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Origin of giant dielectric response in LiCuNb3O9 distorted perovskite ceramics
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Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics Xiuli Chen, Dandan Ma, Fen He, Guisheng Huang, Huanfu Zhou ∗ Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi Key Laboratory in Universities of Clean Metallurgy and Comprehensive Utilization for Non-ferrous Metals Resources, Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 30 November 2016 Accepted 9 December 2016 Available online xxx Keywords: LiCuNb3 O9 Distorted perovskite Relaxation behaviour Giant dielectric constant
a b s t r a c t LiCuNb3 O9 ceramics with the distorted cubic perovskite structure were prepared by a solid-state reaction method. The ceramic exhibited a very large value of permittivity (∼4.4 × 104 at 100 kHz) at room temperature (∼300 K) and a low-temperature dielectric relaxation behaviour following the Arrhenius law. The origin of the giant dielectric response of the LiCuNb3 O9 ceramics was correlated with the structure of the ceramics. The barrier layers in the grain boundaries and the mixed-valent structure of Cu+ /Cu2+ were found to contribute to the giant permittivity of the ceramics and confirmed by X-ray spectroscopy and complex impedance spectroscopy analyses. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As a perovskite-type dielectric material, CaCu3 Ti4 O12 exhibits a high value of dielectric constant (up to 105 ), making it useful for applications like high-performance capacitive memories, volume capacitors and voltage dependent capacity devices [1–3]. Recently, other Cu-contained complex perovskites, such as NaCu3 Ru4 O12 [4], Na1/3 Ca1/3 Bi1/3 Cu3 Ti4 O12 [5], Na1/2 Bi1/2 Cu3 Ti4 O12 [6] and B2/3 Cu3 Ti4 O12 (B = La, Y and Bi) [7,8], have also attracted increased scientific attention because of their intriguing mechanisms and potential applications. Until now, extensive attention has been focused on revealing the origin of giant dielectric response in CaCu3 Ti4 O12 . Mukherjee et al. [9] proposed that the giant permittivity of material might be due to a Maxwell-Wagner type contribution of depletion layers at the interface between sample and electrode. West et al. [10] and Romero et al. [11] suggested that an internal barrier layer capacitor structure consisted of semiconducting grains separated by thin insulating grain boundaries leads to high values of permittivity. Cohen et al. [12] reported that the giant permittivity of CaCu3 Ti4 O12 arises from the spatial inhomogeneity of its local dielectric response. However, none of
the above mentioned hypotheses have been widely accepted, and the mechanism for the origin of the giant dielectric response is still not clearly understood. In addition, the dielectric behaviours of the Cu-contained perovskite ceramics are similar to those of AFe1/2 B1/2 O3 (A = Ba, Sr; B = Nb, Ta), and LuFe2 O4 ceramics; they exhibit a Debye-like relaxation, in which their permittivities are nearly independent of the frequency and the temperature below the relaxation frequency [8,13]. Recently, studies based on the AFe1/2 B1/2 O3 (A = Ba,Sr; B = Nb,Ta) [14–16] and LuFe2 O4 [17] ceramics have provided important information regarding the origin of the giant dielectric response. Apart from the above-mentioned Cu-containing compounds, Sarkar et al. [18] reported that CuO shows extraordinarily high permittivity (’ ∼ 104 ). On this basis, we surmise that LiCuNb3 O9 may also show interesting dielectric properties. The first report on the distorted perovskite structure of LiCuNb3 O9 (a = 7.5286(1) Å with I23 space group) was published by Sato et al. [19]. However, there are very few studies on the dielectric LiCuNb3 O9 ceramics and their dielectric properties. In the present work, the giant dielectric property and dielectric relaxation of LiCuNb3 O9 have been studied. Furthermore, the relationship between the giant dielectric response and the structure was investigated in detail.
∗ Corresponding author. E-mail address: [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019
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Fig. 2. XRD pattern of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. Fig 1. Variation in the bulk density of the LiCuNb3 O9 ceramics with sintering temperature.
