Scholarly article on topic 'Variation of ferroelectric hysteresis loop with temperature in (SrxBa1−x)Nb2O6 unfilled tungsten bronze ceramics'

Variation of ferroelectric hysteresis loop with temperature in (SrxBa1−x)Nb2O6 unfilled tungsten bronze ceramics Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Chen Jiao Huang, Kun Li, Shu Ya Wu, Xiao Li Zhu, Xiang Ming Chen

Abstract The ferroelectric switching properties of (Sr x Ba1−x )Nb2O6 (x = 0.25, 0.5, 0.75) unfilled tungsten bronze ceramics have been investigated over a broad temperature range. At room temperature, the saturated polarization-electric field (P-E) loop is determined for x = 0.25, and it becomes slimmer with increasing x, which indicates the evolution from normal ferroelectric to relaxor ferroelectric. Studies on domain structure by piezoresponse force microscopy (PFM) well confirm this conclusion. Under the same electric field for evaluating P-E loops, both the maximum and remanent polarizations tend to decrease and vanish at low temperatures for all compositions. This phenomenon is quite like the re-entrant relaxor behavior reported in some perovskite-type ferroelectrics. However, with increasing the electric field, the saturated P-E loops can be obtained at low temperatures for all compositions. This suggests that the decline of switchable polarization at low temperatures in (Sr x Ba1−x )Nb2O6 ceramics is due to the increase of coercive field rather than the decrease of the long-rang interaction of structural dipoles on cooling.

Academic research paper on topic "Variation of ferroelectric hysteresis loop with temperature in (SrxBa1−x)Nb2O6 unfilled tungsten bronze ceramics"

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Journal of Materiomics xx (2015) 1—6

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Variation of ferroelectric hysteresis loop with temperature in (SrxBai_x) Nb2O6 unfilled tungsten bronze ceramics

Chen Jiao Huang, Kun Li, Shu Ya Wu, Xiao Li Zhu*, Xiang Ming Chen*

Laboratory of Dielectric Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 28 December 2014; revised 7 January 2015; accepted 20 February 2015 Available online ■ ■ ■

Abstract

The ferroelectric switching properties of (SrxBa1_x)Nb2O6 (x = 0.25, 0.5, 0.75) unfilled tungsten bronze ceramics have been investigated over a broad temperature range. At room temperature, the saturated polarization-electric field (P-E) loop is determined for x = 0.25, and it becomes slimmer with increasing x, which indicates the evolution from normal ferroelectric to relaxor ferroelectric. Studies on domain structure by piezoresponse force microscopy (PFM) well confirm this conclusion. Under the same electric field for evaluating P-E loops, both the maximum and remanent polarizations tend to decrease and vanish at low temperatures for all compositions. This phenomenon is quite like the re-entrant relaxor behavior reported in some perovskite-type ferroelectrics. However, with increasing the electric field, the saturated P-E loops can be obtained at low temperatures for all compositions. This suggests that the decline of switchable polarization at low temperatures in (SrxBa!_x) Nb2O6 ceramics is due to the increase of coercive field rather than the decrease of the long-rang interaction of structural dipoles on cooling. © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: (SrxBa1_x)Nb2O6; Ferroelectric hysteresis loop; Low temperature

1. Introduction

Tetragonal tungsten bronze (TTB) ceramics are an exciting class of materials with a huge diversity of properties due to their complex and tunable crystal structures. The TTB structure can be regarded as a corner sharing BO6 octahedra which create three different interstitial sites (pentagonal A1, square A2 and trigonal C sites) available for different cation inclusion [1,2]. (SrxBa1_x)Nb2O6 is one of the most studied ferroelectric materials with this structure [3—5]. In (SrxBa1_x)Nb2Og, there are five Sr or Ba ions for six A1 and A2 positions, thus one of the A-sites remains empty. (SrxBa1_x)Nb2O6 exhibits interesting dielectric and ferroelectric behaviors. Studies have shown that on increase of the Sr/Ba ratio, a transformation

* Corresponding authors. E-mail addresses: xiaolizi0618@zju.edu.cn (X.L. Zhu), xmchen59@zju. edu.cn (X.M. Chen).

Peer review under responsibility of The Chinese Ceramic Society.

from ferroelectric to relaxor behavior takes place [6,7]. We have revealed that the A1/A2-sites occupancy is the primary parameter governing this variation tendency and the incommensurate oxygen octahedral tilting and A-site random distribution are considered to be the structure origins for the relaxor ferroelectricity and low temperature dielectric relaxations [8].

