Two kinds of anomalous dielectric phenomena in Pr-doped SrTiO3 ceramics: The Debye-like and ferroelectric-like behaviors

Physica B 405 (2010) 4881–4885

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Two kinds of anomalous dielectric phenomena in Pr-doped SrTiO3 ceramics: The Debye-like and ferroelectric-like behaviors Cheng Liu, Peng Liu n College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, PR China

a r t i c l e in fo

abstract

Article history: Received 30 June 2010 Received in revised form 22 September 2010 Accepted 22 September 2010

Sr1  xPrxTiO3 ceramics (0.00 r x r 0.07) were investigated over a broad temperature and frequency range for their interesting dielectric behaviors. Two kinds of anomalous dielectric behaviors were observed, i.e. Debye-like relaxation behaviors for the specimens of x r 0.03 and ferroelectric-like dielectric peaks appearing in x Z 0.04. According to our experiments, the Debye-like relaxation, as well as the CDC behavior (er  3000) detected in x ¼ 0.01, was closely related to the IBLC mechanism. The anomalous dielectric peaks appearing in x Z0.04 were ascribed to an electron transportation process instead of a TF–P. & 2010 Elsevier B.V. All rights reserved.

Keywords: Ceramics Dielectric properties Ferroelectricity Electrical conductivity

1. Introduction Perovskite structure oxides, ABO3, have been investigated widely due to their dielectric, semi-conducting, conducting and superconducting behaviors [1]. SrTiO3 (ST), a so-called quantum paraelectrics belonging to such a kind of material, is applied extensively in electronic, mechanical and ceramic industries for its low loss tangent, good temperature stability and high permittivity. Its simple cubic pervoskite structure transforms to tetragonal structure (CaTiO3-type) with strong elastic anomaly after down to 110 K, which yields an incipient ferroelectric phase [2,3]. The dielectric permittivity of ST increases monotonously as temperature decreases, and then levels off at a saturated value without any emergence of permittivity peaks until at extremely low temperatures. These interesting properties – structural phase transition and quantum paraelectric behavior – were attributed to the restraint of the ferroelectric state by zero-point quantum fluctuations of the lattice [3]. Recently, many investigations have been carried out in pure and doped ST systems experimentally and theoretically, aiming at the intrinsic mechanism of ST for its interesting behaviors [4–18]. Most results revealed that the dielectric properties of doped ST were dominated by the oxygen vacancies as well as doping type (donor/ acceptor) to a large extent. Doping additions, like rare earth ions with hetero-valence, could generate oxygen vacancies and lattice defects, thus affecting the dielectric properties. Ang and Yu [19] observed the coexistence of several dielectric peaks with different

n

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0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.027

physical characteristics in Bi doped ST ceramics, and some of them disappeared with increase in Bi concentration. Dura´n et al. [17] observed a permittivity anomalous peak in Pr-doped ST ceramics around 238 1C, which suggested that the ferroelectric state could exist at room temperature in this system. Ranjan et al. [18] also reported a high-temperature relaxor ferroelectric behavior in the Pr-doped ST system. All these interesting and distinct results in doped ST ceramics illustrate a complex coupling effect of doped ST ceramics [7,20–22], and have stimulated further research in this direction for its intrinsic mechanism. In this study, we investigated Pr-doped ST ceramics for their interesting dielectric behaviors as mentioned above. Two kinds of anomalous dielectric behaviors were found in our experiment, i.e., the Debye-like relaxation behaviors observed in xr0.03 and the anomalous dielectric peaks detected in the xZ0.04 specimens. Dc conductivity, temperature and frequency-dependent dielectric performance measurements were carried out to elucidate these phenomena. The impedance spectrum within a wide frequency range (40 Hz–100 MHz) was also obtained. In addition, we provide a detailed analysis through the Arrhenius law and the frequencydependent response within an equivalent-circuit picture depicted by the internal barrier layer capacitance (IBLC) model.

