- Email: [email protected]
Dielectric and impedance analysis of La doped-TbMnO3
Journal of Alloys and Compounds 549 (2013) 358–361
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric and impedance analysis of La doped-TbMnO3 Yingtang Zhang a,b,⇑, Ting Tong b, William Kinsman c, Peng Jiang b, Guilai Yin a, Shengtao Li a a
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China School of Material Science & Engineering, Institute of Functional Material, Shaanxi University of Technology, Hanzhong 723003, China c Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, United States b
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 6 July 2012 Accepted 2 September 2012 Available online 21 September 2012 Keywords: High dielectric constant IBLC Maxwell–Wagner polarization
a b s t r a c t In the present work, the authors report the results of the dielectric properties of La doped TbMnO3 samples, which have remain relatively uninvestigated. With the complex impedance analysis technique adopted, dielectric properties of the samples as a function of temperature (110 K T 300 K) and frequency (1 Hz f 10 MHz) were measured. The high dielectric constant (HDC) effect was observed at the high temperature and low frequency regions. Two relaxation peaks emerged at the low and highfrequency regions, respectively. Generally, it would be inferred that the peaks correlated with the HDC behavior are due to the Maxwell–Wagner (M–W) polarization effect. We consider that the HDC effect is tightly linked with the grain and grain boundaries’ M–W polarization, often referred to as the internal barrier-layer capacitor (IBLC), and the electrode-bulk interfacial M–W polarization on the low and high temperature regions, respectively. Our results are helpful in providing further insight into the origin of dielectric properties in the perovskite oxide system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, the multiferroic and magnetoelectric material, TbMnO3 (TMO), has attracted considerable interest because of its intriguing physics and potential application in memories, sensors, and transducers [1–4]. Ferroelectricity coupled to spiral magnetic order was discovered in TMO at temperatures between 27 and 41 K [5]. In the past several years, a majority of investigations only focused on magnetoelectric properties of TMO. However, some authors had found that the dielectric properties of TMO at room temperature exhibits the HDC behavior [6], similar to that of CaCu3Ti4O12 (CCTO) [7]. The HDC materials have become increasingly important due to the strong technological needs for the further dimensional size reduction and the performance enhancement of capacitance-based components like capacitors, resonators and filters. Until present, several series of dielectric materials have been investigated, such as CCTO7, (M,N)-doped NiO (M = Li, Na, K and N = Ti, Al, Si, Ta) [8], CuO [9], Bi2/3Cu3Ti4O12, A (Fe1/2B1/2) O3 (A = Ba, Sr, Ca and B = Nb, Ta, Sb) [10], and BaTi1x(Ni1/2W1/2)xO3 [11]. In order to interpret the observed HDC phenomenon, several possible mechanisms including intrinsic and extrinsic viewpoints have been proposed [12,13], albeit controversial. Wang et al. [6] performed an impedance spectroscopy measurement demonstrating that TMO is electrically heterogeneous and consists of ⇑ Corresponding author at: State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China. Tel.: +86 916 2291226. E-mail address: [email protected] (Y. Zhang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.005
semiconducting grains and insulating grain boundaries. They attributed the origin of the HDC phenomenon to an internal barrier-layer capacitance (IBLC) effect at higher temperatures and charge carrier hopping at lower temperatures. While some authors strongly suggest that the HDC phenomenon originated from the Maxwell–Wagner interfacial relaxation [14,15], other authors explain it by surface barrier layer capacitor at high temperature [16]. In the letter, we studied the dielectric behavior properties of La doped-TMO ceramic. La doped-TMO is chosen due to the higher order of vacancies present over non-doped-TMO. The vacancies play an important role on the HDC effect. Our results reveal that La doped-TMO exhibits the HDC behavior that closely links with Maxwell–Wagner polarization from grain–grain boundaries and the electrode interface. 2. Experimental Single phase La doped-TMO used for dielectric measurements was prepared by solid-state reaction. Details about the sintering processes were reported in our preceding paper [17]. Electrical transportation properties were measured by a standard four-probe method. Dielectric properties were performed using a Novocontrol Concept 40 broadband dielectric spectrometer in the frequency range of 1 Hz–100 MHz from 110 K to 300 K. The top and the bottom Au electrodes were fabricated by the pulsed laser deposition technique with a XeCl excimer laser beam (308 nm, 27 ns, 4 Hz).
