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Phase transformation and dielectric properties in Ba1−xLaxZr0.1Ti0.9O3 ceramics
Journal of Alloys and Compounds 592 (2014) 105–108
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Phase transformation and dielectric properties in Ba1xLaxZr0.1Ti0.9O3 ceramics Xia Huang, Jingji Zhang ⇑, Ludong Ji, Hongfang Qi, Jiangying Wang ⇑ College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 3 January 2014 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Perovskite structure Phase transformation Relaxor behavior Strain behavior
a b s t r a c t Ba1xLaxZr0.1Ti0.9O3 ceramics have been prepared by the mixed-oxide route, and their structure and dielectric properties have been investigated. Results showed that La3+ ions entered the unit cell maintaining the perovskite structure. The evolution of dielectric behavior from a sharp dielectric peak with pinched phase transitions (x = 0.00) to a broad dielectric peak with frequency dependence (0.01 6 x 6 0.02) and successively to a ferroelectric relaxor (x = 0.05) was observed with increasing La concentration. Furthermore, remnant polarization and strain decreased due to phase transformation from ferroelectric tetragonal to paraelectric pseudocubic at room temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction BaZr1xTixO3 (BZT) ferroelectric has been attracting an increasingly fundamental and practical interest for potential application in commercial capacitor and tunable microwave devices, due to high dielectric permittivity, high voltage resistance, strong dielectric nonlinearity and composition-dependent Curie temperature (TC) [1–4]. The dielectric response of BZT ceramics suggests a ferroelectric behavior with three phase transitions for the compositions with x 6 0.08, a broad dielectric peak without frequency dependence for 0.15 6 x 6 0.25 and ferroelectric relaxor behavior for x = 0.3 [5]. It was reported that greatly enhanced room temperature piezoelectric and electromechanical responses of d33 = 420 pC/N, d31 = 138 pC/N, and kp = 49% was obtained at a composition x = 0.06 [6]. As is well known, impurity-doping in BZT electroceramics has become an effective and common way to improve the material performance [7–10]. Rare earth (RE) is often referred to as ‘‘vitamin of the electroceramic industry’’. So, RE is used as a modifier to improve the performance of BZT system [11,12]. Recently, based on the vacancy defect compensation models, trivalent RE ionic substitution at A- [13,14] and B-site[15] of BZT ceramics has been widely investigated, due to their moderate ionic radii and ability to substitute both A- and B-site. It was reported that electric field induced strain increased from 0.08% to 0.18% (at 40 kV/cm applied field) for Ba1x/2Lax/3(Zr0.05Ti0.95)O3 with x = 0.02 compared to undoped BaTiO3 [16]. Additionally, it was found that the temperature at ⇑ Corresponding authors. Tel./fax: +86 571 86875613. E-mail addresses: [email protected] (J. Zhang), [email protected] (J. Wang). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.015
the dielectric maxima (Tm) of B-site deficient La-doped BZT (Ba1x LaxZr0.2Ti0.8x/4O3) ceramics decreased more markedly than that of A-site deficient La-doped BZT (Ba1xLa2x/3Zr0.09Ti0.91O3) ceramics with increasing La concentration [17,18]. In Ba1xLa2x/3Zr0.09Ti0.91O3 and Ba1xLaxZr0.2Ti0.8x/4O3 system, the effect of La substitution for Ba can be expressed by Kröger–Vink notation as, respectively,
BaO þ La2 O3 ! BaBa þ 2LaBa þ V 00Ba þ 4OO ;
ð1Þ
1 0000 BaO þ La2 O3 ! BaBa þ 2LaBa þ VTi þ 4OO : 2
ð2Þ 0000
Obviously, the number of V 00Ba increases much more than that of VTi with increasing La3+ concentration. Thus, the variation of Tm in REdoped BZT ceramics with the concentration of RE could not be related to the number of vacancies. However, the doping mechanisms in RE-doped BZT ceramics have not yet well been understood. Therefore, the focus of our research is set to the Ba1xLaxZr0.1Ti0.9O3 system, where the effect of La substitution for Ba on the dielectric, ferroelectric and piezoelectric properties is investigated. 2. Experimental procedures Ba1xLaxZr0.1Ti0.9O3 ceramics, where x = 0.00, 0.01, 0.02, 0.05 and 0.10, were prepared through the solid-state reaction method using starting chemicals of BaCO3 (99.0%), TiO2 (98.0%), ZrO2 (99.0%) and La2O3 (99.99%). Mixtures based on the compositions of Ba1xLaxZr0.1Ti0.9O3 were ball-milled with zirconia media in ethanol for 24 h and dried at 110 °C for 12 h. After drying, the powders were calcined at 1200 °C for 4 h and then re-milled for 24 h. The calcined powders, mixed with 8 wt% polyvinyl alcohol, were pressed into pellets at 100 MPa. The green pellets were kept at 550 °C for 6 h to remove the solvent and the binder. Ba1xLaxZr0.1Ti0.9O3 ceramics were sintered at 1500 °C for 4 h in air.
