the polar R3c to antipolar and nonpolar phases have recently been reported, for example, in rare earth modified BiFeO3 multiferroics.30,31 The coexistence of the rhombohedral (43%) and tetragonal (57%) phases was observed for the x = 0.15 compounds (Figure 1c). The chemical substitution driven tilting of Ti−O octahedron associated with the rhombohedralto-tetragonal phase transition reduces the number of equivalent positions of the Bi3+/Na+/Ba2+/Ca2+/Li+ cations in the A-site and the Ti4+ cations in the B-site of the perovskite from 6 to 2 owing to a remarkable difference in ionic radius of substituting Li+ ion (∼1.24 Å)32 and substituted Bi3+/Na+/Ba2+/Ca2+ ions (∼1.34−1.61 Å).32,33 The tetragonal P4bm structure stabilizes Ti4+ ions at the center of its octahedron (reduced offcentering), resulting in a gradual transformation of the polar

Figure 3. Frequency dependence of the dielectric permittivity and loss highlighting the composition-induced structural transitions between the rhombohedral phase and the tetragonal phase.

100 Hz to 1 MHz. The dielectric constants of the three materials decreased with increasing frequency, which can be attributed to the fact that at higher frequencies fewer dipoles can follow the applied alternating electric field. The dielectric permittivity first increases and then decreases with increasing Li concentration for the entire range of frequency studied, while the loss shows similar values (Figure 3). The fraction of rhombohedral phase was estimated as 93%, 86%, and 43% in ceramics x = 0, 0.05, and 0.15, respectively. The maximum permittivity value observed in x = 0.05 is attributed to the most favorable coexistence of polar and weakly polar phases (among 8566

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Figure 4. Temperature dependence of dielectric permittivity and loss for (a) x = 0, (b) x = 0.05, and (c) x = 0.15 highlighting the compositioninduced phase transitions’ influence on the temperature dependence for the materials investigated.

reported for the system (0.935 − x)Bi 0.5 Na 0.5 TiO 3 − 0.065BaTiO3−xSrTiO3 which exhibits a structural modification from rhombohedral to pseudocubic with increasing x.36 Figure 5 shows the I−P−E loops of the three ceramics, generated at different temperatures in the range 25−150 °C with the application of an alternating electric field of 70 kV/cm and a frequency of 10 Hz. Ceramics with x = 0 and x = 0.05 behave as ferroelectrics up to 75 °C, as evidenced by the presence of domain switching current peaks at the coercive field Ec.29 The coercive field decreases with increasing temperature as typically occurs in ferroelectrics. In the temperature range T < 75 °C, the I−E loops of x = 0.15 ceramics show four current peaks at ±EF and ±EB. The amplitude dependence of the P−E and I−E loops of ceramic x = 0.15 at 25 °C shows a clear step change above a certain electric field amplitude (E > 50 kV/cm see Figure 6) above which the four current peaks appear (Figure 6b). The P−E loops generated when E ≤ 50 kV/cm do not lie within those generated at higher fields (see for instance the light blue curve in Figure 6a). This is thought to be due to the occurrence of an additional polarization mechanism at E > 50 kV/cm as previously reported.3 The multiple current peaks can be interpreted as follows: the subscripts F and B stand for forward and backward. In x = 0.15 ceramic at low temperature, the current peaks corresponding to ±EF and ±EB both appear during electrical loading. At cycling regime conditions, the polarization effects produced in correspondence with +EF and −EF are recovered during electric field reversal at −EB and +EB. This is suggested by the fact that the condition P = 0 is reestablished in the current valley between ±EB and ±EF.3 To further prove that the current peaks at ±EB only appear after the polar phase has been induced, two electric field bursts of 60 kV/cm and 10 Hz were applied on a virgin sample, and the current was monitored for the entire test. The correspondent I−E plot in Figure 6c shows that the peak at +EB is absent in the very first electrical cycle (interval a−b), while the current peaks at ±EB appear in the cycling regime conditions after the polar phase was induced at +EF in the very first cycle (interval