3. Results and discussion The inset of Fig. 1(a) presents the change in resistivity of the sample as a function of temperature. It can be seen that the sample
359
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
(a)
110 K 115 K 120 K 125 K 135 K 145 K 155 K 175 K 195 K 235 K 300 K
20
ln ( )
8
16
6 12
4
0.003
0.006 -1
0.009
-1
T (K )
2
6 1 Hz 10 Hz 100 Hz 2 KHz 10 KHz
4
2
(a) 0
0 0
1
2
3
4
5
6
100
7
150
6
6
4
5
2
4
ln ( (Hz))
Log(Frequency (Hz))
5
200
10 8 6 4 2 0 5
6
7
8
9
-1
1000/Tp (K ) 2
rature
tan
4
6
8
1000/T (K )
increa
4
10
-1
tan
tempe
300
1 Hz 10 Hz 100 Hz 2 KHz 10 KHz
0
3
250
Temperature (K)
se
3
2 2 1
1
(b)
(b) 100
0
1
2
3
4
5
6
7
150
200
250
300
Temperature (K)
Log(Frequency (Hz)) Fig. 1. Frequency dependence of e and tan d from 110 to 300 K. The inset of Fig. 1(a) shows the temperature of resistivity for the sample. The inset of Fig. 1(b) shows the Arrhenius plots (ln fp versus 1=T).
exhibits an obvious semiconducting behavior within a measurable temperature range. Meanwhile, the resistance of the sample (about 10 MX at 150 °C) is smaller than that of pure TMO (about 40 MX at 150 °C) reported by Wang et al. [6]. The change in resistivity that results from temperature can be formally described
ln r ¼ ln r0 þ ðEg =2kÞ=T
ð1Þ
where r is resistivity (X cm2), T temperature (K), Eg as forbidden band width that was determined to be 0.311 eV, and k Boltzmann constant. Fig. 1 reports the e (e is the real part of dielectric permittivity) and tan d dependence of frequency at different temperature from 110 to 300 K. It can be seen from Fig. 1(a) that the value of e reaches 104 in the lower range frequency at the broad temperature range, which is similar to that of CCTO. The value of e decreases with temperature decreasing, but it decreases as frequency increases. Fig. 1(b) presents tan d f curves at different temperatures. There are two peaks, which has same results seen in the curves tan d T (Fig. 2(b)). We reckon that the relaxation peaks originate from the different M–W polarization behavior which are IBLC at high temperature and interface at low temperature. Hence, the dielectric constant of La doped TMO is larger than that of undoped TMO on the low frequency region, because there are more carriers of La doped TMO than those of undoped TMO.
Fig. 2. The temperature dependence of the real part e of the dielectric permittivity of the sample at different frequency from 1 to 10000 Hz. The inset of Fig. 2(b) plots the Arrhenius relation of the measuring frequency f versus the reciprocal of the peak temperature T P .
The inset of Fig. 1(b) shows the ln fp 1=T plots (fp is frequency at the peaks of the curves) for the low and high frequency relaxations. The activation energy (E) and the preexponential were determined to be 0.17 eV and 2.1 107 Hz, 0.08 eV and 1 1010 Hz, respectively. The three magnitude of order decrease in characteristic relaxation frequency compared to pure TMO is perhaps due to the Lanthanum substitution providing more the carriers, more easily inducing a polarization. Fig. 2(a) reports e dependence of temperature at different frequency from 1 to 10000 Hz. e shows an increase with decreasing frequency and increasing temperature. It is shown that e reaches a value as high as 104. This is in 1=T P agreement with the Fig. 1. Fig 2(b) illustrates tan d as a function of temperature. The value of tan d clearly shows the Debye-like relaxation peaks, shifting to higher frequency with increasing temperature. The inset of Fig. 2(b) plots f versus the reciprocal of the peak temperature T P . It indicates that a linear relation may be obtained by ln f versus. The activation energy (E) and the preexponential were determined to be 0.17 eV and 2.1 107 Hz. Based on above results, it is clear that HDC effect is closely related to the M–W polarization. In order to illustrate these results, we performed impedance analysis to separate the different dielectric relaxations caused by the intergrains and electrodes of the ceramic materials. What is shown in the Fig. 3 is the complex impedance plot (Z 0 vs Z 00 ) for La doped-TMO at different temperature. It can be
360
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
0.012
300 K
0.20
195 K
0.15
0.008
0.10 0.004 1 Hz
10 MHz
0.05
0.000 0.00 0.00
Z (
)
6
0.01
0.02
250 0.0
0.03
145 K
200
4
0.1
0.2
0.3
0.4
0.5
0.6
110 K
150 100
2
50 0
0 0
4
8
12
16
0
100
200
300
400
500
600
Z (
700
)
Fig. 3. The complex impedance plot for La doped-TMO at different temperaturez.