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X. Huang et al. / Journal of Alloys and Compounds 592 (2014) 105–108
Phase compositions of the ceramics were investigated by means of X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with Cu Ka radiation. Permittivity and loss tangent as functions of temperature were measured at frequencies from 1 to 1 MHz in temperature range of 125–450 K, using a HP 4284A precision LCR meter (Agilent, Palo Alto, CA). Polarization versus electric field (P–E) hysteresis loops were measured in a silicon oil bath by applying an electric field of triangular waveform at 10 Hz by means of a ferroelectric testing system (Radiant Precision Premier II Technology). MTI 2100 photonic sensor was used for strain measurement.
3. Results and discussion XRD patterns of the Ba1xLaxZr0.1Ti0.9O3 ceramics are shown in Fig. 1. All samples exhibit single perovskite phase, indicating the formation of Ba1xLaxZr0.1Ti0.9O3 solid solution. It can be seen from Fig. 1(b) that the compositions with x 6 0.02 show the tetragonal perovskite structure at room temperature, while those with x P 0.05 are the pseudocubic one. Lattice parameters calculated from the diffraction spectra fitted by space groups P4mm (x 6 0.02) or Pm3m (x P 0.05) are presented in the inset of Fig. 1(a). The lattice parameters decrease with increasing x. It can be explained that the ionic radius of La3+ (1.32 Å) in 12-fold coordination is smaller than that of Ba2+ (1.61 Å) [19], which led to lattice shrinkage. Temperature dependence of permittivity of all samples measured at different frequencies, varying from 1 to 1 MHz, is displayed in Fig. 2. As x increases, the dielectric peaks with paraelectric–ferroelectric transition of the Ba1xLaxZr0.1Ti0.9O3 ceramics are suppressed and broadened and shifted toward low temperature, which is attributed to the deterioration of ferroelectric long-range order [20]. Tm measured at 10 kHz decreases monotonically with increasing x, as shown in Fig. 3, which can be defined approximately by the relation Tm(K) = 2907x + 355. According to Refs. [17,18], the variation of Tm in the Ba1xLaxZr0.2Ti0.8x/4O3 and Ba1xLa2x/3Zr0.09Ti0.91O3 ceramics with x is also plotted in Fig. 3 for comparison. It is found that the decrement of Tm in the present Ba1xLaxZr0.1Ti0.9O3 system is a little higher than that of Ba1xLa2x/3Zr0.09Ti0.91O3 system, whereas it is only half of that of Ba1xLaxZr0.2Ti0.8x/4O3 system. This is because that, in the case of the same x, there exist more La3+ in the Ba1xLaxZr0.1Ti0.9O3 crystal lattice than that of the Ba1xLa2x/3Zr0.09Ti0.91O3 crystal lattice. The obvious decrement of Tm in the Ba1xLaxZr0.2Ti0.8x/4O3 system is mainly due to the increase in the Zr/Ti ratio with increasing x. An evident dielectric diffuseness represented by dielectric peak, reflecting a strong frequency dispersion feature, is also shown in Fig. 2. A strong and sharp dielectric peak at 346 K without obvious
Fig. 2. Temperature dependence of permittivity for the Ba1xLaxZr0.1Ti0.9O3 ceramics.