the studied compositions) with the polar phase being dominant. The ferroelectric domains in this ceramic easily vibrate under the AC field as the connections between the domains are weak, and these are thought to be surrounded by a weakly polar tetragonal phase. The lower permittivity of ceramic x = 0.15 compared to x = 0.05 can be related to the fact that the main phase is tetragonal P4bm which is weakly polar. Figure 4 shows the temperature dependence of permittivity and the loss of the three compositions from room temperature to 600 °C at five different frequencies in the range 1 kHz to 1 MHz. There is no clear shift of the broad permittivity peak at Ts for all three ceramics which is probably because all three materials investigated have a similar distortion in the rhombohedral polar phase (Table 1). It was previously proposed that the temperature dependence of the dielectric permittivity in BNT-based materials reflects a crossover between two dielectric relaxation processes with increasing temperature, which involve polar nanoregions of different phases and which have different temperature evolutions.34 The frequency dispersive anomaly at Ts can be associated with the relaxation of the polar nanoregion of the rhombohedral phase.34 The anomaly at Tm instead was deconvoluted in two different processes taking place with increasing temperature, which include two events: (a) the disappearance of the rhombohedral phase (frequency-independent anomaly in permittivity)34 and (b) the relaxation process of the polar nanoregions with tetragonal symmetry.34,35 The present dielectric data can be interpreted within this framework. Interestingly, the permittivity maximum becomes so much broader with increasing lithium content that the difference between the temperatures Ts and Tm can no longer be defined (Figure 4). This is related to the increase of tetragonal phase fraction with increasing lithium content in a way that the relaxation process of the rhombohedral polar nanoregion is progressively swamped in that of the tetragonal polar nanoregions, making the distinction of Ts and Tm more difficult from the dielectric data. Similar features have previously been 8567

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Figure 5. I−P−E loops of ceramics at different temperatures at applied field E = 70 kV/cm and 10 Hz frequency.

tetragonal phase reversibly transforms into a polar order during the application of the electric field. The temperature at which such transitions take place is related to the relative amount of polar and weakly polar phase initially present in the material, and this temperature decreases with increasing amount of tetragonal phase. In fact, according to our XRD analysis, the fraction of the weakly polar tetragonal phase P4bm increases with increasing lithium content in the compositions studied. This is the reason why in ceramic x = 0.15 the temperature at which the weakly polar-to-polar transitions take place is lower than in the case of x = 0 and x = 0.05.

a−b). The presence of multiple current peaks during electrical loading at ±EF and ±EB is also visible in ceramic x = 0 and x = 0.05 at 100 °C (Figure 4). With further temperature increase, the current peak corresponding to ±EB appears during electric field unloading indicating that the polarization effects produced by ±EF can be recovered during unloading. The threshold fields −EB (+EB) and +EF (−EF) move further away from each other with increasing temperature and the electric field induced transition becomes less hysteretic (the threshold fields ±EF and ±EB become closer). In the plots associated with ceramics x = 0.05, it can be clearly seen that ±EF and ±EB both increase with increasing temperature. In particular, the increase of ±EF with increasing temperature suggests that the polarization mechanisms taking place at ±EF become increasingly hindered with increasing temperature. These effects are all related to the increasing stability of the weakly polar phase with increasing temperature. The present results support the existence of electric field induced transitions in Li-doped BNT-based systems linked with electrical current peaks. The weakly polar

4. CONCLUSIONS In summary, the coexistence of polar rhombohedral R3c and weakly polar tetragonal P4bm phases was found in 0.95[0.94(Bi0.5Na(0.5−x)Lix)TiO3−0.06BaTiO3]−0.05CaTiO3 lead-free systems. The substitution of sodium for lithium increased the relative fraction of the weakly polar phase over the polar one. Both rhombohedral and tetragonal lattice distortions stayed at 8568