2. Experimental The LiCuNb3 O9 ceramics were prepared by the conventional mixed oxide route from the oxide powders (≥99%, Guo-Yao Co. Ltd., Shanghai, China) of Li2 CO3 , CuO and Nb2 O5 . Stoichiometric proportions of the above raw materials were mixed in alcohol (≥99.7%) medium with zirconia media for 4 h, and then calcined at 900 ◦ C for 4 h in air. The resultant powders were mixed with 5 wt % polyvinyl alcohol (PVA) and pressed into pellets of 12 mm in diameter and ∼2.0 mm in thickness. The samples were heated to 550 ◦ C in air for 24 h to burn out the PVA binder. CuO is a sintering aid with low-melting point, which leads to a low sintering temperature of CuO/TiO2 (500 ◦ C) [20,21]. Therefore, the samples were sintered at temperatures in the range of 975–1075 ◦ C for 4 h. The crystal structure of the ceramic was determined using Xray diffractometer (XRD) (Cu K␣1 , 1.54059A, Model X’Pert PRO, PANalytical, Almelo, Holland). The microstructures of the sintered samples were studied using a scanning electron microscope (SEM, Model JSM6380LV, JEOL, Tokyo, Japan). The dielectric properties were measured using an impedance analyzer (HP4284A) at different temperatures in the range of 73–423 K and 300–823 K.
Fig. 3. SEM micrograph of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h.
3. Results and Discussion Fig. 1 presents the bulk density of the LiCuNb3 O9 ceramics as a function of the sintering temperature. As the sintering temperature increases from 975 ◦ C to 1050 ◦ C, the bulk density increases from ∼4.85 g/cm3 to 4.96 g/cm3 , which is equivalent to ∼97% of the theoretical density. The theoretical density of LiCuNb3 O9 ceramic is calculated by the Rietveld refinement using the formula: theory = (n × Mr )/(Vc × NA ), where n is the number of unit cell molecules in an unit cell, Vc is the volume per unit, Mr is the atomic mass in g/mol, and NA is the Avorgado’s constant (6.023 × 1023 ). With further increase in the sintering temperature, the bulk density of the LiCuNb3 O9 ceramic decreases slightly due to oversintering. This result indicates that the sintering temperature for densification of the LiCuNb3 O9 ceramic is ∼1050 ◦ C. Fig. 2 shows the XRD pattern of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. All the peak positions match those of standard LiCuNb3 O9 (PDF: 44-0591) and no other phases are detected. LiCuNb3 O9 ceramic belongs to the cubic crystal structure with space group I23, and the refined cell parameters are a = b = c = 7.542406 Å, ˛ = ˇ = = 90◦ , V = 429.1 Å3 . Fig. 3 shows the SEM image of the polished and etched LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. The ceramic is dense, and the grain sizes are in the range of 0.5–2 m. Some sub-grains are observed inside the large grains. The EDS analysis showed that the ratio of the atomic number of the elements, Cu and Nb is 1:3,
Fig. 4. XPS spectrum of Cu 2p regions in the LiCuNb3 O9 ceramic.
indicating the existence of LiCuNb3 O9 , which agrees well with the XRD result. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the existence of the mixed-valent structure of the polyvalent ions (Cu) in the ceramic. The XPS spectrum of Cu 2p for the LiCuNb3 O9 ceramic is displayed in Fig. 4. All the peak positions were corrected according to the standard C 1s peak at 284.8 eV. The Gaussian-Lorentzian was used to fit Cu 2p3/2 peak. It is evident that the Cu 2p3/2 peak can be deconvoluted into two peaks, showing the existence of Cu+ and Cu2+ . The kinetic energies of Cu2p3/2 for Cu2+ and Cu+ are 917.85, and 916.15 eV, respectively. Similar results have been reported by Chen [22].
Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019
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Fig. 6. Complex impedance spectra of the LiCuNb3 O9 ceramic at the various temperatures. The inset shows the Arrhenius plots of the resistance of the bulk and grain boundary of the sample.
Fig. 5. (a) Temperature dependence of the real part permittivity of LiCuNb3 O9 ceramics at various frequencies ranging from 100 to 100 kHz. (b) Temperature dependence of the imaginary part of permittivity of LiCuNb3 O9 ceramics. The inset shows the frequency dependence of the peak temperature of the imaginary permittivity during the low-temperature dielectric relaxation of the LiCuNb3 O9 ceramics. The solid symbols represent the experimental data and the lines represent the Arrhenius fitting.