Fig. 3. Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at 10 kHz as a function of x. The results of Ba1xLa2x/3Zr0.09Ti0.91O3 and Ba1xLaxZr0.2Ti0.8x/4O3 ceramics are also presented for comparison.
frequency dependence is observed for the sample with x = 0.00. As x increases, the strong and sharp dielectric peak gradually becomes a broad dielectric peak with frequency dispersion. A small shift in Tm toward high temperature with increasing frequency is observed in the composition with x = 0.05, indicating a distinct ferroelectric relaxor behavior. The relaxor state can be induced by a relatively low substitution levels, which is similar to the Ba1xLax[Ti1x(Mg0.5Ti0.5)x]O3 system [21]. While for isovalentsubstituted Ba(ZrxTi1x)O3 system, the relaxor state is only observed at a large substitution levels (x P 0.25). According to Shvartsma et al. [22], the relaxor behavior of BaTiO3-based compositions originates from quenched random electric fields and randomly interacting polar nanoregions at temperatures much higher than Tm [23,24]. Thus it is believed that the relaxor state can be induced by a relatively low substitution levels in the present system, which is mainly due to quenched intense random fields. The relationship between the probe frequency f and Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics can be described by the Vögel–Fulcher (VF) formula [25]:
f ¼ f0 exp
Fig. 1. XRD patterns of (a) the Ba1xLaxZr0.1Ti0.9O3 ceramics and (b) magnified (0 0 2)/(2 0 0) diffraction peak in the ranges of 2h from 43° to 47° as a function of x. The inset presents the variation of the lattice parameters as a function of x.
Ea ; kB ðT m T VF Þ
ð3Þ
where f0 is the Debye frequency, Ea the activation energy, and TVF the freezing temperature of the polarization fluctuation. Frequency dependence of Tm for the composition with x = 0.05 well fits to the VF formula, whose fitting parameters are presented in Fig. 4. The obtained parameters are similar to the results (Ea = 0.001 eV,
X. Huang et al. / Journal of Alloys and Compounds 592 (2014) 105–108
Fig. 4. Frequency dependent 1/Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics with x = 0.05. The symbols indicate the experimental data and the solid lines are the fitting curves using VF relation.
7
TVF = 228 K and f0 = 1.2 10 Hz) for Ba(Zr0.3Ti0.7)O3 composition [22]. In Ba1xLaxZr0.1Ti0.9O3 ceramics, the effect of La substitution for Ba can be expressed by Kröger–Vink notation as 3þ0
BaO þ La2 O3 þ TiO2 ! BaBa þ 2LaBa þ TiTi4þ þ 6OO :
107
Fig. 6. S–E curves of the Ba1xLaxZr0.1Ti0.9O3 ceramics.
is possibly associated with domain switching [27]. As x increases, the strain is decreased, accompanied by decreasing hysteresis. The compositions with high substitution levels (x P 0.05) do not show any strain-field hysteresis, but exhibit just electrostriction typical for paraelectric state.
