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Evolution of polar order in Ba(Ti1−xSnx)O3 ceramics
Solid State Communications 149 (2009) 1877–1880
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Evolution of polar order in Ba(Ti1−x Snx )O3 ceramics Le Wang, Xiaoli Wang ∗ , Bo Li 1 MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, Xi’an 710049, China
article
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Article history: Received 22 July 2009 Accepted 4 August 2009 by A.H. MacDonald Available online 11 August 2009 PACS: 77.22.Ej 77.80.Dj 77.84.Dy Keywords: A. Ferroelectrics D. Hysteresis loop D. Phase transition
abstract The ferroelectric hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 solid solution ceramics covering the temperature Tm of a dielectric permittivity peak were investigated. At temperatures ∼100 K lower than their respective Tm , the two compositions present clear ferroelectricity. Comparing with the parent ferroelectric BaTiO3 , the tiny hysteresis loop of both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics at temperatures above Tm reveals the existence of polar order regions or microdomains. The dielectric properties of the two solid solutions are different: the former shows the character of diffuse ferroelectric phase transition, while the latter displays the feature of relaxor ferroelectrics. According to their ferroelectric properties, however, Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 compositions exhibit the diffuse transition from low-temperature ferroelectric domains to polar nanoregions during the ε(T ) peak area. A difference between the two compositions is that the diffuse transition of the former takes place in a much narrower temperature range than that of the latter. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction BaTiO3 is one of the most studied ferroelectric materials. Some BaTiO3 -based solid solutions with isovalent substitution transform from normal ferroelectrics to ferroelectrics with diffuse phase transition (DPT), then to relaxor ferroelectrics (relaxors) with increasing content of substitutions [1–7]. Ba(Ti1−x Snx )O3 solid solution system is one of the solid solution systems. Researches focusing on the crossover between ferroelectrics with DPT and relaxors have been reported, and different presumptions were proposed according to the dielectric spectroscopy. Wei et al. believed that DPT already correspond to the polar nanoregions, and the distinction between frequency-independent DPT and frequencydependent relaxors was due to the different sizes of the polar nanoregions with the consequence of different relaxation frequencies [8]. Mueller et al. attributed the DPT to a ferroelectric phase transition based on the observation of domains in compositions with Sn content less than 13 at.% [9]. Lei et al. showed a similar opinion about DPT as Mueller et al., and ascribed the crossover from DPT to relaxors to the appearance of the additional dielectric contribution arising from the flipping of the local polarization of the polar clusters [7]. Wang et al. suggested that defect dipoles trapped by random electric fields Er were the origins of the dielectric audio-frequency dispersion, and the temperature Tm of maximum permittivity εm becomes frequency-dependent
∗
Corresponding author. E-mail address: [email protected] (X. Wang).
1 Present address: Henan institute of metrology, Zhengzhou, China. 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.08.005
when the permittivity contributed by the defect dipoles was large enough [10]. The controversies illustrate that other types of experimental data are necessary to approach the origin of relaxor behavior in Ba(Ti1−x Snx )O3 solid solutions. A distinct domain structure of Ba(T0.85 Sn0.15 )O3 at 295 K was observed by piezoresponse force microscopy, though its Tm ≈ 283 K [11]. It manifests that Ba(Ti1−x Snx )O3 ceramics with x below threshold xc (≈0.19) presents diffuse ferroelectric phase transition. In this article, we reported on investigations of the ferroelectric properties of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 in the temperature range covering their permittivity-temperature anomaly. Of the two compositions, Ba(Ti0.85 Sn0.15 )O3 is located at DPT side, and Ba(Ti0.75 Sn0.25 )O3 at relaxor side. 2. Experimental Ceramics Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 were prepared using mixed oxide method. For comparison, BaTiO3 ceramics were synthesized too. Stoichiometric amounts of reagents BaCO3 , TiO2 and SnO2 powders were wet mixed by ball milling and then presintered between temperatures of 1373 K and 1423 K for 2 h. The presintered powder was ball milled and dried. Pellets 12 mm in diameter and ∼1 mm thick were pressed using 10% PVA binder. The pellets were fired between temperatures of 1573 K and 1623 K for 2 to 3 h. For dielectric measurements, ceramic samples were ground and then painted with silver paste as electrodes on two flat sides after being fired at 823 K for 15 m. The dielectric permittivity were measured on an automated system, wherein a
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Fig. 3. Field dependences of current density J and polarization P for BaTiO3 ceramic at 406 K. Fig. 1. Dielectric permittivity of BaTiO3 , Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 as a function of temperature at different frequencies.