2. Experimental details 2.1. Sample preparation Sr1  xPrxTiO3 ceramics (0.00 rxr0.07) were prepared by a traditional solid-state reaction method, using precursors such as Pr6O11 (99.9%), SrCO3 (99.61%) and TiO2 (99.99%). Starting

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powders were ball-milled in ethyl alcohol for 10 h using polypropylene bottle and zirconia balls. After drying at 100 1C for 12 h, the mixed powders were calcined in alumina crucibles at 1050 1C for 3 h at a heating rate of 3 1C/min in air atmosphere. Then the calcined powders were mixed with polyvinyl alcohol (PVA) and compacted into disks (10 mm in diameter and 1 mm in thickness) at a pressure of 1.12  108 kg/m2 after a second ball milling. The green pellets were sintered at 1325 1C in air for 10 h. 2.2. Sample characterization Powder X-ray diffraction proved Pr-doped ST to be a single cubic phase (not shown here). Dielectric performance measurements were carried out after the parallel surfaces were coated with silver electrodes (fired at 600 1C for 30 min). Dc bulk resistivity was measured by a high resistance meter (Agilent 4339B, USA). Temperature dependent dielectric spectra were obtained in a temperature range of 20–650 1C under various frequencies (100 Hz–1 MHz) through a precision LCR meter (Agilent E4980A, USA). Frequency-dependent dielectric and impedance spectra were obtained in a frequency range of 40 Hz–100 MHz at room temperature through a precision impedance analyzer (Agilent 4294A, USA).

3. Results and discussion Fig. 1 shows the variation of the dc resistivity and conductivity of Sr1  xPrxTiO3 specimens with x. The resisitivity value of specimens with xr0.03 is much higher (exceeding 108 O cm) than that of the samples with x Z0.04 (106–107 O cm). The dc conductivity shown in the inset of Fig. 1 exhibits high conductance more intuitively when xZ0.04. Fig. 2(a1)–(a8) and (b1)–(b8) shows temperature dependence of the dielectric constants er and loss tangents tan d of Sr1  xPrxTiO3 specimens (0.00 rxr0.07) within a temperature range of 20–650 1C under various frequencies (100 Hz–1 MHz). For the pure ST ceramic, er exhibits a step-like increase within a temperature range of 187–540 1C in Fig. 2(a1), where typical dielectric relaxation loss peaks are observed in Fig. 2(b1). After x increases to 0.01, two relaxation processes located around 20–150 and 130–400 1C emerge, accompanied by two sets of relaxation loss peaks (LPs for short, marked as A and B) obviously, as shown in Fig. 2(a2) and (b2). Such relaxation behavior still exists even with x up to 0.03 (see Fig. 2(a3), (a4), (b3) and (b4)). It is notable that the permittivity of x¼ 0.01 attains a value of 3000 at room temperature and 1 kHz, exhibiting a colossal dielectric constant (CDC) behavior. Then the er value decreases to 501 and 532 for

Fig. 1. Dc resistivity and conductivity of Sr1  xPrxTiO3 ceramics (0.00 r xr 0.07).