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REFERENCES

(1) Du, H.; Wohlrab, S.; Kaskel, S. Synthesis, Structure, and Properties of Bi3.25Pr0.75Ti2.97V0.03O12 Ferroelectric Ceramics. J. Phys. Chem. C 2007, 111, 11095−11103. (2) Atamanik, E.; Thangadurai, V. Dielectric Properties of Ga-Doped Na0.5K0.5NbO3. J. Phys. Chem. C 2009, 113, 4648−4653. (3) Lomax, J. F.; Fontanella, J. J.; Edmondson, C. A.; Wintersgill, M. C.; Westgate, M. A.; Eker, S. Size Effects Observed via the Electrical Response of BaTiO3 Nanoparticles in a Cavity. J. Phys. Chem. C 2012, 116, 23742−23748. (4) Zhang, S. T.; Kounga, A. B.; Jo, W.; Jamin, C.; Seifert, K.; Granzow, T.; Rodel, J.; Damjanovic, D. High-Strain Lead-Free Antiferroelectric Electrostrictors. Adv. Mater. 2009, 21, 4716−4720. (5) Viola, G.; Ning, H.; Reece, M. J.; Wilson, R.; Correia, T. M.; Weaver, P.; Cain, M.; Yan, H. Reversibility in Electric Field-Induced Transitions and Energy Storage Properties of Bismuth-Based Perovskite Ceramics. J. Phys. D: Appl. Phys. 2012, 45, 355302/1−355302/7. (6) Viola, G.; Ning, H.; Wei, X.; Deluca, M.; Adomkevicius, A.; Khaliq, J.; Reece, M. J.; Yan, H. Dielectric Relaxation, Lattice Dynamics and Polarization Mechanisms in Bi0.5Na0.5TiO3-Based Lead-Free Ceramics. J. Appl. Phys. 2013, 114, 014107/1−014107/9. (7) Xu, C.; Lin, D.; Kwok, K. W. Structure, Electrical Properties and Depolarization Temperature of (Bi0.5Na0.5)TiO3−BaTiO3 Lead-Free Piezoelectric Ceramics. Solid State Sci. 2008, 10, 934−940. (8) Hiruma, Y.; Nagata, H.; Takenaka, T. Phase Diagrams and Electrical Properties of (Bi1/2Na1/2)TiO3-Based Solid Solutions. J. Appl. Phys. 2008, 104, 124106/1−124106/7. (9) Kounga, A. B.; Zhang, S. T.; Jo, W.; Granzow, T.; Rödel, J. Morphotropic Phase Boundary in (1−x)Bi 0 . 5 Na 0 . 5 TiO 3 − xK0.5Na0.5NbO3 Lead-Free Piezoceramics. Appl. Phys. Lett. 2008, 92, 222902/1−222902/3. (10) Zhang, S. T.; Kounga, A. B.; Aulbach, E.; Ehrenberg, H.; Rödel, J. Giant Strain in Lead-Free Piezoceramics Bi0.5Na0.5TiO3−BaTiO3− K0.5Na0.5NbO3 System. Appl. Phys. Lett. 2007, 91, 112906/1−112906/ 3. (11) Zhang, S. T.; Kounga, A. B.; Aulbach, E.; Jo, W.; Granzow, T.; Ehrenberg, H.; Rödel, J. Lead-Free Piezoceramics with Giant Strain in the System Bi0.5Na0.5TiO3−BaTiO3−K0.5Na0.5NbO3. II. Temperature Dependent Properties. J. Appl. Phys. 2008, 103, 034108/1−034108/7. (12) Jo, W.; Granzow, T.; Aulbach, E.; Rödel, J. Origin of the Large Strain Response in K0.5Na0.5NbO3-Modified Bi0.5Na0.5TiO3−BaTiO3 Lead-Free Piezoceramics. J. Appl. Phys. 2009, 105, 094102/1−094102/ 5. (13) Nagata, H.; Yoshida, M.; Makiuki, Y.; Takenaka, T. Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate-Bismuth Potassium Titanate-Barium Titanate near the Morphotropic Phase Boundary. Jpn. J. Appl. Phys. 2003, 42, 7401− 7403. (14) Zhang, S. T.; Yang, B.; Cao, W. The Temperature-Dependent Electrical Properties of Bi0.5Na0.5TiO3−BaTiO3−Bi0.5K0.5TiO3 near the Morphotropic Phase Boundary. Acta Mater. 2012, 60, 469−475. (15) Jones, G. O.; Thomas, P. A. Investigation of the Structure and Phase Transitions in the Novel A-Site Substituted Distorted Perovskite Compound Na0.5Bi0.5TiO3. Acta Crystallogr., B 2002, 58, 168−178. (16) Dorcet, V.; Trolliard, G.; Boullay, P. Reinvestigation of Phase Transitions in Na0.5Bi0.5TiO3 by TEM. Part I: First Order Rhombohedral to Orthorhombic Phase Transition. Chem. Mater. 2008, 20, 5061−5073. (17) Ma, C.; Tan, X. In Situ Transmission Electron Microscopy Study on the Phase Transitions in Lead-Free (1−x)(Bi1/2Na1/2)TiO3− xBaTiO3 Ceramics. J. Am. Ceram. Soc. 2011, 94, 4040−4044. (18) Gorfman, S.; Thomas, P. A. Evidence for a Non-Rhombohedral Average Structure in the Lead-Free Piezoelectric Material Na0.5Bi0.5TiO3. J. Appl. Crystallogr. 2010, 43, 1409−1414. (19) Aksel, E.; Forrester, J. S.; Jones, J. L.; Thomas, P. A.; Page, K.; Suchomel, M. R. Monoclinic Crystal Structure of Polycrystalline Na0.5Bi0.5TiO3. Appl. Phys. Lett. 2011, 98, 152901/1−152901/3.