ð4Þ
The higher Ea of the composition with x = 0.05 than that of 3þ0 Ba(Zr0.3Ti0.7)O3 may be ascribed to the defect pair [ LaBa TiTi4þ ].P–E ferroelectric hysteresis loops of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at room temperature and a field of 30 kV/cm with various La concentration are exhibited in Fig. 5. It is clear that S-shaped loops with high remnant polarization (Pr) are observed for the samples with x 6 0.02. As x increases, the loops start to tilt and become slim. The profiles of P–E curve are closely related to crystalline structure of the samples. The compositions with x 6 0.02 have ferroelectric tetragonal phase at room temperature, as shown in Fig. 2, thus saturated polarization loops with high Pr are obtained. For those with x P 0.05, there exists paraelectric pseudocubic phase and Tm is below room temperature. A nonlinear P–E curve implies a ferroelectric relaxor behavior due to the existence of polar nanoregions above Tm [26]. Moreover, La substitution for Ba results in lattice shrinkage, which make interspace of oxygen octahedron smaller so that the Pr decreases. The bipolar strain–electric field (S–E) curves of the Ba1xLax Zr0.1Ti0.9O3 ceramics, as shown in Fig. 6, are similar to the trend of P–E hysteresis loops. The strain level is 0.11% at 30 kV/cm for the composition with x = 0.00. The hysteretic strain
Fig. 5. P–E hysteresis loops of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at room temperature.
4. Conclusions Only single perovskite structure was observed in the Ba1xLax Zr0.1Ti0.9O3 ceramics with 0.00 6 x 6 0.10, which transformed from tetragonal to pseudocubic structure at room temperature with increasing x. Meanwhile, temperature and frequency dependences of permittivity showed a crossover from ferroelectric to relaxor behavior. For the composition with x = 0.05, the permittivity exhibited relaxor behavior with the frequency dependence of Tm obeying the VF formula, which may be ascribed to quenched intense random fields. Remnant polarization and strain decreased with increasing x due to the change in crystalline structure. Acknowledgments This research was financially supported by the Natural Science Foundation of China (Grant No. 51202234), the Zhejiang Provincial Natural Science Foundation of China (No. LY13E020005) and the Undergraduate Innovative Project of Zhejiang Province (No. 2013R409033). References [1] D. Hennings, A. Schnell, G. Simon, J. Am. Ceram. Soc. 65 (1982) 539–544. [2] Z. Yu, C. Ang, R. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 1489–1493. [3] S. Miao, J. Pokorny, U.M. Pasha, O.P. Thakur, D.C. Sinclair, I.M. Reaney, J. Appl. Phys. 106 (2009) 114111. [4] B. Shen, Q. Zhang, J. Zhai, Z. Xu, Ceram. Int. 39 (2013) S9–S13. [5] Z. Yu, C. Ang, R. Guo, A. Bhalla, Mater. Lett. 61 (2007) 326–329. [6] L. Dong, D.S. Stone, R.S. Lakes, J. Appl. Phys. 111 (2012) 084107. [7] W. Cao, J. Xiong, J. Sun, Mater. Chem. Phys. 106 (2007) 338–342. [8] P. Zheng, J. Zhang, S. Shao, Y. Tan, C. Wang, Appl. Phys. Lett. 94 (2009) 032902. [9] W. Liu, X. Ren, Phys. Rev. Lett. 103 (2009) 257602. [10] F. Bahri, H. Khemakhem, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/ j.jallcom.2013.12.118. [11] Y. Li, R. Wang, X. Ma, Z. Li, R. Sang, Y. Qu, Mater. Res. Bull. 49 (2014) 601–607. [12] Z. Sun, Y. Pu, Z. Dong, Y. Hu, X. Liu, P. Wang, M. Ge, Mater. Lett. 118 (2013) 1–4. [13] S. Bhaskar Reddy, K. Prasad Rao, M.S. Ramachandra Rao, J. Alloys Comp. 481 (2009) 692–696. [14] R. Sagar, R.L. Raibagkar, J. Alloys Comp. 549 (2013) 206–212. [15] X. Chou, J. Zhai, H. Jiang, X. Yao, J. Appl. Phys. 102 (2007) 084106. [16] S. Mahajan, D. Haridas, K. Sreenivas, O.P. Thakur, C. Prakash, Mater. Lett. 97 (2013) 40–43. [17] C. Ostos, L. Mestres, M.L. Martínez-Sarrión, J.E. García, A. Albareda, R. Perez, Solid State Sci. 11 (2009) 1016–1022. [18] X. Chou, J. Zhai, X. Yao, Mater. Chem. Phys. 109 (2008) 125–130.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Phase transformation and dielectric properties in Ba1xLaxZr0.1Ti0.9O3 ceramics Xia Huang, Jingji Zhang ⇑, Ludong Ji, Hongfang Qi, Jiangying Wang ⇑ College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 3 January 2014 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Perovskite structure Phase transformation Relaxor behavior Strain behavior