Fig. 4. Hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 ceramic at different temperatures. Fig. 2. Hysteresis loops of BaTiO3 ceramic at different temperatures.
temperature control sample chamber and an Agilent 4284A inductance–capacitance–resistance (LCR) meter, which can cover a frequency range from 20 Hz to 1 MHz, were controlled by a personal computer. For ferroelectric hysteresis loop measurements, a sinusoidal signal of 1 Hz, generated by a personal computer with a PCI6221 Data Acquisition (DAQ) card, was amplified through a Trek 610E high-voltage supply/amplifier/controller and applied to the sample. Current through the sample was collected by the DAQ card, and converted to a digital signal wherein. The hysteresis loop was obtained through charge integration. 3. Results Fig. 1 shows dielectric permittivity ε from 0.1 kHz to 100 kHz as a function of temperature for ceramics of BaTiO3 , Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 . The Tc of BaTiO3 is 403 K, and the temperature Tm of maximum permittivity εm of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 is 285 K and ∼190 K, respectively. The ε(T ) peak becomes broader with an increasing content of Sn4+ ions. The full width at half εm for Ba(Ti0.75 Sn0.25 )O3 is more than 80 K, which is much wider than that of Ba(Ti0.85 Sn0.15 )O3 (∼40 K). For Ba(Ti0.85 Sn0.15 )O3 , Tm is frequency-independent, but for Ba(Ti0.75 Sn0.25 )O3 , Tm is 187 K at 0.1 kHz and 192 K at 100 kHz. Hysteresis loops of BaTiO3 ceramic measured at different temperatures are depicted in Fig. 2. BaTiO3 ceramic exhibits a thorough paraelectric state at 417 K (14 K higher than Tc = 403 K) and above under the AC maximum field of 3.5 MV m−1 . Upon cooling, a double-hysteresis loop was observed for BaTiO3 at 406 K
(3 K higher than Tc , see Fig. 3). There is a discharge peak on the opposite side of the charge peak in the loop of current density J versus electric field E. It indicates that the double-hysteresis loop is caused by a field-induced phase transition between paraelectric and ferroelectric states. The induced ferroelectric states vanish as applied electric field decreases to zero. At temperatures 393 K and 288 K of the ferroelectric state, the remanent polarization Pr of BaTiO3 is 0.043 C m−2 and 0.095 C m−2 , respectively. Fig. 4 shows the hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 ceramic. At 333 K (48 K higher than Tm = 285 K) and above, the induced polarization by electric field is reversible, and there is no hysteresis effect. The hysteresis behavior appears when the sample is cooled to ∼313 K. Very small remnant polarization indicates the existence of some microdomains in Ba(Ti0.85 Sn0.15 )O3 at 313 K. In addition, a special polarization behavior under high field is observed just above Tm . Fig. 5 plots the field dependences of current density J and polarization P for Ba(Ti0.85 Sn0.15 )O3 ceramic at 288 K under AC maximum fields of 2 MV m−1 and 3.6 MV m−1 , respectively. The shape of the two loops of J versus E at different temperatures is quite alike. The J (E ) peaks are composed of discharge and charge current density, and the peak points are located at the increasing field side, which implies there are net charge parts. Nevertheless, the corresponding hysteresis loops exhibit distinct difference. The hysteresis loop is normal under Em of 2 MV m−1 . But when the AC maximum field increases to 3.6 MV m−1 , the hysteresis loop displays a double-like shape. Since there is no corresponding charge and discharge peaks in the J (E ) loop, the double-like hysteresis loop should not stem from the field-induced transition between paraelectric and ferroelectric states. Another difference from that of the parent ferroelectric BaTiO3 , the doublelike hysteresis loop of Ba(Ti0.85 Sn0.15 )O3 presents some remanent
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Fig. 6. Hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at different temperatures.