The temperature dependences of the real and the imaginary parts of the permittivity of the LiCuNb3 O9 sample are demonstrated in Fig. 5. The giant permittivity (∼4.4 × 104 at 100 kHz) is observed at room temperature (∼300 K). The sample exhibited dielectric relaxation behaviour at 125–300 K (as shown in Fig. 5(a)). The permittivity increased significantly with increasing in the temperature above 150 K. In order to determine the dielectric relaxation behaviour, the variation in the critical temperature with frequency in the ´ıı´-T plots was investigated (as shown in Fig. 5(b)). The relationship between the critical temperature and the frequency can be expressed by the Arrhenius law, f = f 0 exp(-E a /kT ),
(1)
where f0 , Ea and k are the pre-exponential term, the activation energy and the Boltzmann constant, respectively. The value of Ea was found to be 0.146 eV by fitting the data to Eq. (1). In the case of the CaCu3 Ti4 O12 ceramic, Chen et al. [22] reported that the giant dielectric behaviour along with a low-temperature dielectric relaxation is due to the electronic ferroelectricity, resulting from the ordering of Cu+ /Cu2+ or Ti3+ /Ti4+ in the arrangement of polar symmetry. Similar phenomenon was observed in LuFe2 O4 ceramics [17]. In the present work, the XPS spectrum in Fig. 4 confirmed the coexistence of Cu+ and Cu2+ in the LiCuNb3 O9 ceramics. Therefore,
the low-temperature dielectric relaxation of the LiCuNb3 O9 ceramics can be associated with the polar arrangement of the electrons in the mixed-valent structure of Cu+ /Cu2+ . Complex impedance spectroscopy analysis is an effective method to identify the contribution from the bulk and/or grain boundary to the electrical characteristics in polycrystalline materials. Fig. 6 presents the impedance spectra of the LiCuNb3 O9 ceramic at various temperatures. The presence of only one arc in the whole temperature range indicates that the contribution came from the grain boundary. The resistances of the bulk and the grain boundary in the samples at different temperatures can be calculated from the intercepts of each semicircular arc with the real axis. The resistances as a function of the reciprocal temperatures are plotted, and they fit the Arrhenius relation which can now be expressed as: R = R0 exp(E a /kB T ),
(2)
Here, R0 is the pre-exponential term and Ea is the activation energy. The relationship between the natural logarithms of the R values and the reciprocal of the temperature is observed to be linear in the temperature range from 523 to 683 K. From the inset of Fig. 6, we can obtain the resistance of the bulk and the grain boundary. Accordingly, the Ea values for the grain boundaries and the grain interiors were calculated to be 0.372 and 0.107 eV, respectively. The Ea of the grain boundary is much larger than that of the bulk, indicating that the bulk and the grain boundary exhibit different characteristics of electrical transport. In general, the grain boundary affects the electrical conductivity due to the potential barrier originating from the Cu-rich phases at the grain boundary [23–25]. Furthermore, the Maxwell-Wagner polarization is usually observed in materials that consist of grains separated by more insulating inter-grain barriers. Therefore, the significantly giant dielectric response of the LiCuNb3 O9 ceramic is related not only to the mixed-valent structure of Cu+ /Cu2+ , but also to the Maxwell-Wagner polarisation mechanism. 4. Conclusions LiCuNb3 O9 ceramics were prepared by a solid-state reaction method. The structure, dielectric properties, and giant dielectric response of LiCuNb3 O9 ceramics have been investigated. The LiCuNb3 O9 ceramic with the distorted perovskite structure exhibited giant permittivity and dielectric relaxation behaviour, which are correlated to the mixed-valent structure of Cu+ /Cu2+ and the Maxwell-Wagner polarisation mechanism. The results of the
Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019