200.0k
Z ( )
150.0k
100.0k R0
50.0k
Ri Qi
0.0
Oxide/electrode R-Q element
0
100k
200k
Rd1
Rd2
Rd3
Qd1
Qd2
Qd3
Internal barrier-layer R-Q elements
300k
400k
500k
600k
Z ( ) Fig. 4. The results that simulate using equivalent circuits of the complex impedance plot at 195 K. The inset presents the equivalent fitting circuit.
seen obviously that there are two types of relaxation behaviors on low and high frequency regions at low temperature, as seen in Fig. 3. However, the two types of relaxation behavior are of no significant difference at 300 K. It is often concluded that the two relaxation behaviors here originate from IBLC and electrode interfacial M–W polarization behavior, respectively. On low temperature region, there are many electron phases which likely are La-rich and La-poor phases between grain boundaries. These phases generate various kinds of relaxation behaviors. To clarify the origination of HDC, the complex impedance data were modeled using equivalent circuits based on a two-part RCPE unit. One part is the internal barrier layer, and the other part
is the oxide/electrode barrier layer. each part contains a resistor (R) and a constant phase element (CPE) in parallel. The impedance of the element is defined by 1=Z CPE ¼ Q ðjxÞn where xð¼ 2pf Þ is the angular frequency, j is the square root of 1, Q and nð0 n 1Þ are adjustable parameters independent of the frequency. Fig. 4 shows the fitting results using equivalent circuits. It can be seen that the experimental complex impedance spectrum is perfectly modeled with a series of three parallel R-CPE unit circuits. In the R-CPE unit of the internal barrier-layer, there are three, two and one R-CPE unit for 195, 235 and 300 K (The fitting results using equivalent circuits at 235 and 300 K are not shown), respectively. Every R-CPE unit represents a different intergrain which generates the IBLC effect. The simulated data for R and Q are shown in table 1. The results indicate that the HDC effect at high temperatures is predominately ascribed to the Maxwell–Wagner effect, originating from the interfacial polarization of the oxide/electrode. This relaxation behavior can generate the large dielectric loss. This fact can be proved by Fig. 1(b) and Fig. 2(b). With the increase of temperature, the localized carriers become unfrozen, which elevate the dipolar effects. At the same time, the unfreezing process leads to a large elevation in the number of hopping carriers, which greatly decreases the resistance, as confirmed by the temperature dependence of the dc resistance shown on the inset of Fig. 1(a) and table 1. It is well known that when the carriers hop to the vicinity of blocking grain boundaries or electrodes and form space charges, the relaxation of the space charges will result in an apparent giant dielectric constant. Due to more carriers yielded by the distorted lattice in La-doped TMO than that in undoped TMO, the dielectric constant of La-doped TMO is larger than that of undoped TMO at 300 K [6]. On the low temperature region, there are lots of space charges between grain boundaries that result in the dielectric relaxation (IBLC effect). With the increase of temperature the number of carriers increases, and ultimately
Table 1 The simulated data for R and Q using the equivalent circuit model. Temperature (K)
R0
Ri
Qi
Rd3
Qd3
Rd2
Qd2
Rd1
Qd1
195 235 300
5.987 0.01 26.28
4.1 104 4827 353.2
2.9 109 8.2 1010 1.1 1010
4.5 104
2 108
1.1 104 1.2 105
4.3 1011 3.3 108
4.8 105 0.01 3.3 104
2.2 108 239.9 3.7 108
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
accumulate on the interface of the oxide/electrode. Therefore, the interfacial polarization becomes gradually enhanced, and the interfacial Maxwell–Wagner polarization plays a predominant role on the HDC behavior. These results and the proposed model are helpful for comprehending the origination of the HDC phenomenon of the oxide. This comprehension can provide at least some hints on the highpermittivity dielectric materials on optimization and application microelectronics. 4. Conclusions In conclusion, the dielectric behaviors of La doped TMO have been investigated systematically. Our results indicate that the dielectric properties in Lanthanum doped Terbium Manganese Oxide perovskites primarily emerge from the IBLC and interfacial M–W polarization on the low temperature region and the high temperature regions. Acknowledgments We acknowledge the financial support from the Special Program for State Key Laboratory of Electrical Insulation and Power Equipment (EIPE11207) and Special Program for Education Bureau of Province (12JK0953). This work was also supported by the China Postdoctoral Science Foundation (2011M501454).