a b s t r a c t Ba1xLaxZr0.1Ti0.9O3 ceramics have been prepared by the mixed-oxide route, and their structure and dielectric properties have been investigated. Results showed that La3+ ions entered the unit cell maintaining the perovskite structure. The evolution of dielectric behavior from a sharp dielectric peak with pinched phase transitions (x = 0.00) to a broad dielectric peak with frequency dependence (0.01 6 x 6 0.02) and successively to a ferroelectric relaxor (x = 0.05) was observed with increasing La concentration. Furthermore, remnant polarization and strain decreased due to phase transformation from ferroelectric tetragonal to paraelectric pseudocubic at room temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction BaZr1xTixO3 (BZT) ferroelectric has been attracting an increasingly fundamental and practical interest for potential application in commercial capacitor and tunable microwave devices, due to high dielectric permittivity, high voltage resistance, strong dielectric nonlinearity and composition-dependent Curie temperature (TC) [1–4]. The dielectric response of BZT ceramics suggests a ferroelectric behavior with three phase transitions for the compositions with x 6 0.08, a broad dielectric peak without frequency dependence for 0.15 6 x 6 0.25 and ferroelectric relaxor behavior for x = 0.3 [5]. It was reported that greatly enhanced room temperature piezoelectric and electromechanical responses of d33 = 420 pC/N, d31 = 138 pC/N, and kp = 49% was obtained at a composition x = 0.06 [6]. As is well known, impurity-doping in BZT electroceramics has become an effective and common way to improve the material performance [7–10]. Rare earth (RE) is often referred to as ‘‘vitamin of the electroceramic industry’’. So, RE is used as a modifier to improve the performance of BZT system [11,12]. Recently, based on the vacancy defect compensation models, trivalent RE ionic substitution at A- [13,14] and B-site[15] of BZT ceramics has been widely investigated, due to their moderate ionic radii and ability to substitute both A- and B-site. It was reported that electric field induced strain increased from 0.08% to 0.18% (at 40 kV/cm applied field) for Ba1x/2Lax/3(Zr0.05Ti0.95)O3 with x = 0.02 compared to undoped BaTiO3 [16]. Additionally, it was found that the temperature at ⇑ Corresponding authors. Tel./fax: +86 571 86875613. E-mail addresses: [email protected] (J. Zhang), [email protected] (J. Wang). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.015
the dielectric maxima (Tm) of B-site deficient La-doped BZT (Ba1x LaxZr0.2Ti0.8x/4O3) ceramics decreased more markedly than that of A-site deficient La-doped BZT (Ba1xLa2x/3Zr0.09Ti0.91O3) ceramics with increasing La concentration [17,18]. In Ba1xLa2x/3Zr0.09Ti0.91O3 and Ba1xLaxZr0.2Ti0.8x/4O3 system, the effect of La substitution for Ba can be expressed by Kröger–Vink notation as, respectively,
BaO þ La2 O3 ! BaBa þ 2LaBa þ V 00Ba þ 4OO ;
ð1Þ
1 0000 BaO þ La2 O3 ! BaBa þ 2LaBa þ VTi þ 4OO : 2
ð2Þ 0000
Obviously, the number of V 00Ba increases much more than that of VTi with increasing La3+ concentration. Thus, the variation of Tm in REdoped BZT ceramics with the concentration of RE could not be related to the number of vacancies. However, the doping mechanisms in RE-doped BZT ceramics have not yet well been understood. Therefore, the focus of our research is set to the Ba1xLaxZr0.1Ti0.9O3 system, where the effect of La substitution for Ba on the dielectric, ferroelectric and piezoelectric properties is investigated. 2. Experimental procedures Ba1xLaxZr0.1Ti0.9O3 ceramics, where x = 0.00, 0.01, 0.02, 0.05 and 0.10, were prepared through the solid-state reaction method using starting chemicals of BaCO3 (99.0%), TiO2 (98.0%), ZrO2 (99.0%) and La2O3 (99.99%). Mixtures based on the compositions of Ba1xLaxZr0.1Ti0.9O3 were ball-milled with zirconia media in ethanol for 24 h and dried at 110 °C for 12 h. After drying, the powders were calcined at 1200 °C for 4 h and then re-milled for 24 h. The calcined powders, mixed with 8 wt% polyvinyl alcohol, were pressed into pellets at 100 MPa. The green pellets were kept at 550 °C for 6 h to remove the solvent and the binder. Ba1xLaxZr0.1Ti0.9O3 ceramics were sintered at 1500 °C for 4 h in air.