Fig. 5. Field dependences of current density J (a) and polarization P (b) for Ba(Ti0.85 Sn0.15 )O3 ceramic at 288 K under different ac maximum fields.
polarization, which implies that there exist some microdomains. The remanent polarizations measured at the two AC maximum fields are same. The thin hysteresis loop exists in Ba(Ti0.85 Sn0.15 )O3 ceramic above Tm until ∼333 K. Below Tm , the increase of Pr accelerates obviously with decreasing temperature. The hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at different temperatures are represented in Fig. 6. Tm of Ba(Ti0.75 Sn0.25 )O3 at 100 Hz is 187 K, and tiny hysteresis loop can be measured till to ∼290 K. Comparing with Ba(Ti0.85 Sn0.15 )O3 , Pr of Ba(Ti0.75 Sn0.25 )O3 becomes smaller and increases slowly on cooling at the corresponding temperature range. Unlike Ba(Ti0.85 Sn0.15 )O3 , the hysteresis loop of Ba(Ti0.75 Sn0.25 )O3 does not show a double-like anomaly under a higher AC field at temperatures just above Tm (see Fig. 7). In addition, the remnant polarization also stays constant when the AC maximum field increases. For further understanding the evolution of polar order in Ba(Ti1−x Snx )O3 ceramics, the hysteresis loops measured at ∼100 K lower than their respective Tm are shown in Fig. 8. The hysteresis loop of BaTiO3 at 288 K is also represented for comparison. At about 100 K lower than Tm , both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 display enhanced ferroelectricity with Pr is 0.087 C m−2 and 0.045 C m−2 , respectively. It demonstrates that the concentration and scales of the polar order regions in both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 compositions increase with further decreasing temperature. There are some ferroelectric states in Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics at low temperatures beyond the ε(T ) peak area. 4. Discussion In Ba(Ti1−x Snx )O3 solid solutions, Ti4+ ions are ferroelectric active, but Sn4+ ions are not [12]. Since BaTiO3 does not show remanent polarization after undergoing a high AC electric field at temperatures just above Tc (see Fig. 3), it means that the field-induced ferroelectric phase from the paraelectric state or polar nanoregions of BaTiO3 cannot persist when the applied
Fig. 7. Hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at 193 K under different AC maximum fields.
Fig. 8. Hysteresis loops of BaTiO3 at 288 K (A), Ba(Ti0.85 Sn0.15 )O3 at 153 K (B), and Ba(Ti0.75 Sn0.25 )O3 at 83 K (C).
field decreases to zero. Accordingly, the thin hysteresis loop of either Ba(Ti0.85 Sn0.15 )O3 or Ba(Ti0.75 Sn0.25 )O3 at temperatures above Tm implies the existence of microdomains in the solid solutions. The evolution of the polar order in Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 with a decreasing temperature can be estimated according to the relationship of remanent polarization Pr versus T − Tm (see Fig. 9). In the temperature range of 50 K above Tm , the remanent polarization of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 exhibits a similar variation, and Pr of the former is some lower than that of the latter. At the temperature about 50 K higher than Tm , Ba(Ti0.85 Sn0.15 )O3 does not show a hysteresis loop with Pr = 0, but Ba(Ti0.75 Sn0.25 )O3 still shows a very tiny hysteresis loop with
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than that of the latter. The volumes of polar nanoregions in Ba(Ti0.75 Sn0.25 )O3 should be less than that in Ba(Ti0.85 Sn0.15 )O3 around Tm , because the former contains more ferroelectric inactive Sn4+ ions than the latter. As a result, the hysteresis loop of Ba(Ti0.75 Sn0.25 )O3 dose not present a double-like hysteresis loop in the vicinity of Tm under a high AC field. On the other hand, Ba(Ti0.75 Sn0.25 )O3 ceramics have a higher concentration of microdomains than Ba(Ti0.85 Sn0.15 )O3 ceramics around Tm . 5. Conclusion
Fig. 9. Remnant polarization Pr versus temperature difference T − Tm for Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics.