x¼0.02 and 0.03, respectively, but is still higher than that of the pure ST ceramics (er  300) and the specimens with x Z0.04 (er  150–200) (see Fig. 2(e)), which can be attributed to reduction of grain size [23]. With x increasing to a higher level (x Z0.04), relaxation processes A and B disappear; instead, anomalous dielectric permittivity peaks start to rise around 265 1C (see Fig. 2(a5)–(a8)). The location of these dielectric peaks is close to the reported location around 238 1C, which was considered a ferroelectric–paraelectric transition peak (TF–P) [17,18]. Remarkably, another three LPs (marked as C, D and E) are detected within the temperature range of 180–324 1C (C), 140–240 1C (D) and 260–360 1C (E) in Fig. 2(b5)–(b8), whose locations are not in complete accordance with that of the permittivity peaks. As known, typical TF–P is always accompanied by a maximum value of er and tan d around the phase transition temperature, such as Ba0.6Sr0.4TiO3 ceramics—a typical ferroelectrics adopted here for comparison, whose dielectric temperature spectra are exhibited in Fig. 2(c) and (d). Therefore, the anomalous dielectric peaks observed in Sr1  xPrxTiO3 specimens with high Pr level (xZ0.04) are controversial and need to be investigated deeply. In Fig. 3 the frequency dependence of dielectric constant (a, c) and the loss tangent (c, d) of Sr1  xPrxTiO3 ceramics (0.00rx r0.07) at room temperature are shown. Both the pure ST ceramic and the samples of x Z0.04 exhibit good stability within a wide frequency range of 40 Hz–100 MHz, except a relatively high tan d value of specimens with xZ0.04 at low frequencies (see Fig. 3(c) and (d)). In Fig. 3(a), the er value represents a decline at low frequencies ( o104 Hz) at first, then exhibits a relatively flat step during the mid frequencies (104– 106 Hz), and descends again with frequency increasing to above  106 Hz, corresponding to two loss peaks (marked as I and II in Fig. 3(b)) located at around 103 and 106 Hz. Such behavior seemingly accords well with a Debye-like relaxation [24,25]. Now, the two anomalous behaviors, i.e. the Debye-like relaxation behaviors in low Pr level specimens (0ox r0.03) and the anomalous er peaks in highly Pr-doped ones (xZ0.04), are detected. Hence there are two questions that need to be answered necessarily: what is the origin of the Debye-like relaxation in Pr-doped ST ceramics with low Pr level (0ox r0.03)? What are the anomalous er peaks found in highly Pr-doped specimens (x Z0.04)? To further corroborate these pendent questions, activation energy analysis is adopted in the investigation of the intrinsic mechanism. We calculated the Ea values of the different relaxation processes (A–E) occurring in different Sr1  xPrxTiO3 ceramic specimens. The corresponding fp variation with temperature in tan d–frequency curves, as shown in Fig. 4(a), obeys the Arrhenius law well: fp ¼ f0 expðEa =kB TÞ

ð1Þ

where fp is the dielectric characteristic frequency, f0 the preexponential factor, Ea the activation energy, kB the Boltzmann constant and T the absolute temperature. Six different relaxation processes (marked as LPs of the pure ST and LPs A–E) are classified clearly in different colors in Fig. 4(a). In Fig. 4(b), the Ea value of the LPs of the pure ST ceramic is 0.93 eV, existing well with the activation energy 0.98 eV for diffusion of the doubly ionized oxygen vacancies VO in ST crystal [26]. Thus the LPs of the pure ST ceramics are caused by the oxygen vacancies. Oxygen vacancies can be easily formed by loss of oxygen in the high-sintering process, yet they can be restrained by a small amount of rare earth donor dopant [27]. This can be used to explain the higher loss peak values appearing in pure ST than those of the Pr-doped samples. However, two new relaxation processes (LPs A and B) are induced in specimens of 0 ox r0.03. From Fig. 4(b), the Ea values

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Fig. 2. Temperature dependence of dielectric constant (a1–a8, c) and loss tangent (b1–b8, d) of Sr1  xPrxTiO3 ceramics (0.00 rx r0.07) and Ba0.6Sr0.4TiO3 ceramics, respectively. Permittivity as a function of x under 1 kHz at room temperature is indicated in frame (e).