361
References [1] S. Ishiwata, Y. Taguchi, H. Murakawa, Y. Onose, Y. Tokura, Science 319 (2008) 1643. [2] S.W. Cheong, M. Mostovoy, Nature Mater. 7 (2007) 13. [3] Y. Yamasaki, H. Sagayama, T. Goto, M. Matsuura, K. Hirota, T. Arima, Y. Tokura, Phys. Rev. Lett. 98 (2007) 147204. [4] A.M. Shuvaev, V.D. Travkin, V.Y. Ivanov, A.A. Mukhin, A. Pimenov, Phys. Rev. Lett. 104 (2010) 097202. [5] T. Kimura1, T. Goto1, H. Shintani, K. Ishizaka, T. Arima, Y. Tokura1, Nature Mater. 426 (2003) 54. [6] C.C. Wang, Y.M. Cui, L.W. Zhang, Appl. Phys. Lett. 90 (2007) 012904. [7] D.C. Sinclair, T.B. Adams, F.D. Morrison, A.R. West, Appl. Phys. Lett. 80 (2002) 2153. [8] J.P. Kumar, B.K. Chaudhuri, Appl. Phys. Lett. 88 (2006) 182901. [9] S. Sarkar, J.P. Kumar, B.K. Chaudhuri, H. Sakata, Appl. Phys. Lett. 89 (2006) 212905. [10] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A.W. Sleight, J. Solid State Chem. 151 (2000) 323. [11] F. Zhao, Zh.X. Yue, J. Pei, D.H. Yang, Zh.L. Gui, L.T. Li, Appl. Phys. Lett. 91 (2007) 052903. [12] W. Li, O. Auciello, R.N. Premnath, B. Kabius, Appl. Phys. Lett. 96 (2010) 162907. [13] D.C. Sinclair, T.B. Adams, F.D. Morrison, A.R. West, Appl. Phys. Lett. 80 (2002) 2153. [14] M. Viviani, M. Bassoli, V. Buscaglia1, M.T. Buscaglia, P. Nanni, J. Phys. D: Appl. Phys. 42 (2009) 175407. [15] W. Li, R.W. Schwartz, Phys. Rev. B 75 (2007) 012104. [16] S. Krohns, P. Lunkenheimer, S.G. Ebbinghaus, A. Loidl, J. Appl. Phys. 103 (2008) 084107. [17] Y.T. Zhang, Z.Y. Chen, C.C. Wang, Q. Jie, H.B. Lu, J. Magn. Magn. Mater. 321 (2009) 1199.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric and impedance analysis of La doped-TbMnO3 Yingtang Zhang a,b,⇑, Ting Tong b, William Kinsman c, Peng Jiang b, Guilai Yin a, Shengtao Li a a
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China School of Material Science & Engineering, Institute of Functional Material, Shaanxi University of Technology, Hanzhong 723003, China c Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, United States b
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 6 July 2012 Accepted 2 September 2012 Available online 21 September 2012 Keywords: High dielectric constant IBLC Maxwell–Wagner polarization
a b s t r a c t In the present work, the authors report the results of the dielectric properties of La doped TbMnO3 samples, which have remain relatively uninvestigated. With the complex impedance analysis technique adopted, dielectric properties of the samples as a function of temperature (110 K T 300 K) and frequency (1 Hz f 10 MHz) were measured. The high dielectric constant (HDC) effect was observed at the high temperature and low frequency regions. Two relaxation peaks emerged at the low and highfrequency regions, respectively. Generally, it would be inferred that the peaks correlated with the HDC behavior are due to the Maxwell–Wagner (M–W) polarization effect. We consider that the HDC effect is tightly linked with the grain and grain boundaries’ M–W polarization, often referred to as the internal barrier-layer capacitor (IBLC), and the electrode-bulk interfacial M–W polarization on the low and high temperature regions, respectively. Our results are helpful in providing further insight into the origin of dielectric properties in the perovskite oxide system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, the multiferroic and magnetoelectric material, TbMnO3 (TMO), has attracted considerable interest because of its intriguing physics and potential application in memories, sensors, and transducers [1–4]. Ferroelectricity coupled to spiral magnetic order was discovered in TMO at temperatures between 27 and 41 K [5]. In the past several years, a majority of investigations only focused on magnetoelectric properties of TMO. However, some authors had found that the dielectric properties of TMO at room temperature exhibits the HDC behavior [6], similar to that of CaCu3Ti4O12 (CCTO) [7]. The HDC materials have become increasingly important due to the strong technological needs for the further dimensional size reduction and the performance enhancement of capacitance-based components like capacitors, resonators and filters. Until present, several series of dielectric materials have been investigated, such as CCTO7, (M,N)-doped NiO (M = Li, Na, K and N = Ti, Al, Si, Ta) [8], CuO [9], Bi2/3Cu3Ti4O12, A (Fe1/2B1/2) O3 (A = Ba, Sr, Ca and B = Nb, Ta, Sb) [10], and BaTi1x(Ni1/2W1/2)xO3 [11]. In order to interpret the observed HDC phenomenon, several possible mechanisms including intrinsic and extrinsic viewpoints have been proposed [12,13], albeit controversial. Wang et al. [6] performed an impedance spectroscopy measurement demonstrating that TMO is electrically heterogeneous and consists of ⇑ Corresponding author at: State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China. Tel.: +86 916 2291226. E-mail address: [email protected] (Y. Zhang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.005
semiconducting grains and insulating grain boundaries. They attributed the origin of the HDC phenomenon to an internal barrier-layer capacitance (IBLC) effect at higher temperatures and charge carrier hopping at lower temperatures. While some authors strongly suggest that the HDC phenomenon originated from the Maxwell–Wagner interfacial relaxation [14,15], other authors explain it by surface barrier layer capacitor at high temperature [16]. In the letter, we studied the dielectric behavior properties of La doped-TMO ceramic. La doped-TMO is chosen due to the higher order of vacancies present over non-doped-TMO. The vacancies play an important role on the HDC effect. Our results reveal that La doped-TMO exhibits the HDC behavior that closely links with Maxwell–Wagner polarization from grain–grain boundaries and the electrode interface. 2. Experimental Single phase La doped-TMO used for dielectric measurements was prepared by solid-state reaction. Details about the sintering processes were reported in our preceding paper [17]. Electrical transportation properties were measured by a standard four-probe method. Dielectric properties were performed using a Novocontrol Concept 40 broadband dielectric spectrometer in the frequency range of 1 Hz–100 MHz from 110 K to 300 K. The top and the bottom Au electrodes were fabricated by the pulsed laser deposition technique with a XeCl excimer laser beam (308 nm, 27 ns, 4 Hz).
3. Results and discussion The inset of Fig. 1(a) presents the change in resistivity of the sample as a function of temperature. It can be seen that the sample
359
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
(a)
110 K 115 K 120 K 125 K 135 K 145 K 155 K 175 K 195 K 235 K 300 K
20
ln ( )
8
16
6 12
4
0.003
0.006 -1
0.009
-1
T (K )
2
6 1 Hz 10 Hz 100 Hz 2 KHz 10 KHz
4
2
(a) 0
0 0
1
2
3
4
5
6
100
7
150
6
6
4
5
2
4
ln ( (Hz))
Log(Frequency (Hz))
5
200
10 8 6 4 2 0 5
6
7
8
9
-1
1000/Tp (K ) 2
rature
tan
4
6
8
1000/T (K )
increa
4
10
-1
tan
tempe
300
1 Hz 10 Hz 100 Hz 2 KHz 10 KHz
0
3
250
Temperature (K)
se
3
2 2 1
1
(b)
(b) 100
0
1
2
3
4
5
6
7
150
200
250
300
Temperature (K)
Log(Frequency (Hz)) Fig. 1. Frequency dependence of e and tan d from 110 to 300 K. The inset of Fig. 1(a) shows the temperature of resistivity for the sample. The inset of Fig. 1(b) shows the Arrhenius plots (ln fp versus 1=T).