Pr = 0.003 C m−2 . In the vicinity of Tm , Pr of Ba(Ti0.85 Sn0.15 )O3 starts a quick increase on cooling. Whereas the increase rate of Pr of Ba(Ti0.75 Sn0.25 )O3 stays similar in the two temperature ranges of 50 K above and below Tm . The ±50 K temperature ranges around Tm are just the whole range of the ε(T ) peak of Ba(Ti0.85 Sn0.15 )O3 . In the light of the polarization characteristics described above, the ε(T ) peak could be ascribed to the diffuse transition from macrodomains to microdomains, polar nanoregions (or called ergodic polar cluster state [7,13]), and paraelectric states with an increasing temperature. There should be volumes of polar nanoregions besides some microdomains around the Tm range. Under a higher AC field, the polarizations of polar nanoregions orientate along the applied field direction, the strong Coulomb force among them makes the existence of hysteresis of field-induced polarization with decreasing field, and results in a double-like hysteresis loop (see Fig. 5). The whole ε(T ) peak of Ba(Ti0.75 Sn0.25 )O3 extends ±100 K from Tm . At 293 K (about 100 K higher than Tm ) Ba(Ti0.75 Sn0.25 )O3 does not show a hysteresis loop with Pr = 0. At 83 K (about 100 K lower than Tm ) the remnant polarization is just same as that of Ba(Ti0.85 Sn0.15 )O3 at 233 K (52 K lower than Tm ). Therefore, the origin of the ε(T ) peak of Ba(Ti0.75 Sn0.25 )O3 should be similar to that of Ba(Ti0.85 Sn0.15 )O3 . The difference is that the diffuse transition from macrodomains to microdomains, polar nanoregions and paraelectric states of the former with increasing temperature takes place in a much wider temperature range
The dielectric properties of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics are different, the former shows the character of diffuse ferroelectric phase transition, while the latter displays the feature of relaxor ferroelectrics. According to their ferroelectric properties, however, the two compositions exhibit the diffuse transition from low-temperature ferroelectric domains to polar nanoregions during the ε(T ) peak area. The concentration of polar order regions in Ba(Ti0.75 Sn0.25 )O3 ceramics is still higher than that in Ba(Ti0.85 Sn0.15 )O3 ceramics in the analogous temperature range above Tm . At temperatures ∼100 K lower than their respective Tm , both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics exhibit distinct ferroelectricity. Acknowledgment This work was supported by the Funds of National Natural Science Foundation of China (project no 50772087). References [1] V.S. Tiwari, N. Singh, D. Pandey, J. Phys. Condens. Matter 7 (1995) 1441. [2] N. Singhy, A.P. Singhz, C.D. Prasadx, D. Pandey, J. Phys. Condens. Matter 8 (1996) 7813. [3] V.V. Lemanov, E.P. Smirnova, P.P. Syrnikov, E.A. Tarakanov, Phys. Rev. B 54 (1996) 3151. [4] Z. Yu, R. Guo, A.S. Bhalla, J. Appl. Phys. 88 (2000) 410. [5] C. Ménoret, J.M. Kiat, B. Dkhil, M. Dunlop, H. Dammak, O. Hernandez, Phys. Rev. B 65 (2002) 224104. [6] C. Ang, Z. Jing, Z. Yu, J. Phys. Condens. Matter 14 (2002) 8901. [7] C. Lei, A.A. Bokov, Z-G. Ye, J. Appl. Phys. 101 (2007) 084105. [8] X. Wei, Y. Feng, X. Yao, Appl. Phys. Lett. 83 (2003) 2031. [9] V. Mueller, H. Beige, H.-P. Abicht, Appl. Phys. Lett. 84 (2004) 1341. [10] X. Wang, B. Li, Solid State Commun. 149 (2009) 537. [11] V.V. Shvartsman, W. Kleemann, J. Dec, Z.K. Xu, S.G. Lu, J. Appl. Phys. 99 (2006) 124111. [12] É. Bévillon, A. Chesnaud, Y. Wang, G. Dezanneau, G. Geneste, J. Phys. Condens. Matter 20 (2008) 145217. [13] A.A. Bokov, Z.-G. Ye, J. Mater. Sci. 41 (2006) 31.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Evolution of polar order in Ba(Ti1−x Snx )O3 ceramics Le Wang, Xiaoli Wang ∗ , Bo Li 1 MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, Xi’an 710049, China
article
info
Article history: Received 22 July 2009 Accepted 4 August 2009 by A.H. MacDonald Available online 11 August 2009 PACS: 77.22.Ej 77.80.Dj 77.84.Dy Keywords: A. Ferroelectrics D. Hysteresis loop D. Phase transition