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present study are expected to contribute to the development of high-performance giant dielectric materials for capacitor applications. Acknowledgments This work was supported by Natural Science Foundation of China (Nos. 11664008, 11464009 and 11364012), Natural Science Foundation of Guangxi (Nos. 2015GXNSFDA139033, 2014GXNSFAA118312, 2013GXNSFAA019291 and 2014GXNSFAA118326), Research Start-up Funds Doctor of Guilin University of Technology (Nos. 002401003281 and 002401003282) and Project of Outstanding Young Teachers’ Training in Higher Education Institutions of Guangxi. References [1] P. Thomas, K. Dwarakanath, K.B.R. Varma, Effect of calcium stoichiometry on the dielectric response of CaCu3 Ti4 O12 ceramics, J. Eur. Ceram. Soc. 32 (8) (2012) 1681–1690. [2] X.J. Luo, Y.S. Liu, C.P. Yang, S.S. Chen, S.L. Tang, K. Bärnere, Oxygen vacancy related defect dipoles in CaCu3Ti4O12 : Detected byelectron paramagnetic resonance spectroscopy, J. Eur. Ceram. Soc. 35 (7) (2015) 2073–2081. [3] R. Schmidt, M.C. Stennett, N.C. Hyatt, J. Pokorny, J. Prado-Gonjal, M. Li, D.C. Sinclair, Effects of sintering temperature on the internal barrier layer capacitor (IBLC) structure in CaCu3 Ti4 O12 (CCTO) ceramics, J. Eur. Ceram. Soc. 32 (12) (2012) 3313–3323. [4] M.A. Subramanian, A.W. Sleight, ACu3 Ti4 O12 and ACu3 Ru4 O12 perovskites: high dielectric constants and valence degeneracy, Solid State Sci. 4 (3) (2002) 347–351. [5] P. Kumonsa, P. Thongbai, B. Putasaeng, T. Yamwong, S. Maensiri, Na1/3 Ca1/3 Bi1/3 Cu3 Ti4 O12 : a new giant dielectric perovskite ceramic in ACu3 Ti4 O12 compounds, J. Eur. Ceram. Soc. 35 (5) (2015) 1441–1447. [6] M.C. Ferrarelli, T.B. Adams, A. Feteira, D.C. Sinclair, A.R. West, High intrinsic permittivity in Na1/2 Bi1/2 Cu3 Ti4 O12 , Appl. Phys. Lett. 89 (21) (2006) 212904. [7] W. Hao, J. Zhang, Y. Tan, W. Su, Giant dielectric-permittivity phenomena of compositionally and structurally CaCu3 Ti4 O12 -like oxide ceramics, J. Am. Ceram. Soc. 92 (12) (2009) 2937–2943. [8] J. Liu, C.G. Duan, W.G. Yin, W.N. Mei, R.W. Smith, J.R. Hardy, Large dielectric constant and Maxwell-Wagner relaxation in Bi2/3 Cu3 Ti4 O12 , Phys. Rev. B 70 (14) (2004) 144106. [9] R. Mukherjee, G. Lawes, B. Nadgorny, Enhancement of high dielectric permittivity in CaCu3 Ti4 O12 /RuO2 composites in the vicinity of the percolation threshold, Appl. Phys. Lett. 105 (7) (2014) 072901.
[10] A.R. West, T.B. Adams, F.D. Morrison, D.C. Sinclair, Novel high capacitance materials: BaTiO3 : La and CaCu3 Ti4 O12 , J. Eur. Ceram. Soc. 24 (6) (2004) 1439–1448. [11] J.J. Romero, P. Leret, F. Rubio-Marcos, A. Quesada, J.F. Fernández, Evolution of the intergranular phase during sintering of CaCu3 Ti4 O12 ceramics, J. Eur. Ceram. Soc. 30 (3) (2010) 737–742. [12] M.H. Cohen, J.B. Neaton, L.X. He, D. Vanderbilt, Extrinsic models for the dielectric response of CaCu3 Ti4 O12 , J. Appl. Phys. 94 (5) (2003) 3299–3306. [13] J. Liu, C.G. Duan, W.N. Mei, R.W. Smith, J.R. Hardy, Dielectric properties and Maxwell-Wagner relaxation of compounds ACu3 Ti4 O12 (A = Ca, Bi2/3 , Y2/3 , La2/3 ), J. Appl. Phys. 98 (9) (2005) 093703. [14] S. Saha, T.P. Sinha, Low-temperature scaling behavior of BaFe0.5 Nb0.5 O3 , Phys. Rev. B 65 (13) (2002) 134103. [15] Y.Y. Liu, X.M. Chen, X.Q. Liu, L. Li, Giant dielectric response and relaxor behaviors induced by charge and defect ordering in Sr(Fe1/2 Nb1/2 )O3 ceramics, Appl. Phys. Lett. 90 (19) (2007) 192905. [16] Z. Wang, X.M. Chen, L. Ni, X.Q. Liu, Y.Y. Liu, Dielectric relaxations in Ba(Fe1/2 Ta1/2 )O3 giant dielectric constant ceramics, Appl. Phys. Lett. 90 (10) (2007) 102905. [17] N. Ikeda, H. Ohsumi, K. Ohwada, K. Ishii, T. Inami, K. Kakurai, Y. Murakami, K. Yoshii, S. Mori, Y. Horibe, H. Kito, Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2 O4 , Nature (London) 436 (7054) (2005) 1136–1138. [18] S. Sarkar, P.K. Jana, B.K. Chaudhuri, H. Sakata, Copper (II) oxide as a giant dielectric material, Appl. Phys. Lett. 89 (21) (2006) 212905. [19] M. Satoand, Y. Hama, Structure of new perovskite-related compounds, LiCuM3 O9 (M = Nb, Ta), J. Mater. Chem. 3 (3) (1993) 233–236. [20] L.L. Zhu, M.H. Hong, G.W. Ho, Fabrication of wheat grain textured TiO2 /CuO composite nanofibers for enhanced