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X. Huang et al. / Journal of Alloys and Compounds 592 (2014) 105–108
Phase compositions of the ceramics were investigated by means of X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with Cu Ka radiation. Permittivity and loss tangent as functions of temperature were measured at frequencies from 1 to 1 MHz in temperature range of 125–450 K, using a HP 4284A precision LCR meter (Agilent, Palo Alto, CA). Polarization versus electric field (P–E) hysteresis loops were measured in a silicon oil bath by applying an electric field of triangular waveform at 10 Hz by means of a ferroelectric testing system (Radiant Precision Premier II Technology). MTI 2100 photonic sensor was used for strain measurement.
3. Results and discussion XRD patterns of the Ba1xLaxZr0.1Ti0.9O3 ceramics are shown in Fig. 1. All samples exhibit single perovskite phase, indicating the formation of Ba1xLaxZr0.1Ti0.9O3 solid solution. It can be seen from Fig. 1(b) that the compositions with x 6 0.02 show the tetragonal perovskite structure at room temperature, while those with x P 0.05 are the pseudocubic one. Lattice parameters calculated from the diffraction spectra fitted by space groups P4mm (x 6 0.02) or Pm3m (x P 0.05) are presented in the inset of Fig. 1(a). The lattice parameters decrease with increasing x. It can be explained that the ionic radius of La3+ (1.32 Å) in 12-fold coordination is smaller than that of Ba2+ (1.61 Å) [19], which led to lattice shrinkage. Temperature dependence of permittivity of all samples measured at different frequencies, varying from 1 to 1 MHz, is displayed in Fig. 2. As x increases, the dielectric peaks with paraelectric–ferroelectric transition of the Ba1xLaxZr0.1Ti0.9O3 ceramics are suppressed and broadened and shifted toward low temperature, which is attributed to the deterioration of ferroelectric long-range order [20]. Tm measured at 10 kHz decreases monotonically with increasing x, as shown in Fig. 3, which can be defined approximately by the relation Tm(K) = 2907x + 355. According to Refs. [17,18], the variation of Tm in the Ba1xLaxZr0.2Ti0.8x/4O3 and Ba1xLa2x/3Zr0.09Ti0.91O3 ceramics with x is also plotted in Fig. 3 for comparison. It is found that the decrement of Tm in the present Ba1xLaxZr0.1Ti0.9O3 system is a little higher than that of Ba1xLa2x/3Zr0.09Ti0.91O3 system, whereas it is only half of that of Ba1xLaxZr0.2Ti0.8x/4O3 system. This is because that, in the case of the same x, there exist more La3+ in the Ba1xLaxZr0.1Ti0.9O3 crystal lattice than that of the Ba1xLa2x/3Zr0.09Ti0.91O3 crystal lattice. The obvious decrement of Tm in the Ba1xLaxZr0.2Ti0.8x/4O3 system is mainly due to the increase in the Zr/Ti ratio with increasing x. An evident dielectric diffuseness represented by dielectric peak, reflecting a strong frequency dispersion feature, is also shown in Fig. 2. A strong and sharp dielectric peak at 346 K without obvious
Fig. 2. Temperature dependence of permittivity for the Ba1xLaxZr0.1Ti0.9O3 ceramics.