exhibits an obvious semiconducting behavior within a measurable temperature range. Meanwhile, the resistance of the sample (about 10 MX at 150 °C) is smaller than that of pure TMO (about 40 MX at 150 °C) reported by Wang et al. [6]. The change in resistivity that results from temperature can be formally described
ln r ¼ ln r0 þ ðEg =2kÞ=T
ð1Þ
where r is resistivity (X cm2), T temperature (K), Eg as forbidden band width that was determined to be 0.311 eV, and k Boltzmann constant. Fig. 1 reports the e (e is the real part of dielectric permittivity) and tan d dependence of frequency at different temperature from 110 to 300 K. It can be seen from Fig. 1(a) that the value of e reaches 104 in the lower range frequency at the broad temperature range, which is similar to that of CCTO. The value of e decreases with temperature decreasing, but it decreases as frequency increases. Fig. 1(b) presents tan d f curves at different temperatures. There are two peaks, which has same results seen in the curves tan d T (Fig. 2(b)). We reckon that the relaxation peaks originate from the different M–W polarization behavior which are IBLC at high temperature and interface at low temperature. Hence, the dielectric constant of La doped TMO is larger than that of undoped TMO on the low frequency region, because there are more carriers of La doped TMO than those of undoped TMO.
Fig. 2. The temperature dependence of the real part e of the dielectric permittivity of the sample at different frequency from 1 to 10000 Hz. The inset of Fig. 2(b) plots the Arrhenius relation of the measuring frequency f versus the reciprocal of the peak temperature T P .
The inset of Fig. 1(b) shows the ln fp 1=T plots (fp is frequency at the peaks of the curves) for the low and high frequency relaxations. The activation energy (E) and the preexponential were determined to be 0.17 eV and 2.1 107 Hz, 0.08 eV and 1 1010 Hz, respectively. The three magnitude of order decrease in characteristic relaxation frequency compared to pure TMO is perhaps due to the Lanthanum substitution providing more the carriers, more easily inducing a polarization. Fig. 2(a) reports e dependence of temperature at different frequency from 1 to 10000 Hz. e shows an increase with decreasing frequency and increasing temperature. It is shown that e reaches a value as high as 104. This is in 1=T P agreement with the Fig. 1. Fig 2(b) illustrates tan d as a function of temperature. The value of tan d clearly shows the Debye-like relaxation peaks, shifting to higher frequency with increasing temperature. The inset of Fig. 2(b) plots f versus the reciprocal of the peak temperature T P . It indicates that a linear relation may be obtained by ln f versus. The activation energy (E) and the preexponential were determined to be 0.17 eV and 2.1 107 Hz. Based on above results, it is clear that HDC effect is closely related to the M–W polarization. In order to illustrate these results, we performed impedance analysis to separate the different dielectric relaxations caused by the intergrains and electrodes of the ceramic materials. What is shown in the Fig. 3 is the complex impedance plot (Z 0 vs Z 00 ) for La doped-TMO at different temperature. It can be
360
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
0.012
300 K
0.20
195 K
0.15
0.008
0.10 0.004 1 Hz
10 MHz
0.05
0.000 0.00 0.00
Z (
)
6
0.01
0.02
250 0.0
0.03
145 K
200
4
0.1
0.2
0.3
0.4
0.5
0.6
110 K
150 100
2
50 0
0 0
4
8
12
16
0
100
200
300
400
500
600
Z (
700
)
Fig. 3. The complex impedance plot for La doped-TMO at different temperaturez.