solar H2 generation and degradation performance, Nano Energy 11 (2015) 28–37. [21] F. Forcade, R. Snyders, G. Guisbiers, B. Gonzalez, X. Noirfalise, E. Vigil, Impact of the chemical precursor on the crystalline constitution of nano-CuO/TiO2 films, Mater Res. Bull. 70 (1) (2015) 248–253. [22] L. Ni, X.M. Chen, Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics, Appl. Phys. Lett. 91 (12) (2007) 122905. [23] R. Schmidt, S. Pandey, P. Fiorenza, D.C. Sinclair, Non-stoichiometry in CaCu3 Ti4 O12 (CCTO) ceramics, RSC Adv. 3 (34) (2013) 14580–14589. [24] T.B. Adams, D.C. Sinclair, A.R. West, Giant barrier layer capacitance effects in CaCu3 Ti4 O12 ceramics, Adv. Mater. 14 (18) (2002) 1321–1323. [25] L. Singh, I.W. Kim, B.C. Sin, K.D. Mandal, U.S. Rai, A. Ullah, H. Chung, Y. Lee, Dielectric studies of a nano-crystalline CaCu2.9 Zn0.1 Ti4 O12 electro-ceramic by one pot glycine assisted synthesis from inexpensive TiO2 for energy storage capacitors, RSC Adv. 4 (95) (2014) 52770–52784.
Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019
ARTICLE IN PRESS
JECS-10971; No. of Pages 4
Journal of the European Ceramic Society xxx (2016) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics Xiuli Chen, Dandan Ma, Fen He, Guisheng Huang, Huanfu Zhou ∗ Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi Key Laboratory in Universities of Clean Metallurgy and Comprehensive Utilization for Non-ferrous Metals Resources, Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 30 November 2016 Accepted 9 December 2016 Available online xxx Keywords: LiCuNb3 O9 Distorted perovskite Relaxation behaviour Giant dielectric constant
a b s t r a c t LiCuNb3 O9 ceramics with the distorted cubic perovskite structure were prepared by a solid-state reaction method. The ceramic exhibited a very large value of permittivity (∼4.4 × 104 at 100 kHz) at room temperature (∼300 K) and a low-temperature dielectric relaxation behaviour following the Arrhenius law. The origin of the giant dielectric response of the LiCuNb3 O9 ceramics was correlated with the structure of the ceramics. The barrier layers in the grain boundaries and the mixed-valent structure of Cu+ /Cu2+ were found to contribute to the giant permittivity of the ceramics and confirmed by X-ray spectroscopy and complex impedance spectroscopy analyses. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As a perovskite-type dielectric material, CaCu3 Ti4 O12 exhibits a high value of dielectric constant (up to 105 ), making it useful for applications like high-performance capacitive memories, volume capacitors and voltage dependent capacity devices [1–3]. Recently, other Cu-contained complex perovskites, such as NaCu3 Ru4 O12 [4], Na1/3 Ca1/3 Bi1/3 Cu3 Ti4 O12 [5], Na1/2 Bi1/2 Cu3 Ti4 O12 [6] and B2/3 Cu3 Ti4 O12 (B = La, Y and Bi) [7,8], have also attracted increased scientific attention because of their intriguing mechanisms and potential applications. Until now, extensive attention has been focused on revealing the origin of giant dielectric response in CaCu3 Ti4 O12 . Mukherjee et al. [9] proposed that the giant permittivity of material might be due to a Maxwell-Wagner type contribution of depletion layers at the interface between sample and electrode. West et al. [10] and Romero et al. [11] suggested that an internal barrier layer capacitor structure consisted of semiconducting grains separated by thin insulating grain boundaries leads to high values of permittivity. Cohen et al. [12] reported that the giant permittivity of CaCu3 Ti4 O12 arises from the spatial inhomogeneity of its local dielectric response. However, none of