Fig. 3. Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at 10 kHz as a function of x. The results of Ba1xLa2x/3Zr0.09Ti0.91O3 and Ba1xLaxZr0.2Ti0.8x/4O3 ceramics are also presented for comparison.
frequency dependence is observed for the sample with x = 0.00. As x increases, the strong and sharp dielectric peak gradually becomes a broad dielectric peak with frequency dispersion. A small shift in Tm toward high temperature with increasing frequency is observed in the composition with x = 0.05, indicating a distinct ferroelectric relaxor behavior. The relaxor state can be induced by a relatively low substitution levels, which is similar to the Ba1xLax[Ti1x(Mg0.5Ti0.5)x]O3 system [21]. While for isovalentsubstituted Ba(ZrxTi1x)O3 system, the relaxor state is only observed at a large substitution levels (x P 0.25). According to Shvartsma et al. [22], the relaxor behavior of BaTiO3-based compositions originates from quenched random electric fields and randomly interacting polar nanoregions at temperatures much higher than Tm [23,24]. Thus it is believed that the relaxor state can be induced by a relatively low substitution levels in the present system, which is mainly due to quenched intense random fields. The relationship between the probe frequency f and Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics can be described by the Vögel–Fulcher (VF) formula [25]:
f ¼ f0 exp
Fig. 1. XRD patterns of (a) the Ba1xLaxZr0.1Ti0.9O3 ceramics and (b) magnified (0 0 2)/(2 0 0) diffraction peak in the ranges of 2h from 43° to 47° as a function of x. The inset presents the variation of the lattice parameters as a function of x.
Ea ; kB ðT m T VF Þ
ð3Þ
where f0 is the Debye frequency, Ea the activation energy, and TVF the freezing temperature of the polarization fluctuation. Frequency dependence of Tm for the composition with x = 0.05 well fits to the VF formula, whose fitting parameters are presented in Fig. 4. The obtained parameters are similar to the results (Ea = 0.001 eV,
X. Huang et al. / Journal of Alloys and Compounds 592 (2014) 105–108
Fig. 4. Frequency dependent 1/Tm of the Ba1xLaxZr0.1Ti0.9O3 ceramics with x = 0.05. The symbols indicate the experimental data and the solid lines are the fitting curves using VF relation.
7
TVF = 228 K and f0 = 1.2 10 Hz) for Ba(Zr0.3Ti0.7)O3 composition [22]. In Ba1xLaxZr0.1Ti0.9O3 ceramics, the effect of La substitution for Ba can be expressed by Kröger–Vink notation as 3þ0
BaO þ La2 O3 þ TiO2 ! BaBa þ 2LaBa þ TiTi4þ þ 6OO :
107
Fig. 6. S–E curves of the Ba1xLaxZr0.1Ti0.9O3 ceramics.
is possibly associated with domain switching [27]. As x increases, the strain is decreased, accompanied by decreasing hysteresis. The compositions with high substitution levels (x P 0.05) do not show any strain-field hysteresis, but exhibit just electrostriction typical for paraelectric state.
ð4Þ
The higher Ea of the composition with x = 0.05 than that of 3þ0 Ba(Zr0.3Ti0.7)O3 may be ascribed to the defect pair [ LaBa TiTi4þ ].P–E ferroelectric hysteresis loops of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at room temperature and a field of 30 kV/cm with various La concentration are exhibited in Fig. 5. It is clear that S-shaped loops with high remnant polarization (Pr) are observed for the samples with x 6 0.02. As x increases, the loops start to tilt and become slim. The profiles of P–E curve are closely related to crystalline structure of the samples. The compositions with x 6 0.02 have ferroelectric tetragonal phase at room temperature, as shown in Fig. 2, thus saturated polarization loops with high Pr are obtained. For those with x P 0.05, there exists paraelectric pseudocubic phase and Tm is below room temperature. A nonlinear P–E curve implies a ferroelectric relaxor behavior due to the existence of polar nanoregions above Tm [26]. Moreover, La substitution for Ba results in lattice shrinkage, which make interspace of oxygen octahedron smaller so that the Pr decreases. The bipolar strain–electric field (S–E) curves of the Ba1xLax Zr0.1Ti0.9O3 ceramics, as shown in Fig. 6, are similar to the trend of P–E hysteresis loops. The strain level is 0.11% at 30 kV/cm for the composition with x = 0.00. The hysteretic strain
Fig. 5. P–E hysteresis loops of the Ba1xLaxZr0.1Ti0.9O3 ceramics measured at room temperature.