of LPs A and B are 0.59, 0.55, 0.61 and 0.78, 0.75, 0.79 eV, respectively, for x ¼0.01, 0.02, 0.03. The lower values 0.59, 0.55 and 0.61 eV of LPs A approach the activation energy of grain boundaries 0.54 eV [28], indicating that the LPs A can be attributed to the effect of interfacial layers. The higher values 0.78, 0.75 and 0.79 eV of LPs B illuminate a transition of localized charge carriers stimulated during a heating process [29]. These two relaxation processes correlate well with the Debye-like relaxation behaviors detected in Fig. 3(a) and (b) for that the loss peak I located around 103 Hz is already ascribed to a transition of localized charge carriers stimulated during a heating process, while the loss peak II around 106 Hz originates from the grain boundary effect [24]. Fig. 4(c) shows the impedance complex plane plots for the Sr1  xPrxTiO3 ceramics (0.00 rxr0.03) at room temperature. The impedance measurement results are analyzed by an equivalent circuit shown in the inset of Fig. 4(c), consisting of resistor– capacitor (RC) elements as described in other reports [30–32]. The complex impedance of this equivalent circuit can be expressed

accurately by Z ¼

1 R1 g þ ioCg

þ

1 R1 gb þ ioCgb

¼ Z 0 iZ00

ð2Þ

in which o represents the angular frequency, and (Rg,Cg) and (Rgb,Cgb) represent the resistance and capacitance related to the grains and grain boundaries, respectively. According to the IBLC model, the relationships, Rg 5Rgb and Cg 5Cgb, are widely accepted. The Rgb value is determined by the curvature radius of the RgbCgb semicircle. From Fig. 4(c), the Rgb value of x ¼0.01 ceramics is higher than that of the pure ST and the compositions with x ¼0.02 and 0.03. The inset of Fig. 4(c) shows the magnification of the Z0  Z00 in high frequencies, from which the Rg value of x¼ 0.01 is less than the other two compositions. The higher Rgb value—the insulation of grain boundaries (GB) can be attributed to the reoxidation of GB during the cooling process in air [33,34]. The lower Rg value—the semi-conduction of grains is ascribed to the formation of defects by donor doping in the ST

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Fig. 3. Frequency dependence of dielectric constant (a, c) and loss tangent (c, d) of Sr1  xPrxTiO3 ceramics (0.00 rx r 0.07) at room temperature.

Fig. 4. (a) ln fp as a function of 1000/T on the relaxation loss peaks A–E of Sr1  xPrxTiO3 ceramics (0.00 r x r0.07). (b) Ea values of different relaxation processes attained from the Arrhenius law. (c) Impedance complex plane plots for the Sr1  xPrxTiO3 ceramics (0.00 r x r0.03) at room temperature.

matrix [35]. The conductive ions may arise from SrTiO3

Pr!PrSr þ e0

ð3Þ

which is given by the electroneutrality condition (ENC): n  ½Pr 

ð4Þ

Hence the decrease of the Rgb value and increase of the Rg value with x lead to er value reduction from 3000 of x¼0.01 to 501

and 532 of x¼ 0.02 and 0.03, respectively. The higher Rgb and lower Rg value also correspond to the higher dc resistivity values for the compositions with x ¼0.01, 0.02 and 0.03, as shown in Fig. 1. These results indicate that the Debye-like relaxation behavior can be explained by the IBLC mechanism. In Fig. 4(b), the Ea value of LPs C is 0.81 eV, indicating a similar procedure of LPs B and dilution of the interfacial layer effects. It also suggests a transition state from the Debye-like relaxation to the ferroelectric-like behavior when x¼ 0.04. With x increasing to