the above mentioned hypotheses have been widely accepted, and the mechanism for the origin of the giant dielectric response is still not clearly understood. In addition, the dielectric behaviours of the Cu-contained perovskite ceramics are similar to those of AFe1/2 B1/2 O3 (A = Ba, Sr; B = Nb, Ta), and LuFe2 O4 ceramics; they exhibit a Debye-like relaxation, in which their permittivities are nearly independent of the frequency and the temperature below the relaxation frequency [8,13]. Recently, studies based on the AFe1/2 B1/2 O3 (A = Ba,Sr; B = Nb,Ta) [14–16] and LuFe2 O4 [17] ceramics have provided important information regarding the origin of the giant dielectric response. Apart from the above-mentioned Cu-containing compounds, Sarkar et al. [18] reported that CuO shows extraordinarily high permittivity (’ ∼ 104 ). On this basis, we surmise that LiCuNb3 O9 may also show interesting dielectric properties. The first report on the distorted perovskite structure of LiCuNb3 O9 (a = 7.5286(1) Å with I23 space group) was published by Sato et al. [19]. However, there are very few studies on the dielectric LiCuNb3 O9 ceramics and their dielectric properties. In the present work, the giant dielectric property and dielectric relaxation of LiCuNb3 O9 have been studied. Furthermore, the relationship between the giant dielectric response and the structure was investigated in detail.
∗ Corresponding author. E-mail address: [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019
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Fig. 2. XRD pattern of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. Fig 1. Variation in the bulk density of the LiCuNb3 O9 ceramics with sintering temperature.
2. Experimental The LiCuNb3 O9 ceramics were prepared by the conventional mixed oxide route from the oxide powders (≥99%, Guo-Yao Co. Ltd., Shanghai, China) of Li2 CO3 , CuO and Nb2 O5 . Stoichiometric proportions of the above raw materials were mixed in alcohol (≥99.7%) medium with zirconia media for 4 h, and then calcined at 900 ◦ C for 4 h in air. The resultant powders were mixed with 5 wt % polyvinyl alcohol (PVA) and pressed into pellets of 12 mm in diameter and ∼2.0 mm in thickness. The samples were heated to 550 ◦ C in air for 24 h to burn out the PVA binder. CuO is a sintering aid with low-melting point, which leads to a low sintering temperature of CuO/TiO2 (500 ◦ C) [20,21]. Therefore, the samples were sintered at temperatures in the range of 975–1075 ◦ C for 4 h. The crystal structure of the ceramic was determined using Xray diffractometer (XRD) (Cu K␣1 , 1.54059A, Model X’Pert PRO, PANalytical, Almelo, Holland). The microstructures of the sintered samples were studied using a scanning electron microscope (SEM, Model JSM6380LV, JEOL, Tokyo, Japan). The dielectric properties were measured using an impedance analyzer (HP4284A) at different temperatures in the range of 73–423 K and 300–823 K.
Fig. 3. SEM micrograph of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h.
3. Results and Discussion Fig. 1 presents the bulk density of the LiCuNb3 O9 ceramics as a function of the sintering temperature. As the sintering temperature increases from 975 ◦ C to 1050 ◦ C, the bulk density increases from ∼4.85 g/cm3 to 4.96 g/cm3 , which is equivalent to ∼97% of the theoretical density. The theoretical density of LiCuNb3 O9 ceramic is calculated by the Rietveld refinement using the formula: theory = (n × Mr )/(Vc × NA ), where n is the number of unit cell molecules in an unit cell, Vc is the volume per unit, Mr is the atomic mass in g/mol, and NA is the Avorgado’s constant (6.023 × 1023 ). With further increase in the sintering temperature, the bulk density of the LiCuNb3 O9 ceramic decreases slightly due to oversintering. This result indicates that the sintering temperature for densification of the LiCuNb3 O9 ceramic is ∼1050 ◦ C. Fig. 2 shows the XRD pattern of the LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. All the peak positions match those of standard LiCuNb3 O9 (PDF: 44-0591) and no other phases are detected. LiCuNb3 O9 ceramic belongs to the cubic crystal structure with space group I23, and the refined cell parameters are a = b = c = 7.542406 Å, ˛ = ˇ = = 90◦ , V = 429.1 Å3 . Fig. 3 shows the SEM image of the polished and etched LiCuNb3 O9 ceramic sintered at 1050 ◦ C for 4 h. The ceramic is dense, and the grain sizes are in the range of 0.5–2 m. Some sub-grains are observed inside the large grains. The EDS analysis showed that the ratio of the atomic number of the elements, Cu and Nb is 1:3,
Fig. 4. XPS spectrum of Cu 2p regions in the LiCuNb3 O9 ceramic.