abstract The ferroelectric hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 solid solution ceramics covering the temperature Tm of a dielectric permittivity peak were investigated. At temperatures ∼100 K lower than their respective Tm , the two compositions present clear ferroelectricity. Comparing with the parent ferroelectric BaTiO3 , the tiny hysteresis loop of both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics at temperatures above Tm reveals the existence of polar order regions or microdomains. The dielectric properties of the two solid solutions are different: the former shows the character of diffuse ferroelectric phase transition, while the latter displays the feature of relaxor ferroelectrics. According to their ferroelectric properties, however, Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 compositions exhibit the diffuse transition from low-temperature ferroelectric domains to polar nanoregions during the ε(T ) peak area. A difference between the two compositions is that the diffuse transition of the former takes place in a much narrower temperature range than that of the latter. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction BaTiO3 is one of the most studied ferroelectric materials. Some BaTiO3 -based solid solutions with isovalent substitution transform from normal ferroelectrics to ferroelectrics with diffuse phase transition (DPT), then to relaxor ferroelectrics (relaxors) with increasing content of substitutions [1–7]. Ba(Ti1−x Snx )O3 solid solution system is one of the solid solution systems. Researches focusing on the crossover between ferroelectrics with DPT and relaxors have been reported, and different presumptions were proposed according to the dielectric spectroscopy. Wei et al. believed that DPT already correspond to the polar nanoregions, and the distinction between frequency-independent DPT and frequencydependent relaxors was due to the different sizes of the polar nanoregions with the consequence of different relaxation frequencies [8]. Mueller et al. attributed the DPT to a ferroelectric phase transition based on the observation of domains in compositions with Sn content less than 13 at.% [9]. Lei et al. showed a similar opinion about DPT as Mueller et al., and ascribed the crossover from DPT to relaxors to the appearance of the additional dielectric contribution arising from the flipping of the local polarization of the polar clusters [7]. Wang et al. suggested that defect dipoles trapped by random electric fields Er were the origins of the dielectric audio-frequency dispersion, and the temperature Tm of maximum permittivity εm becomes frequency-dependent
∗
Corresponding author. E-mail address: [email protected] (X. Wang).
1 Present address: Henan institute of metrology, Zhengzhou, China. 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.08.005
when the permittivity contributed by the defect dipoles was large enough [10]. The controversies illustrate that other types of experimental data are necessary to approach the origin of relaxor behavior in Ba(Ti1−x Snx )O3 solid solutions. A distinct domain structure of Ba(T0.85 Sn0.15 )O3 at 295 K was observed by piezoresponse force microscopy, though its Tm ≈ 283 K [11]. It manifests that Ba(Ti1−x Snx )O3 ceramics with x below threshold xc (≈0.19) presents diffuse ferroelectric phase transition. In this article, we reported on investigations of the ferroelectric properties of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 in the temperature range covering their permittivity-temperature anomaly. Of the two compositions, Ba(Ti0.85 Sn0.15 )O3 is located at DPT side, and Ba(Ti0.75 Sn0.25 )O3 at relaxor side. 2. Experimental Ceramics Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 were prepared using mixed oxide method. For comparison, BaTiO3 ceramics were synthesized too. Stoichiometric amounts of reagents BaCO3 , TiO2 and SnO2 powders were wet mixed by ball milling and then presintered between temperatures of 1373 K and 1423 K for 2 h. The presintered powder was ball milled and dried. Pellets 12 mm in diameter and ∼1 mm thick were pressed using 10% PVA binder. The pellets were fired between temperatures of 1573 K and 1623 K for 2 to 3 h. For dielectric measurements, ceramic samples were ground and then painted with silver paste as electrodes on two flat sides after being fired at 823 K for 15 m. The dielectric permittivity were measured on an automated system, wherein a
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Fig. 3. Field dependences of current density J and polarization P for BaTiO3 ceramic at 406 K. Fig. 1. Dielectric permittivity of BaTiO3 , Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 as a function of temperature at different frequencies.