On the other hand, so-called re-entrant relaxor behavior was observed in some perovskite-type materials such as 0.99BaTi03-0.01AgNb03 [9] and (1-x)BaTiO3-xBiScO3 solid solutions [10—12], which was characterized by the dielectric anomalies under the ferroelectric transition temperature and the decrease of switchable polarization on cooling. This behavior is always ascribed to a decrease of the long-rang interaction of structural dipoles, but the origin behind it has not been well understood. For tungsten bronze ferroelectrics, some subtle structural effects, including the oxygen octahedron tilting [13,14], the random cation distribution in A sites [15—17] and A-site vacancy [1] would disturb the ferroelectric

http://dx.doi.org/10.1016/j.jmat.2015.02.004

2352-8478/© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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■30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30

E (kV/cm) E (kV/cm) E (kV/cm)

Fig. 1. Polarization-electric field (P—E) and current-electric field (J—E) curves of 10 Hz at various temperatures for SrxBai_xNb2O6 ceramics: (a) x = 0.25, (b) x = 0.50, (c) x = 0.75.

Fig. 2. Temperature dependence of remanent polarization (Pr) and maximum polarization (Pmax) of SrxBa1_xNb2O6 ceramics: (a) x = 0.25, (b) x = 0.50, (c) x = 0.75.

long-range ordering, which seems to make the re-entrant relaxor behavior easier to occur. Actually, the re-entrant phenomenon has been reported in some filled tungsten bronzes such as Sr2NaNb5O15 [18] and Ba5RTi3Nb7O30 (R = La, Nd, Sm) [19].

As mentioned above, (SrxBa1_x)Nb2Og ceramics exhibit composition dependent dielectric and ferroelectric behavior, changing from normal ferroelectric to relaxor ferroelectric with increasing x. An alternative important issue for these materials is to investigate the switchable polarization at low temperatures, and then to understand the evolution tendency of ferroelectric ordering with decreasing temperature and to determine the possible re-entrant relaxor behavior.

In the present work, the ferroelectric hysteresis loop of (SrxBa1_x)Nb2Og (x = 0.25, 0.5, 0.75) ceramics have been investigated over a broad temperature range, and the effects of electric field upon the ferroelectric hysteresis loop are determined to distinguish the re-entrant relaxor behavior and the

phenomena due to the increased coercive field with decreasing temperature. The domain structures have been also studied by the piezoresponse force microscopy in order to understand the ferroelectric behavior.

2. Experimental procedure

(SrxBa1_x)Nb2Og ceramics with x = 0.25, x = 0.50 and x = 0.75 were prepared by a standard solid state reaction process, and the details of the preparation process could be found elsewhere [8]. The variation of ferroelectric polarization with temperature was evaluated by a precision materials analyzer (RT Premier II, Radient Technologies, Inc., Albuquerque, New Mexico). The behaviors of polarization and domain back-switching of (SrxBa1_x)Nb2Og ceramics were characterized using a commercial piezoresponse force microscopy (Cypher S, Asylum Research, Santa Barbara, California). A Pt-coated cantilever (Olympus AC240, nominal spring

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Fig. 3. PFM domain patterns of SrxBa1_xNb2O6 ceramics: (a) x = 0.25, (b) x = 0.50, (c) x = 0.75. The left, middle and right columns are the topography, amplitude, and phase images, respectively. Top to bottom rows correspond to compositions with x = 0.25, 0.50, and 0.75.

constant ~2 N/m, resonant frequency ~70 kHz) was used with a scanning rate of 2 Hz.

3. Results and discussion

Fig. 1 shows the ferroelectric hysteresis loops (P-E loops) and the current electric field curves (J-E curves) of the present ceramics at different temperatures under the same electric field. At room temperature, the saturated polarization-electric field (P-E ) loop is determined for x = 0.25, and it becomes slimmer with increasing x, which indicates the evolution process from normal ferroelectric to relaxor ferroelectric. The electric peaks appearing at the coercive fields in the J-E curves suggest the contribution of ferroelectric domain switching [20]. For each composition, as temperature decreases, the hysteresis loop becomes fatter and exhibits stronger nonlinear response with more obvious electric peak in the J-E curve. However, on cooling to the temperature far below Tm (Tm is the temperature at which the maximum dielectric constant reaches [8]), the hysteresis loops become slim again and exhibit almost linear response for all compositions. Moreover, in the J-E curves, the domain switching current peaks almost disappear, and it seems that the ferroelectric character is losing at such low temperatures.

