Ergodicity and Nonergodicity in La-doped Bi 1/2 (Na 0.82 K 0.18 ) 1/2 TiO 3 Relaxors

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1 Journal of the Korean Physical Society, Vol. 66, No. 7, April 2015, pp Ergodicity and Nonergodicity in La-doped Bi 1/2 (Na 0.82 K 0.18 ) 1/2 TiO 3 Relaxors Thi Hinh Dinh, Hyoung-Su Han and Jae-Shin Lee School of Materials Science and Engineering, University of Ulsan, Ulsan , Korea Chang-Won Ahn and Ill-Won Kim Department of Physics, University of Ulsan, Ulsan , Korea Mohammad Reza Bafandeh Department of Metallurgy, University of Kashan, Kashan, Iran (Received 28 April 2014, in final form 26 November 2014) The phase transition of La-doped [Bi 1/2 (Na 0.82K 0.18) 1/2 ]TiO 3 (BNKT) ceramics was investigated by using high-temperature X-ray diffraction and temperature-dependent dielectric measurements. Undoped BNKT was found to be a nonergodic relaxor, which was evidenced by the presence of a depolarization temperature below which polar nanoregions were frozen. However, La-doped BNKT ceramics are believed to be composites consisting of ergodic and nonergodic relaxors. The results suggest that a nonergodic relaxor with tetragonal symmetry might be distributed in an ergodic relaxor matrix with pseudocubic symmetry. PACS numbers: j, d, Je, n Keywords: Lead-free piezoelectric ceramics, Relaxor, Phase transition, La doping DOI: /jkps I. INTRODUCTION Recently, lead-free piezoelectric ceramics have attracted much attention due to the increasing demand for environmentally friendly materials in the electronics industry [1 3]. Among them, Bi 1/2 Na 1/2 TiO 3 - Bi 1/2 K 1/2 TiO 3 (BNT-BKT) solid solutions have been intensively studied since the first report on this system by C. F. Buhrer [4] in Later, Pronin et al. [5] studied the crystal structure of the [Bi 1/2 (Na 1 x K x ) 1/2 ]TiO 3 (BNKT) system and found phase transitions between rhombohedral (x<0.18), pseudocubic (x = ), and tetragonal (x >0.40) phases as a function of the BKT content. In 1999, Sasaki et al. [6] reported the dielectric and the piezoelectric properties of the BNKT system. Their X- ray investigation showed the existence of a morphotropic phase boundary (MPB) between rhombohedral BNT and tetragonal BKT near x = The electromechanical coupling factor and the dielectric constant peaked at the MPB, resulting in k p = 31.4%, k t = 42.3% at x =0.16and(ε T 33)/ε o = 1030 at x = Yoshii et al. [7] also reported that the highest electromechanical coupling factor (k 33 ) and piezoelectric constant (d 33 )were jslee@ulsan.ac.kr; Tel: ; Fax: obtained when x = In addition, Izumi et al. [8] prepared BNKT single crystals within the composition range of x = by using a flux method and reported the polarization hysteresis and the electricfield-induced strain (EFIS) for this system. They found that a small amount of BKT substitution suppressed the remnant polarization (P r )from38μc/cm 2 at x =0 down to 15 μc/cm 2 at x = 0.02 and that the piezoelectric strain constant (d 33) was enhanced up to 297 pm/v at x = Recently, a giant electric-field-induced strain (EFIS) was reported by Zhang et al. [9] in Bi 1/2 Na 1/2 TiO 3 BaTiO 3 (K 1/2 Na 1/2 )NbO 3 (BNT-BT-KNN) ceramics. Thenceforth, many modified BNT-based systems were found to show excellent piezoelectric responses: BNKT BiAlO 3 [10] and BNKT LiSbO 3 [11] for their ferroelectric and piezoelectric properties; Nbdoped BNKT [12], Ta-doped BNKT [13], Sn-doped BNKT [14, 15], Li- and Ta-codoped BNKT [16], and BNKT-LaFeO 3 [17] for their EFIS properties. One of the most interesting features in these systems is that a giant EFIS is observed above a certain threshold electric field involving a considerably large hysteresis and it is only available when macroscopic piezoelectricity nearly vanishes [18]. From this background, these BNT-based materials at the relaxor-ferroelectric phase boundary might be classified as incipient piezoceramics [18], in which the materials is not macroscopically piezoelectric but be-