Fig. 4. Hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 ceramic at different temperatures. Fig. 2. Hysteresis loops of BaTiO3 ceramic at different temperatures.
temperature control sample chamber and an Agilent 4284A inductance–capacitance–resistance (LCR) meter, which can cover a frequency range from 20 Hz to 1 MHz, were controlled by a personal computer. For ferroelectric hysteresis loop measurements, a sinusoidal signal of 1 Hz, generated by a personal computer with a PCI6221 Data Acquisition (DAQ) card, was amplified through a Trek 610E high-voltage supply/amplifier/controller and applied to the sample. Current through the sample was collected by the DAQ card, and converted to a digital signal wherein. The hysteresis loop was obtained through charge integration. 3. Results Fig. 1 shows dielectric permittivity ε from 0.1 kHz to 100 kHz as a function of temperature for ceramics of BaTiO3 , Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 . The Tc of BaTiO3 is 403 K, and the temperature Tm of maximum permittivity εm of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 is 285 K and ∼190 K, respectively. The ε(T ) peak becomes broader with an increasing content of Sn4+ ions. The full width at half εm for Ba(Ti0.75 Sn0.25 )O3 is more than 80 K, which is much wider than that of Ba(Ti0.85 Sn0.15 )O3 (∼40 K). For Ba(Ti0.85 Sn0.15 )O3 , Tm is frequency-independent, but for Ba(Ti0.75 Sn0.25 )O3 , Tm is 187 K at 0.1 kHz and 192 K at 100 kHz. Hysteresis loops of BaTiO3 ceramic measured at different temperatures are depicted in Fig. 2. BaTiO3 ceramic exhibits a thorough paraelectric state at 417 K (14 K higher than Tc = 403 K) and above under the AC maximum field of 3.5 MV m−1 . Upon cooling, a double-hysteresis loop was observed for BaTiO3 at 406 K
(3 K higher than Tc , see Fig. 3). There is a discharge peak on the opposite side of the charge peak in the loop of current density J versus electric field E. It indicates that the double-hysteresis loop is caused by a field-induced phase transition between paraelectric and ferroelectric states. The induced ferroelectric states vanish as applied electric field decreases to zero. At temperatures 393 K and 288 K of the ferroelectric state, the remanent polarization Pr of BaTiO3 is 0.043 C m−2 and 0.095 C m−2 , respectively. Fig. 4 shows the hysteresis loops of Ba(Ti0.85 Sn0.15 )O3 ceramic. At 333 K (48 K higher than Tm = 285 K) and above, the induced polarization by electric field is reversible, and there is no hysteresis effect. The hysteresis behavior appears when the sample is cooled to ∼313 K. Very small remnant polarization indicates the existence of some microdomains in Ba(Ti0.85 Sn0.15 )O3 at 313 K. In addition, a special polarization behavior under high field is observed just above Tm . Fig. 5 plots the field dependences of current density J and polarization P for Ba(Ti0.85 Sn0.15 )O3 ceramic at 288 K under AC maximum fields of 2 MV m−1 and 3.6 MV m−1 , respectively. The shape of the two loops of J versus E at different temperatures is quite alike. The J (E ) peaks are composed of discharge and charge current density, and the peak points are located at the increasing field side, which implies there are net charge parts. Nevertheless, the corresponding hysteresis loops exhibit distinct difference. The hysteresis loop is normal under Em of 2 MV m−1 . But when the AC maximum field increases to 3.6 MV m−1 , the hysteresis loop displays a double-like shape. Since there is no corresponding charge and discharge peaks in the J (E ) loop, the double-like hysteresis loop should not stem from the field-induced transition between paraelectric and ferroelectric states. Another difference from that of the parent ferroelectric BaTiO3 , the doublelike hysteresis loop of Ba(Ti0.85 Sn0.15 )O3 presents some remanent
L. Wang et al. / Solid State Communications 149 (2009) 1877–1880
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Fig. 6. Hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at different temperatures.