Figure 6. (a) P−E and (b) I−E loops of ceramic x = 0.15 at room temperature at different electric field amplitudes and 10 Hz frequency. (c) The absence of EB in the first electrical cycle (interval a−b) at E = 60 kV/cm.

the same level with increasing lithium concentration. Four current peaks in I−E loops can be attributed to field-induced transitions from weakly polar to polar phases and from polar to weakly polar phases. The onset temperature of the electric field induced transitions decreased with increased lithium content. This decrease is a result of an increased fraction of weakly polar phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Vladimir Koval would like to acknowledge The Grant Agency of the Slovak Academy of Sciences for financial support on Projects No. 2/0053/11 and No. 2/0057/14. 8569

dx.doi.org/10.1021/jp500609h | J. Phys. Chem. C 2014, 118, 8564−8570

The Journal of Physical Chemistry C

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(20) Levin, I.; Reaney, I. M.; Anton, E. M.; Jo, W.; Rodel, J.; Pokorny, J.; Schmitt, L. A.; Kleebe, H. J.; Hinterstein, M.; Jones, J. L. Local Structure, Pseudosymmetry, and Phase Transitions in Na1/2Bi1/2TiO3−K1/2Bi1/2TiO3 Ceramics. Phys. Rev. B 2013, 87, 024113/1−024113/11. (21) Keeble, D. S.; Barney, E. R.; Keen, D. A.; Tucker, M. G.; Kreisel, J.; Thomas, P. A. Bifurcated Polarization Rotation in Bismuth-Based Piezoelectrics. Adv. Funct. Mater. 2013, 23, 185−190. (22) Rao, B. N.; Fitch, A. N.; Ranjan, R. Ferroelectric−Ferroelectric Phase Coexistence in Na1/2Bi1/2TiO3. Phys. Rev. B 2013, 87, 060102(R)/1−060102(R)/5. (23) Kling, J.; Tan, X.; Jo, W.; Kleebe, H. J.; Fuess, H.; Rödel, J. In Situ Transmission Electron Microscopy of Electric Field-Triggered Reversible Domain Formation in Bi-Based Lead-Free Piezoceramics. J. Am. Ceram. Soc. 2010, 93, 2452−2455. (24) Lin, D. M.; Xiao, D. Q.; Zhu, J. G.; Yu, P. Piezoelectric and Ferroelectric Properties of [Bi0.5(Na1−x−yKxLiy)0.5]TiO3 Lead-Free Piezoelectric Ceramics. Appl. Phys. Lett. 2006, 88, 062901/1− 062901/3. (25) Choy, S. H.; Wang, X. X.; Chan, H. L. W.; Choy, C. L. Electromechanical and Ferroelectric Properties of (Bi1/2Na1/2)TiO3(Bi1/2K1/2)TiO3-(Bi1/2Li1/2)TiO3-BaTiO3 Lead-Free Piezoelectric Ceramics for Accelerometer Application. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 775−781. (26) Lei, N.; Zhu, M.; Yang, P.; Wang, L.; Wang, L.; Hou, Y.; Yan, H. Effect of Lattice Occupation Behavior of Li+ Cations on Microstructure and Electrical Properties of (Bi1/2Na1/2)TiO3-Based LeadFree Piezoceramics. J. Appl. Phys. 2011, 109, 054102/1−054102/6. (27) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (28) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Physica B 1993, 192, 55−69. (29) Yan, H.; Inam, F.; Viola, G.; Ning, H.; Zhang, H.; Jiang, Q.; Zhang, T.; Gao, Z.; Reece, M. J. The Contribution of Electrical Conductivity, Dielectric Permittivity and Domain Switching in Ferroelectric Hysteresis Loops. J. Adv. Dielectrics 2011, 1, 107−118. (30) Koval, V.; Skorvanek, I.; Reece, M. J.; Mitoseriu, L.; Yan, H. Effect of Dysprosium Substitution on Crystal Structure and Physical Properties of Multiferroic BiFeO3 Ceramics. J. Eur. Ceram. Soc. 2014, 34, 641−651. (31) Khomchenko, V. A.; Shvartsman, V. V.; Borisov, P.; Kleemann, W.; Kiselev, D. A.; Bdikin, I. K.; Vieira, J. M.; Kholkin, A. L. Effect of Gd Substitution on the Crystal Structure and Multiferroic Properties of BiFeO3. Acta Mater. 2009, 57, 5137−5145. (32) Tai, C. W.; Choy, S. H.; Chan, H. L.W. Ferroelectric Domain Morphology Evolution and Octahedral Tilting in Lead-Free (Bi1/2Na1/2)TiO3−(Bi1/2K1/2)TiO3−(Bi1/2Li1/2)TiO3−BaTiO3 Ceramics at Different Temperatures. J. Am. Ceram. Soc. 2008, 91, 3335−3341. (33) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides. Acta Crystallogr., Sect. A 1976, 32, 751−767. (34) Jo, W.; Schaab, S.; Sapper, E.; Schmitt, L. A.; Kleebe, H. J.; Bell, A. J.; Rodel, J. On the Phase Identity and Its Thermal Evolution of Lead Free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3. J. Appl. Phys. 2011, 110, 074106/1−074106/9. (35) Kling, J.; Jo, W.; Dittmer, R.; Schaab, S.; Kleebe, H. J. Temperature-Dependent Phase Transitions in the Lead-Free Piezoceramics (1 − x − y)(Bi1/2Na1/2)TiO3−xBaTiO3−y(K0.5Na0.5)NbO3 Observed by in Situ Transmission Electron Microscopy and Dielectric Measurements. J. Am. Ceram. Soc. 2013, 96, 3312−3324. (36) Wang, F.; Xu, M.; Tang, Y.; Wang, T.; Shi, W.; Leung, M. C. Large Strain Response in the Ternary Bi0.5Na0.5TiO3−BaTiO3−SrTiO3 Solid Solutions. J. Am. Ceram. Soc. 2012, 95, 1955−1959.

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dx.doi.org/10.1021/jp500609h | J. Phys. Chem. C 2014, 118, 8564−8570