2 Journal of the Korean Physical Society, Vol. 66, No. 7, April 2015 comes piezoelectric when an electric field is applied. More recently, a giant unipolar strain of 715 pm/v was found for the 3 mol% La-doped BNKT system by our group [19]. However, the phase transitions in that system are not yet known. Therefore, we studied the crystal structure and the dielectric properties of La-doped BNKTasafunctionoftemperature.Inparticular,such properties were compared before and after poling treatment. II. EXPERIMENTS Ceramic specimens were prepared by using a conventional ceramic fabrication process. Commerciallyavailable reagent-grade metal-oxide and carbonate powders of Bi 2 O 3, Na 2 CO 3, K 2 CO 3, TiO 2, and La 2 O 3 (99.9%, High Purity Chemicals, Japan) were weighed according to the chemical formula [Bi 1/2 (Na 0.82 K 0.18 ) 1/2 ] 1 x La x TiO 3 (La100x; x = ). The powders were mixed in ethanol with zirconia balls by using ball-milling for 24 h, dried at 80 C for 24 h, and calcined at 850 C for 2 h in an alumina crucible. After calcination, the powder was mixed with polyvinyl alcohol as a binder and was then compacted into green discs with diameters of 12 mm under a uniaxial pressure of 98 MPa. These green pellets were sintered at 1175 C in covered alumina crucibles for 2 h in air. The sintered discs first were lapped to a thickness of 0.5 mm and annealed at 850 C for 30 min to remove any stress from mechanical treatment. A silver paste was screen-printed on both sides of the specimen and subsequently fired at 700 C for 30 min and was then poled in a silicone oil bath at room temperature under a direct current field of 50 kv/cm for 15 min (poled samples). In order to remove the poling effect, a group of samples were heated for 5 h at 850 C (unpoled samples). The electrical polarization (P ) and the electromechanical strain (S) as functions of the external electric field (E) were measured at 0.3 Hz with a 15-μF measurement capacitance by using a modified Sawyer-Tower circuit equipped with an optical sensor (Philtec, Inc., Annapolis, MD, USA). The crystal structures of the poled and the unpoled samples were analyzed with an X-ray diffractometer (XRD, RAD III, Rigaku, Japan) at room temperature. High-temperature X-ray diffraction analyses were carried out with an Ultima IV X-ray diffractometer (Multi Purpose XRD, H-2, Rigaku, Japan). The temperature-dependent dielectric properties of both poled and unpoled specimens were characterized by using an impedance analyzer (HP4192A, Agilent, CA) attached to a computer programmable electric furnace at different frequencies ( MHz) in the temperature range of C under a heating and cooling rate of 2 C/min. Fig. 1. (Color online) (a) P -E and (b) S-E hysteresis loops of La100x ceramics (x =0 0.05). III. RESULTS AND DISCUSSION Figure 1 displays the polarization hysteresis loops and bipolar strain curves of La100x ceramics with x = Undoped BNKT exhibited strong ferroelectricity that could be rationalized by using a definite P r and coercive field (E c )inthep-e loop of Fig. 1(a) and a butterfly-shaped strain curve in Fig. 1(b) [15, 17]. In contrast, as the La doping level was increased, gradual changes in the polarization and the bipolar strain hysteresis loops were observed. More specifically, the maximum polarization (P max ), maximum strain (S max ), remnant polarization (P r ), and negative strain (S neg )decreased with increasing x, indicating that the addition of La suppressed a field-induced ferroelectric order. Interestingly, La doping brought about a significant enhancement in the attainable strain, similarly to previous works [15,17,20]. The disruption of the field-induced ferroelectric order was accompanied by a giant bipolar strain of 650 pm/v in La3, which was believed to be a distinct phenomenon in the incipient piezoelectric material [10, 17]. The X-ray diffraction (XRD) patterns of both unpoled