Fig. 5. Field dependences of current density J (a) and polarization P (b) for Ba(Ti0.85 Sn0.15 )O3 ceramic at 288 K under different ac maximum fields.
polarization, which implies that there exist some microdomains. The remanent polarizations measured at the two AC maximum fields are same. The thin hysteresis loop exists in Ba(Ti0.85 Sn0.15 )O3 ceramic above Tm until ∼333 K. Below Tm , the increase of Pr accelerates obviously with decreasing temperature. The hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at different temperatures are represented in Fig. 6. Tm of Ba(Ti0.75 Sn0.25 )O3 at 100 Hz is 187 K, and tiny hysteresis loop can be measured till to ∼290 K. Comparing with Ba(Ti0.85 Sn0.15 )O3 , Pr of Ba(Ti0.75 Sn0.25 )O3 becomes smaller and increases slowly on cooling at the corresponding temperature range. Unlike Ba(Ti0.85 Sn0.15 )O3 , the hysteresis loop of Ba(Ti0.75 Sn0.25 )O3 does not show a double-like anomaly under a higher AC field at temperatures just above Tm (see Fig. 7). In addition, the remnant polarization also stays constant when the AC maximum field increases. For further understanding the evolution of polar order in Ba(Ti1−x Snx )O3 ceramics, the hysteresis loops measured at ∼100 K lower than their respective Tm are shown in Fig. 8. The hysteresis loop of BaTiO3 at 288 K is also represented for comparison. At about 100 K lower than Tm , both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 display enhanced ferroelectricity with Pr is 0.087 C m−2 and 0.045 C m−2 , respectively. It demonstrates that the concentration and scales of the polar order regions in both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 compositions increase with further decreasing temperature. There are some ferroelectric states in Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics at low temperatures beyond the ε(T ) peak area. 4. Discussion In Ba(Ti1−x Snx )O3 solid solutions, Ti4+ ions are ferroelectric active, but Sn4+ ions are not [12]. Since BaTiO3 does not show remanent polarization after undergoing a high AC electric field at temperatures just above Tc (see Fig. 3), it means that the field-induced ferroelectric phase from the paraelectric state or polar nanoregions of BaTiO3 cannot persist when the applied
Fig. 7. Hysteresis loops of Ba(Ti0.75 Sn0.25 )O3 ceramic at 193 K under different AC maximum fields.
Fig. 8. Hysteresis loops of BaTiO3 at 288 K (A), Ba(Ti0.85 Sn0.15 )O3 at 153 K (B), and Ba(Ti0.75 Sn0.25 )O3 at 83 K (C).
field decreases to zero. Accordingly, the thin hysteresis loop of either Ba(Ti0.85 Sn0.15 )O3 or Ba(Ti0.75 Sn0.25 )O3 at temperatures above Tm implies the existence of microdomains in the solid solutions. The evolution of the polar order in Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 with a decreasing temperature can be estimated according to the relationship of remanent polarization Pr versus T − Tm (see Fig. 9). In the temperature range of 50 K above Tm , the remanent polarization of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 exhibits a similar variation, and Pr of the former is some lower than that of the latter. At the temperature about 50 K higher than Tm , Ba(Ti0.85 Sn0.15 )O3 does not show a hysteresis loop with Pr = 0, but Ba(Ti0.75 Sn0.25 )O3 still shows a very tiny hysteresis loop with
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than that of the latter. The volumes of polar nanoregions in Ba(Ti0.75 Sn0.25 )O3 should be less than that in Ba(Ti0.85 Sn0.15 )O3 around Tm , because the former contains more ferroelectric inactive Sn4+ ions than the latter. As a result, the hysteresis loop of Ba(Ti0.75 Sn0.25 )O3 dose not present a double-like hysteresis loop in the vicinity of Tm under a high AC field. On the other hand, Ba(Ti0.75 Sn0.25 )O3 ceramics have a higher concentration of microdomains than Ba(Ti0.85 Sn0.15 )O3 ceramics around Tm . 5. Conclusion
Fig. 9. Remnant polarization Pr versus temperature difference T − Tm for Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics.