4. Conclusions Only single perovskite structure was observed in the Ba1xLax Zr0.1Ti0.9O3 ceramics with 0.00 6 x 6 0.10, which transformed from tetragonal to pseudocubic structure at room temperature with increasing x. Meanwhile, temperature and frequency dependences of permittivity showed a crossover from ferroelectric to relaxor behavior. For the composition with x = 0.05, the permittivity exhibited relaxor behavior with the frequency dependence of Tm obeying the VF formula, which may be ascribed to quenched intense random fields. Remnant polarization and strain decreased with increasing x due to the change in crystalline structure. Acknowledgments This research was financially supported by the Natural Science Foundation of China (Grant No. 51202234), the Zhejiang Provincial Natural Science Foundation of China (No. LY13E020005) and the Undergraduate Innovative Project of Zhejiang Province (No. 2013R409033). References [1] D. Hennings, A. Schnell, G. Simon, J. Am. Ceram. Soc. 65 (1982) 539–544. [2] Z. Yu, C. Ang, R. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 1489–1493. [3] S. Miao, J. Pokorny, U.M. Pasha, O.P. Thakur, D.C. Sinclair, I.M. Reaney, J. Appl. Phys. 106 (2009) 114111. [4] B. Shen, Q. Zhang, J. Zhai, Z. Xu, Ceram. Int. 39 (2013) S9–S13. [5] Z. Yu, C. Ang, R. Guo, A. Bhalla, Mater. Lett. 61 (2007) 326–329. [6] L. Dong, D.S. Stone, R.S. Lakes, J. Appl. Phys. 111 (2012) 084107. [7] W. Cao, J. Xiong, J. Sun, Mater. Chem. Phys. 106 (2007) 338–342. [8] P. Zheng, J. Zhang, S. Shao, Y. Tan, C. Wang, Appl. Phys. Lett. 94 (2009) 032902. [9] W. Liu, X. Ren, Phys. Rev. Lett. 103 (2009) 257602. [10] F. Bahri, H. Khemakhem, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/ j.jallcom.2013.12.118. [11] Y. Li, R. Wang, X. Ma, Z. Li, R. Sang, Y. Qu, Mater. Res. Bull. 49 (2014) 601–607. [12] Z. Sun, Y. Pu, Z. Dong, Y. Hu, X. Liu, P. Wang, M. Ge, Mater. Lett. 118 (2013) 1–4. [13] S. Bhaskar Reddy, K. Prasad Rao, M.S. Ramachandra Rao, J. Alloys Comp. 481 (2009) 692–696. [14] R. Sagar, R.L. Raibagkar, J. Alloys Comp. 549 (2013) 206–212. [15] X. Chou, J. Zhai, H. Jiang, X. Yao, J. Appl. Phys. 102 (2007) 084106. [16] S. Mahajan, D. Haridas, K. Sreenivas, O.P. Thakur, C. Prakash, Mater. Lett. 97 (2013) 40–43. [17] C. Ostos, L. Mestres, M.L. Martínez-Sarrión, J.E. García, A. Albareda, R. Perez, Solid State Sci. 11 (2009) 1016–1022. [18] X. Chou, J. Zhai, X. Yao, Mater. Chem. Phys. 109 (2008) 125–130.
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