indicating the existence of LiCuNb3 O9 , which agrees well with the XRD result. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the existence of the mixed-valent structure of the polyvalent ions (Cu) in the ceramic. The XPS spectrum of Cu 2p for the LiCuNb3 O9 ceramic is displayed in Fig. 4. All the peak positions were corrected according to the standard C 1s peak at 284.8 eV. The Gaussian-Lorentzian was used to fit Cu 2p3/2 peak. It is evident that the Cu 2p3/2 peak can be deconvoluted into two peaks, showing the existence of Cu+ and Cu2+ . The kinetic energies of Cu2p3/2 for Cu2+ and Cu+ are 917.85, and 916.15 eV, respectively. Similar results have been reported by Chen [22].
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Fig. 6. Complex impedance spectra of the LiCuNb3 O9 ceramic at the various temperatures. The inset shows the Arrhenius plots of the resistance of the bulk and grain boundary of the sample.
Fig. 5. (a) Temperature dependence of the real part permittivity of LiCuNb3 O9 ceramics at various frequencies ranging from 100 to 100 kHz. (b) Temperature dependence of the imaginary part of permittivity of LiCuNb3 O9 ceramics. The inset shows the frequency dependence of the peak temperature of the imaginary permittivity during the low-temperature dielectric relaxation of the LiCuNb3 O9 ceramics. The solid symbols represent the experimental data and the lines represent the Arrhenius fitting.
The temperature dependences of the real and the imaginary parts of the permittivity of the LiCuNb3 O9 sample are demonstrated in Fig. 5. The giant permittivity (∼4.4 × 104 at 100 kHz) is observed at room temperature (∼300 K). The sample exhibited dielectric relaxation behaviour at 125–300 K (as shown in Fig. 5(a)). The permittivity increased significantly with increasing in the temperature above 150 K. In order to determine the dielectric relaxation behaviour, the variation in the critical temperature with frequency in the ´ıı´-T plots was investigated (as shown in Fig. 5(b)). The relationship between the critical temperature and the frequency can be expressed by the Arrhenius law, f = f 0 exp(-E a /kT ),
(1)
where f0 , Ea and k are the pre-exponential term, the activation energy and the Boltzmann constant, respectively. The value of Ea was found to be 0.146 eV by fitting the data to Eq. (1). In the case of the CaCu3 Ti4 O12 ceramic, Chen et al. [22] reported that the giant dielectric behaviour along with a low-temperature dielectric relaxation is due to the electronic ferroelectricity, resulting from the ordering of Cu+ /Cu2+ or Ti3+ /Ti4+ in the arrangement of polar symmetry. Similar phenomenon was observed in LuFe2 O4 ceramics [17]. In the present work, the XPS spectrum in Fig. 4 confirmed the coexistence of Cu+ and Cu2+ in the LiCuNb3 O9 ceramics. Therefore,
the low-temperature dielectric relaxation of the LiCuNb3 O9 ceramics can be associated with the polar arrangement of the electrons in the mixed-valent structure of Cu+ /Cu2+ . Complex impedance spectroscopy analysis is an effective method to identify the contribution from the bulk and/or grain boundary to the electrical characteristics in polycrystalline materials. Fig. 6 presents the impedance spectra of the LiCuNb3 O9 ceramic at various temperatures. The presence of only one arc in the whole temperature range indicates that the contribution came from the grain boundary. The resistances of the bulk and the grain boundary in the samples at different temperatures can be calculated from the intercepts of each semicircular arc with the real axis. The resistances as a function of the reciprocal temperatures are plotted, and they fit the Arrhenius relation which can now be expressed as: R = R0 exp(E a /kB T ),
(2)