The maximum polarization and the remanent polarization of (SrxBa1_x)Nb2O6 ceramics as a function of temperature are given in Fig. 2. Under the same electric field, both the maximum polarization (Pmax) and the remanent polarization (Pr) gradually increase at first, and then they begin to decrease after they reach the maximum value and almost vanish at low temperatures for all compositions. The anomaly high values of Pmax and Pr for x = 0.25 are due to the increase of the leakage current at high temperature [21]. The variation of polarization with temperature seems to be quite like the re-entrant relaxor behavior reported in the perovskite-type ferroelectrics [9—12].

Fig. 3 shows the domain structure of (SrxBa1_x)Nb2Og ceramics imaged by PFM. As illustrated in the images, the average domain size of x = 0.5 is smaller than that of x = 0.25, which corresponds with the increasing relaxor behavior and a strengthening of random fields with increasing Sr content. Although the domain structure of x = 0.75 has been detected in the single crystals [22,23], there is almost no piezoresponse in our PFM measurement. It has been reported that the polar nanoregions in the canonical relaxors [24,25] could have a size in the order of 2—10 nm. This could be smaller in the ceramic materials compared with single crystals. The domain size in the relaxor ceramic sample with

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Fig. 4. PFM domain switching patterns of SrxBa1_xNb2O6 ceramics: (a) x = 0.25, (b) x = 0.50, (c) x = 0.75. The left, middle and right columns are the topography, amplitude, and phase images, respectively. Top to bottom rows correspond to compositions with x = 0.25, 0.50, and 0.75.

Fig. 5. Polarization-electric field (P—E) curves of 10 Hz with various electric fields for SrxBa1_xNb2O6 ceramics: (a) x = 0.25 at room temperature, (b) x = 0.50 at room temperature, (c) x = 0.75 at room temperature, (d) x = 0.25 at 133 K, (e) x = 0.50 at 133 K, (f) x = 0.75 at 133 K.

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x = 0.75 might be smaller than the size of the PFM tip we used, thereby, the domain structure could not be detected. The PFM image of a polarization scanned with a tip under ±9 V (outside region corresponds to polarization poled under —9 V, and the inside region under +9 V) is shown in Fig. 4. The ferroelectric domain back-switching behaviors can be achieved in the samples with x = 0.25 and x = 0.5. There is also no apparent piezoresponse in the samples with x = 0.75 due to the unresolved domain structure.

On the other hand, the hysteresis loops measurement is closely related to the electric field. We enhance the electric field in order to determine its influence on the ferroelectric properties. Fig. 5 shows the polarization-electric field (P-E ) curves of 10 Hz with various electric fields for (SrxBa1—x) Nb2O6 ceramics. At room temperature (Fig. 5(a)—(c)), the hysteresis loop becomes more saturated and the Pmax increases gradually with increasing electric field for all compositions. The low temperature (114 K) ferroelectric hysteresis loops (Fig. 5(d)—(f)) show that, under low electric field, it looks like those of linear dielectric materials without hysteresis for each sample. However, when the applied electric field is high enough, the saturated P-E loops can be obtained for all compositions.

The polarization switching in ferroelectric materials is a thermally activated process. The Gibbs free energy (G) versus electric displacement varies with different temperatures. According to the Landau-Devonshire Theory [26,27], this free energy has the shape of a double well potential with two free energy minima related to the +Ps and —Ps at temperature below T0 (T0 is lower than the Curie Point) for ferroelectrics with first order phase transformation. As the temperature continues to fall, the double well potential of G becomes deeper. From a testing standpoint, we need higher electric field to switch the ferroelectric domains at low temperatures as the coercive field (Ec) increases with decreasing temperature. For (SrxBai—x)Nb2O6 ceramics, the non-linear hysteresis loop can be measured when using higher electric field at low temperature. That is, the decline of switchable polarization at low temperatures shown in Fig. 2 is just due to the increase of Ec rather than a decrease of the long-rang interaction of structural dipoles on cooling. In terms of kinetics, the switching of domains in ferroelectric materials by an applied electric field is believed to occur by the formation of new domain nuclei with favored orientation of polarization, which subsequently expand and grow at the expense of the existing domains [28—30]. Under low temperature, the ferroelectric domains are more difficult to be switched because of the low nucleation rate and grow rate, thus higher electric field is needed [30]. Therefore, the low temperature linear response detected under lower field in (SrxBa1—x)Nb2O6 ceramics is essentially different from the re-entrant relaxor behavior. Actually, the present ceramics all become normal ferroelectric with a high coercive field at low temperatures. We should be careful when we identify the re-entrant relaxor behavior only by the character of a decrease of polarization at low temperature.