C. Liu, P. Liu / Physica B 405 (2010) 4881–4885

a higher level (x 40.04), LPs D and E substitute LPs A–C, and the Ea values of LPs D and E are 1.34, 1.64, 1.9 eV and 1.44, 1.63, 2.01 eV, respectively, for the samples with x ¼0.05, 0.06 and 0.07, much higher than that of the doubly ionized oxygen vacancies VO , localized charge carriers and grain boundaries. It has been reported that the Ea value of electron transport across the grain boundary is between 1.7–2.0 eV [36], which approximates to the Ea values of 1.34–2.01 eV of LPs D and E. Thus the LPs D and E are attributed to such an electron transportation process. In our opinion, the transporting electron can also be derived from the procedure depicted in Eq. (3) and its amount can enhance with x, which will lead to high dc conductivity exhibited in the inset of Fig. 1 for specimens with higher Pr level (xZ0.04). Moreover, according to the Goldschmidt tolerance factor pffiffiffi t ¼ ðrO þ rA Þ= 2ðrO þrB Þ ð5Þ where rO, rA and rB are the ionic radii of O, A and B ions of ABO3 pervoskite oxides [37]. Oxides of t 41 are often ferroelectrics (such as BaTiO3), while materials with t E1 possess full symmetry cubic structures, like BaZrO3, and perovskites with t o1 exhibit distorted non-ferroelectric structures with tilted BO6 octahedra, e.g., perovskite CaTiO3 [38]. The radius of Pr3 + is smaller than that of Sr2 + ðrPr3 þ =rSr2 þ ¼ 0:78Þ. Thereby the substitution of Pr3 + for the Sr2 + site, making tE0.89 ( o1), would induce distorted nonferroelectric structures and suppress the ferroelectric state [39]. All these results suggest that the anomalous dielectric peaks observed in highly Pr-doped samples (xZ0.04) is a complex stimulated conduction process during a heating procedure instead of a TF–P. 4. Conclusions In conclusion, we observed two kinds of anomalous dielectric behaviors in Sr1  xPrxTiO3 (0.00 rx r0.07) ceramics, i.e. the Debye-like relaxation behaviors in specimens with xr0.03 and the ferroelectric-like dielectric peaks appearing in x Z0.04. Six different relaxation processes were distinguished according to the Arrhenius law: LPs A was attributed to the effect of interfacial layers; LPs B and C were caused by the transition of localized charge carriers stimulated at a high temperature; LPs D and E corresponded to the electron transport across the grain boundary during the heating process. Therefore, the Debye-like relaxation behavior, including the CDC behavior, was closely related to the IBLC mechanism. The anomalous dielectric peaks appearing in x Z0.04 were attributed to electron transportation during the heating process.

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Research Funds for the Central Universities (Program no. 2010ZYGX018).

References [1] [2] [3] [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14]

[15] [16]

[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

Acknowledgements

[33] [34] [35] [36]

This work was supported by the National Natural Science Foundation of China (Grant no. 50872078) and the Fundamental

[37] [38] [39]