Pr = 0.003 C m−2 . In the vicinity of Tm , Pr of Ba(Ti0.85 Sn0.15 )O3 starts a quick increase on cooling. Whereas the increase rate of Pr of Ba(Ti0.75 Sn0.25 )O3 stays similar in the two temperature ranges of 50 K above and below Tm . The ±50 K temperature ranges around Tm are just the whole range of the ε(T ) peak of Ba(Ti0.85 Sn0.15 )O3 . In the light of the polarization characteristics described above, the ε(T ) peak could be ascribed to the diffuse transition from macrodomains to microdomains, polar nanoregions (or called ergodic polar cluster state [7,13]), and paraelectric states with an increasing temperature. There should be volumes of polar nanoregions besides some microdomains around the Tm range. Under a higher AC field, the polarizations of polar nanoregions orientate along the applied field direction, the strong Coulomb force among them makes the existence of hysteresis of field-induced polarization with decreasing field, and results in a double-like hysteresis loop (see Fig. 5). The whole ε(T ) peak of Ba(Ti0.75 Sn0.25 )O3 extends ±100 K from Tm . At 293 K (about 100 K higher than Tm ) Ba(Ti0.75 Sn0.25 )O3 does not show a hysteresis loop with Pr = 0. At 83 K (about 100 K lower than Tm ) the remnant polarization is just same as that of Ba(Ti0.85 Sn0.15 )O3 at 233 K (52 K lower than Tm ). Therefore, the origin of the ε(T ) peak of Ba(Ti0.75 Sn0.25 )O3 should be similar to that of Ba(Ti0.85 Sn0.15 )O3 . The difference is that the diffuse transition from macrodomains to microdomains, polar nanoregions and paraelectric states of the former with increasing temperature takes place in a much wider temperature range
The dielectric properties of Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics are different, the former shows the character of diffuse ferroelectric phase transition, while the latter displays the feature of relaxor ferroelectrics. According to their ferroelectric properties, however, the two compositions exhibit the diffuse transition from low-temperature ferroelectric domains to polar nanoregions during the ε(T ) peak area. The concentration of polar order regions in Ba(Ti0.75 Sn0.25 )O3 ceramics is still higher than that in Ba(Ti0.85 Sn0.15 )O3 ceramics in the analogous temperature range above Tm . At temperatures ∼100 K lower than their respective Tm , both Ba(Ti0.85 Sn0.15 )O3 and Ba(Ti0.75 Sn0.25 )O3 ceramics exhibit distinct ferroelectricity. Acknowledgment This work was supported by the Funds of National Natural Science Foundation of China (project no 50772087). References [1] V.S. Tiwari, N. Singh, D. Pandey, J. Phys. Condens. Matter 7 (1995) 1441. [2] N. Singhy, A.P. Singhz, C.D. Prasadx, D. Pandey, J. Phys. Condens. Matter 8 (1996) 7813. [3] V.V. Lemanov, E.P. Smirnova, P.P. Syrnikov, E.A. Tarakanov, Phys. Rev. B 54 (1996) 3151. [4] Z. Yu, R. Guo, A.S. Bhalla, J. Appl. Phys. 88 (2000) 410. [5] C. Ménoret, J.M. Kiat, B. Dkhil, M. Dunlop, H. Dammak, O. Hernandez, Phys. Rev. B 65 (2002) 224104. [6] C. Ang, Z. Jing, Z. Yu, J. Phys. Condens. Matter 14 (2002) 8901. [7] C. Lei, A.A. Bokov, Z-G. Ye, J. Appl. Phys. 101 (2007) 084105. [8] X. Wei, Y. Feng, X. Yao, Appl. Phys. Lett. 83 (2003) 2031. [9] V. Mueller, H. Beige, H.-P. Abicht, Appl. Phys. Lett. 84 (2004) 1341. [10] X. Wang, B. Li, Solid State Commun. 149 (2009) 537. [11] V.V. Shvartsman, W. Kleemann, J. Dec, Z.K. Xu, S.G. Lu, J. Appl. Phys. 99 (2006) 124111. [12] É. Bévillon, A. Chesnaud, Y. Wang, G. Dezanneau, G. Geneste, J. Phys. Condens. Matter 20 (2008) 145217. [13] A.A. Bokov, Z.-G. Ye, J. Mater. Sci. 41 (2006) 31.