3 Ergodicity and Nonergodicity in La-doped Bi1/2 (Na0.82 K0.18 )1/2 TiO3 Relaxors Thi Hinh Dinh et al Fig. 2. (Color online) X-ray diffraction patterns of {111} and {200} reflections for (a) unpoled and (b) poled La100x (x = ) ceramics. and poled samples are contrasted in Fig. 2, which reveals a perovskite structure without any traces of secondary or impurity phases in any of the compositions. In both the unpoled and the poled states, undoped BNKT showed the coexistence of rhombohedral and tetragonal phases, in agreement with the results of previous reports [12 17]. When the La content was increased, the ferroelectric order was decreased by a phase transition to a pseudocubic phase, which was regarded as a relaxor because large strains were obtained under high electric fields, as seen in Fig. 1(b). A phase transition from the coexistence of rhombohedral and tetragonal phases to a pseudocubic phase was also seen to happen at x = 0.03 and x = 0.04 for the unpoled and the poled samples, respectively. In comparison with unpoled samples, poled specimens with x = showed irrecoverable changes in the XRD patterns while the poling treatment had little influence on the XRD pattern for La4 and La5 samples. For undoped BNKT, the poled sample showed an increased (111) peak intensity while the (11 1) peak intensity was decreased, indicating domain reorientation in the rhombohedral phase during the poling process. As the La doping level was increased, such a rhombohedral distortion was weakened, which was confirmed by the merging of the {111} peak split into a pseudocubic (111) reflection at higher doping levels, as seen in Fig. 2. Figure 3 shows temperature-dependent X-ray diffraction patterns for the La0, La2, and La4 samples. The results showed that the crystal structure of the BNKT ceramics was affected not only by La doping but also by temperature. Interestingly, the effect of La doping on the phase transition of BNKT was very similar to that of elevating temperature. Undoped BNKT in Fig. 3(a) revealed the coexistence of rhombohedral and tetragonal phases at room temperature. As the temperature was increased up to 200 C, the rhombohedral phase disappeared, leaving only a tetragonal phase. A further increase in temperature above 250 C resulted in Fig. 3. (Color online) Temperature-dependent X-ray diffraction patterns of La100x ceramics: (a) x = 0, (b) x = 0.02, and (c) x = a pseudocubic phase being developed, which could be deduced by the concurrent lack of peak splits in both of the {111} and the {200} reflections. La doping also induced a phase transition from the coexistence of rhombohedral and tetragonal phases to a tetragonal phase at room temperature, which was found by comparison of the room-temperature XRD patterns of three samples in Fig. 3. On the basis of the results obtained from the P (E) and the S(E) hysteresis loops (Fig. 1) and the temperature-dependent XRD data, we were able to conclude that both La doping and heating promoted the ferroelectric-to-relaxor phase transition in BNKT. To clarify the relaxor behavior in La-doped BNKT, we characterized the temperature-dependent dielectric properties at various La doping levels and frequencies. Figure 4 compares the frequency-dependent dielectric constant measured at temperatures from room temperature up to 400 C for the La0, La2, and La4 samples before and after poling. Regardless of the poling treatment, all samples showed a strong frequencydependence in their dielectric spectra, implying that Ladoped BNKT ceramics could be categorized as relaxors. Unpoled La0 revealed two distinct dielectric anomalies as seen in Fig. 4(a): a broad maximum in the dielectric permittivity at around 275 C and a low temperature shoulder at 175 C. Jo et al. [21] explained the presence

4 Journal of the Korean Physical Society, Vol. 66, No. 7, April 2015 the poled and the unpoled states in Fig. 4(a) clearly demonstrates the presence of a depolarization temperature at 160 C, where the electric-field-induced longrange order breaks down into a short-range one [22,23]. This result shows that La0 is a nonergodic relaxor. We should note that the determination of T d is only valid for poled specimens [7, 15, 17, 21, 24, 25]. On the other hand, the absence of T d in La2 and La4 in Figs. 4(b) and (c) characterize them as ergodic relaxors, in which the field-induced changes are recovered back to their initial state on removal of the electric field [26]. Although the long-range ordered domains could not be detected in the poled states of the La2 (Fig. 4(b)) and the La4 (Fig. 4(c)) samples, but the poling treatment in La2 and La4 still induced an irreversible change at temperatures below 300 C. The difference in the dielectric spectra between the poled and the unpoled states strongly suggests a possibility that frozen PNRs co-exist in the La2 and La4 samples after poling, as in 2-mol% LaFeO 3 -modified BNKT [17]. Hence, we believe that La-modified BNKT ceramics are mixtures of an ergodic and a nonergodic relaxor. Based upon the XRD data shown in Fig. 3, a nonergodic relaxor with a tetragonal symmetry, which is a minor phase, might be distributed in an ergodic relaxor matrix with a pseudocubic symmetry. IV. CONCLUSION The phase transition of La-doped BNKT after poling and depoling was investigated by using high-temperature XRD and temperature-dependent dielectric measurements. Undoped BNKT was found to be a nonergodic relaxor, which showed frozen PNRs at room temperature. However, La-doped BNKT ceramics are believed to be relaxor composites, in which a nonergodic relaxor with a tetragonal symmetry might be distributed in an ergodic relaxor matrix with a pseudocubic symmetry. ACKNOWLEDGMENTS Fig. 4. (Color online) Frequency and temperature dependences of the dielectric constant for poled and unpoled La100x ceramics with (a) x =0,(b)x = 0.02, and (c) x = This work was financially supported by the 2013 Research Fund of the University of Ulsan. REFERENCES of the two anomalies in the BNT-BT system by assuming the coexistence of two different polar nanoregions (PNRs) with different crystal symmetries [21]. More recently, Han et al. [17] provided experimental data for LaFeO 3 -modified BNKT that supported the coexistence of two PNRs at the same time in a single specimen. A comparison of the dielectric spectra of La0 between [1] T. Takenaka and H. Nagata, J. Eur. Ceram. Soc. 25, 2693 (2005). [2] T. R. Shrout and S. J. Zhang, J. Electroceram. 19, 111 (2007). [3] J. Rödel, W. Jo, K. T. P. Seifert, E-M. Anton, T. Granzow and D. Damjanovic, J. Am. Ceram. Soc. 92, 1153 (2009). [4] C. F. Buhrer, J. Chem. Phys. 36, 798 (1962).

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