200.0k
Z ( )
150.0k
100.0k R0
50.0k
Ri Qi
0.0
Oxide/electrode R-Q element
0
100k
200k
Rd1
Rd2
Rd3
Qd1
Qd2
Qd3
Internal barrier-layer R-Q elements
300k
400k
500k
600k
Z ( ) Fig. 4. The results that simulate using equivalent circuits of the complex impedance plot at 195 K. The inset presents the equivalent fitting circuit.
seen obviously that there are two types of relaxation behaviors on low and high frequency regions at low temperature, as seen in Fig. 3. However, the two types of relaxation behavior are of no significant difference at 300 K. It is often concluded that the two relaxation behaviors here originate from IBLC and electrode interfacial M–W polarization behavior, respectively. On low temperature region, there are many electron phases which likely are La-rich and La-poor phases between grain boundaries. These phases generate various kinds of relaxation behaviors. To clarify the origination of HDC, the complex impedance data were modeled using equivalent circuits based on a two-part RCPE unit. One part is the internal barrier layer, and the other part
is the oxide/electrode barrier layer. each part contains a resistor (R) and a constant phase element (CPE) in parallel. The impedance of the element is defined by 1=Z CPE ¼ Q ðjxÞn where xð¼ 2pf Þ is the angular frequency, j is the square root of 1, Q and nð0 n 1Þ are adjustable parameters independent of the frequency. Fig. 4 shows the fitting results using equivalent circuits. It can be seen that the experimental complex impedance spectrum is perfectly modeled with a series of three parallel R-CPE unit circuits. In the R-CPE unit of the internal barrier-layer, there are three, two and one R-CPE unit for 195, 235 and 300 K (The fitting results using equivalent circuits at 235 and 300 K are not shown), respectively. Every R-CPE unit represents a different intergrain which generates the IBLC effect. The simulated data for R and Q are shown in table 1. The results indicate that the HDC effect at high temperatures is predominately ascribed to the Maxwell–Wagner effect, originating from the interfacial polarization of the oxide/electrode. This relaxation behavior can generate the large dielectric loss. This fact can be proved by Fig. 1(b) and Fig. 2(b). With the increase of temperature, the localized carriers become unfrozen, which elevate the dipolar effects. At the same time, the unfreezing process leads to a large elevation in the number of hopping carriers, which greatly decreases the resistance, as confirmed by the temperature dependence of the dc resistance shown on the inset of Fig. 1(a) and table 1. It is well known that when the carriers hop to the vicinity of blocking grain boundaries or electrodes and form space charges, the relaxation of the space charges will result in an apparent giant dielectric constant. Due to more carriers yielded by the distorted lattice in La-doped TMO than that in undoped TMO, the dielectric constant of La-doped TMO is larger than that of undoped TMO at 300 K [6]. On the low temperature region, there are lots of space charges between grain boundaries that result in the dielectric relaxation (IBLC effect). With the increase of temperature the number of carriers increases, and ultimately
Table 1 The simulated data for R and Q using the equivalent circuit model. Temperature (K)
R0
Ri
Qi
Rd3
Qd3
Rd2
Qd2
Rd1
Qd1
195 235 300
5.987 0.01 26.28
4.1 104 4827 353.2
2.9 109 8.2 1010 1.1 1010
4.5 104
2 108
1.1 104 1.2 105
4.3 1011 3.3 108
4.8 105 0.01 3.3 104
2.2 108 239.9 3.7 108
Y. Zhang et al. / Journal of Alloys and Compounds 549 (2013) 358–361
accumulate on the interface of the oxide/electrode. Therefore, the interfacial polarization becomes gradually enhanced, and the interfacial Maxwell–Wagner polarization plays a predominant role on the HDC behavior. These results and the proposed model are helpful for comprehending the origination of the HDC phenomenon of the oxide. This comprehension can provide at least some hints on the highpermittivity dielectric materials on optimization and application microelectronics. 4. Conclusions In conclusion, the dielectric behaviors of La doped TMO have been investigated systematically. Our results indicate that the dielectric properties in Lanthanum doped Terbium Manganese Oxide perovskites primarily emerge from the IBLC and interfacial M–W polarization on the low temperature region and the high temperature regions. Acknowledgments We acknowledge the financial support from the Special Program for State Key Laboratory of Electrical Insulation and Power Equipment (EIPE11207) and Special Program for Education Bureau of Province (12JK0953). This work was also supported by the China Postdoctoral Science Foundation (2011M501454).
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