Chen Ang, Zhi Yu, L.E. Cross, Phys. Rev. B 62 (2000) 228. M.A. Saifi, L.E. Cross, Phys. Rev. B 2 (1970) 677. ¨ K.A. Muller, H. Burkard, Phys. Rev. B 19 (1979) 3593. ¨ J.G. Bednorz, K.A. Muller, Phys. Rev. Lett. 52 (1984) 2289. U. Bianchi, W. Kleemann, J.G. Bednorz, J. Phys.: Condens. Matter 6 (1994) 1229. V.V. Lemanov, E.P. Smirnova, P.P. Syrnikov, E.A. Tarakanov, Phys. Rev. B 54 (1996) 3151. Chen Ang, Zhi Yu, P. Lunkenheimer, J. Hemberger, A. Loidl, Phys. Rev. B 59 (1999) 6670. V. Porokhonskyy, A. Pashkin, V. Bovtun, J. Petzelt, M. Savinov, P. Samoukhina, T. Ostapchuk, J. Pokorny, M. Avdeev, A. Kholkin, P. Vilarinho, Phys. Rev. B 69 (2004) 144104. M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y.-J. Shan, T. Nakamura, Phys. Rev. Lett. 82 (1999) 3540. M. Takesada, M. Itoh, T. Yagi, Phys. Rev. Lett. 96 (2006) 227602. D.E. Grupp, A.M. Goldman, Science 276 (1997) 392. A.A. Sirenko, I.A. Akimov, J.R. Fox, A.M. Clark, H.C. Li, W. Si, X.X. Xi, Phys. Rev. Lett. 82 (1999) 4500. H.C. Li, W. Si, A.D. West, X.X. Xi, Appl. Phys. Lett. 73 (1998) 190. J. Petzelt, T. Ostapchuk, I. Gregora, I. Rychetsky´, S. Hoffmann-Eifert, A.V. Pronin, Y. Yuzyuk, B.P. Gorshunov, S. Kamba, V. Bovtun, J. Pokorny´, M. Savinov, V. Porokhonskyy, D. Rafaja, P. Vanˇek, A. Almeida, M.R. Chaves, A.A. Volkov, M. Dressel, R. Waser, Phys. Rev. B 64 (2001) 184111. G.A. Samara, J. Phys.: Condens. Matter 15 (2003) R367. J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, D.G. Schlom, Nature (London) 430 (2004) 758. A. Dura´n, E. Martı´nez, J.A. Dı´az, J.M. Siqueiros, J. Appl. Phys. 97 (2005) 104109. R. Ranjan, R. Hackl, A. Chandra, E. Schmidbauer, D. Trots, H. Boysen, Phys. Rev. B 76 (2007) 224109. Chen Ang, Zhi Yu, J. Appl. Phys. 91 (2002) 1487. C. Ang, Z. Yu, P.M. Vilarinho, J.L. Baptista, Phys. Rev. B 57 (1998) 7403. C. Ang, J.F. Scott, Z. Yu, H. Ledbetter, J.L. Baptista, Phys. Rev. B 59 (1999) 6661. C. Ang, Z. Yu, J. Hemberger, P. Lunkenheimer, A. Loidl, Phys. Rev. B 59 (1999) 6665. Cheng Liu, Peng Liu, Jian-ping Zhou, Ying He, Li-na Su, Lei Cao, Huai-wu Zhang, J. Appl. Phys. 107 (2010) 094108. Chunhong Mu, Huaiwu Zhang, Ying He, Peng Liu, Jian Shen, Mater. Sci. Eng. B 162 (2009) 195. S. Krohns, P. Lunkenheimer, S.G. Ebbinghaus, Appl. Phys. Lett. 91 (2007) 022910. A.E. Paladino, J. Am. Ceram. Soc. 48 (1965) 476. D.M. Smyth, in: The Defect Chemistry of Metal Oxides, Oxford University Press, Oxford, 2000. R.K. Grubbs, E.L. Venturini, P.G. Clem, J.J. Richardson, B.A. Tuttle, G.A. Samara, Phys. Rev. B 72 (2005) 104111. W. Li, R.W. Schwartza, Appl. Phys. Lett. 89 (2006) 242906. Y.H. Lin, J. Cai, M. Li, C.W. Nan, Appl. Phys. Lett. 88 (2006) 172902. T.B. Adams, D.C. Sinclair, A.R. West, Phys. Rev. B 73 (2006) 094124. Hao Xue, Xiangfeng Guan, Rong Yu, Zhaoxian Xiong, J. Alloys Compd. 482 (2009) L14. Tsang-Tse Fang, Li-Then Mei, J. Am. Ceram. Soc. 90 (2007) 638. Tsang-Tse Fang, Hsu-Kai Shiau, J. Am. Ceram. Soc. 87 (2004) 2072. Ralf Moos, Karl Heinz Hardtl, J. Am. Ceram. Soc. 80 (1997) 2549. Fumimasa Horikiri, Li Qun Han, Atsushi Kaimai, Takanori Otake, Keiji Yashiro, Tatsuya Kawada, Junichiro Mizusaki, Solid State Ionics 177 (2006) 2555. V.M. Goldschmidt, Naturwissenschaften 14 (1926) 477. A.M. Glazer, Acta Crystallogr. Sect. B 28 (1972) 3384. R. Kagimura, M. Suewattana, D.J. Singh, Phys. Rev. B 78 (2008) ) 012103.