Here, R0 is the pre-exponential term and Ea is the activation energy. The relationship between the natural logarithms of the R values and the reciprocal of the temperature is observed to be linear in the temperature range from 523 to 683 K. From the inset of Fig. 6, we can obtain the resistance of the bulk and the grain boundary. Accordingly, the Ea values for the grain boundaries and the grain interiors were calculated to be 0.372 and 0.107 eV, respectively. The Ea of the grain boundary is much larger than that of the bulk, indicating that the bulk and the grain boundary exhibit different characteristics of electrical transport. In general, the grain boundary affects the electrical conductivity due to the potential barrier originating from the Cu-rich phases at the grain boundary [23–25]. Furthermore, the Maxwell-Wagner polarization is usually observed in materials that consist of grains separated by more insulating inter-grain barriers. Therefore, the significantly giant dielectric response of the LiCuNb3 O9 ceramic is related not only to the mixed-valent structure of Cu+ /Cu2+ , but also to the Maxwell-Wagner polarisation mechanism. 4. Conclusions LiCuNb3 O9 ceramics were prepared by a solid-state reaction method. The structure, dielectric properties, and giant dielectric response of LiCuNb3 O9 ceramics have been investigated. The LiCuNb3 O9 ceramic with the distorted perovskite structure exhibited giant permittivity and dielectric relaxation behaviour, which are correlated to the mixed-valent structure of Cu+ /Cu2+ and the Maxwell-Wagner polarisation mechanism. The results of the
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present study are expected to contribute to the development of high-performance giant dielectric materials for capacitor applications. Acknowledgments This work was supported by Natural Science Foundation of China (Nos. 11664008, 11464009 and 11364012), Natural Science Foundation of Guangxi (Nos. 2015GXNSFDA139033, 2014GXNSFAA118312, 2013GXNSFAA019291 and 2014GXNSFAA118326), Research Start-up Funds Doctor of Guilin University of Technology (Nos. 002401003281 and 002401003282) and Project of Outstanding Young Teachers’ Training in Higher Education Institutions of Guangxi. References [1] P. Thomas, K. Dwarakanath, K.B.R. Varma, Effect of calcium stoichiometry on the dielectric response of CaCu3 Ti4 O12 ceramics, J. Eur. Ceram. Soc. 32 (8) (2012) 1681–1690. [2] X.J. Luo, Y.S. Liu, C.P. Yang, S.S. Chen, S.L. Tang, K. Bärnere, Oxygen vacancy related defect dipoles in CaCu3Ti4O12 : Detected byelectron paramagnetic resonance spectroscopy, J. Eur. Ceram. Soc. 35 (7) (2015) 2073–2081. [3] R. Schmidt, M.C. Stennett, N.C. Hyatt, J. Pokorny, J. Prado-Gonjal, M. Li, D.C. Sinclair, Effects of sintering temperature on the internal barrier layer capacitor (IBLC) structure in CaCu3 Ti4 O12 (CCTO) ceramics, J. Eur. Ceram. Soc. 32 (12) (2012) 3313–3323. [4] M.A. Subramanian, A.W. Sleight, ACu3 Ti4 O12 and ACu3 Ru4 O12 perovskites: high dielectric constants and valence degeneracy, Solid State Sci. 4 (3) (2002) 347–351. [5] P. Kumonsa, P. Thongbai, B. Putasaeng, T. Yamwong, S. Maensiri, Na1/3 Ca1/3 Bi1/3 Cu3 Ti4 O12 : a new giant dielectric perovskite ceramic in ACu3 Ti4 O12 compounds, J. Eur. Ceram. Soc. 35 (5) (2015) 1441–1447. [6] M.C. Ferrarelli, T.B. Adams, A. Feteira, D.C. Sinclair, A.R. West, High intrinsic permittivity in Na1/2 Bi1/2 Cu3 Ti4 O12 , Appl. Phys. Lett. 89 (21) (2006) 212904. [7] W. Hao, J. Zhang, Y. Tan, W. Su, Giant dielectric-permittivity phenomena of compositionally and structurally CaCu3 Ti4 O12 -like oxide ceramics, J. Am. Ceram. Soc. 92 (12) (2009) 2937–2943. [8] J. Liu, C.G. Duan, W.G. Yin, W.N. Mei, R.W. Smith, J.R. Hardy, Large dielectric constant and Maxwell-Wagner relaxation in Bi2/3 Cu3 Ti4 O12 , Phys. Rev. B 70 (14) (2004) 144106. [9] R. Mukherjee, G. Lawes, B. Nadgorny, Enhancement of high dielectric permittivity in CaCu3 Ti4 O12 /RuO2 composites in the vicinity of the percolation threshold, Appl. Phys. Lett. 105 (7) (2014) 072901.
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Please cite this article in press as: X. Chen, et al., Origin of giant dielectric response in LiCuNb3 O9 distorted perovskite ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.019