4. Conclusions

In summary, (SrxBa1_x)Nb2O6 unfilled tungsten bronze ceramics exhibit strong composition dependent ferroelectric behavior. The polarization-electric field (P-E ) loop becomes slimmer with increasing x, indicating the evolution from normal ferroelectric to relaxor ferroelectric. Studies on domain structure by piezoresponse force microscopy (PFM) well confirm this tendency. Though the P-E loop becomes slimmer and the maximum and remanent polarizations tend to decrease and vanish at low temperatures under a lower electric field, these phenomena are just due to the increase of coercive field rather than a decrease of the long-rang interaction of structural dipoles on cooling, and they do not indicate the re-entrant relaxor behavior. The saturated P-E loop can be obtained with increasing electric field even at low temperatures for all composition in the present ceramics, indicating that the present ceramics all become ferroelectric with a high coercive field at low temperatures.

Acknowledgments

The present work was financially supported by National Natural Science Foundation of China under grant numbers 51332006, 51102210 and 51202215. The authors would like to thank Yao J. and Liu C.L. from Asylum Research for providing PFM measurements.

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Ms. Chen Jiao Huang is currently a MS student at Department of Materials Science and Engineering, Zhejiang University. She graduated from Department of Materials Science & Engineering, Central South University (Changsha, China) in June 2001. Her current research interests cover the field of ferroelectric materials.

Mr. Kun Li is a PhD student at Department of Materials Science and Engineering, Zhejiang University, China. He was awarded BS degree in materials science and engineering, Central South University, China in 2010. He is author or co-author for 5 peer-reviewed papers. His current research interests are focused on ferroelectric and relaxor ferroelectric materials, especially those with tungsten bronze structure.

Dr. Shu Ya Wu is currently an associate professor at Department of Materials Science and Engineering, Zhejiang University. She was awarded BS degree in chemistry by Lanzhou University in 1994, was awarded MS degree in chemistry by Zhejiang University in 1997, and was awarded PhD degree in Materials Science and Engineering by Zhejiang University in 2008. She is author or co-author of more than 30 peer-reviewed papers. Her primary scientific interests cover the fields of ferroelectric and relxor ferroelectric materials, and ceramic

processing.

Dr. Xiao Li Zhu is currently a lecture at Department of Materials Science and Engineering, Zhejiang University. She was awarded BS degree and PhD degree in materials science and engineering, Zhe-jiang University (Hangzhou, China) in 2005 and 2010, respectively. She worked as a postdoctoral fellow at Department of Materials Science and Engineering, Zhejiang University from July, 2010 to August, 2012, and worked as a postdoctoral fellow at Department of Materials and Ceramics Engineering, University of Aveiro, Portugal from September, 2012 to June, 2014. She is author or co-author of more than 20 peer-reviewed papers. Her primary scientific interests cover the fields of ferroelectric and relxor ferroelectric materials, and ceramic processing.

Dr. Xiang Ming Chen is currently a Cheung Kong Professor at Department of Materials Science and Engineering, Zhejiang University. He graduated from the Department of Materials Science & Engineering, Central South University (Changsha, China) in December 1981, and was awarded the PhD degree by The University of Tokyo (Tokyo, Japan) in March 1991. He spent 3 years at Yokohama R&D Laboratories in The Furukawa Electric Co., Ltd. (Yokohama, Japan) as a research scientist, then he joined the Department of Materials Science & Engineering, Zhejiang University (Hangzhou, China) in July 1994, and he was promoted professor in December 1996. His primary scientific interests cover the fields of 1) microwave dielectric ceramics, 2) multiferroic materials, 3) ferroelectric and relaxor ferroelectric ceramics, and 4)energy storage dielectric ceramics. He has authored and/or co-authored more than 310 papers in the pier-reviewed journals. He received the Okazaki Kiyoshi Award in 2007, and was selected as a fellow of The American Ceramic Society in 2013. He currently serves as an associate editor of J. Am. Ceram. Soc., and on the editorial board of Adv. Appl. Ceram.