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High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics
Accepted Manuscript High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics
S. Huang, K.P. Su, H.O. Wang, S.L. Yuan, D.X. Huo PII:
S0254-0584(17)30377-2
DOI:
10.1016/j.matchemphys.2017.05.016
Reference:
MAC 19689
To appear in:
Materials Chemistry and Physics
Received Date:
22 December 2016
Revised Date:
28 April 2017
Accepted Date:
11 May 2017
Please cite this article as: S. Huang, K.P. Su, H.O. Wang, S.L. Yuan, D.X. Huo, High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.05.016
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ACCEPTED MANUSCRIPT 1. R3Fe5O12 (R = Eu, Gd) ceramics are fabricated via sol-gel method. 2. High dielectric property is observed in R3Fe5O12 (R = Eu, Gd) ceramics. 3. Two types of dielectric relaxations are observed.
ACCEPTED MANUSCRIPT High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics S. Huang1,2,a), K. P. Su1, H. O. Wang1, S. L. Yuan2, and D. X. Huo1,b) 1Institute 2School
of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, P. R. China
of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
Polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics are synthesized by a sol-gel method. The X-ray diffraction patterns show that both materials exhibit the same cubic structure with space group Ia3d. The dielectric behaviors and impedance spectra in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 K to 700 K. The materials exhibit relatively large dielectric permittivity and low dielectric loss in a wide temperature and frequency range. Two dielectric relaxation processes are observed in the measurements. The high frequency relaxation is shown to originate from the electrons hopping between Fe2+ and Fe3+, while the low frequency relaxation most likely arises from the doubly ionized oxygen vacancies. The impedance spectra indicate that the ceramics are electrically heterogeneous consisting of semiconducting grains and insulating grain boundaries (corresponding to the high and low frequency electrical responses, respectively), and the heterogeneous structures play important roles in the dielectric properties. 1. Introduction Materials with considerable high dielectric property have been playing significant roles in microelectronics due to the important application values in devices, such as the miniaturization of capacitive electronic elements and the designing of high energy density storage components [1-3]. The large permittivity is traditionally found in classic ferroelectric materials because of the presence of permanent dipole moments during ferroelectric phase transition [4,5]. However, the high dielectric property can be achieved only in a narrow temperature range around the ferroelectric transition temperature. Recently, Subramanian et al. [6] have found that the non-ferroelectric ceramics CaCu3Ti4O12 and the relevant compounds can also exhibit anomalously large permittivity (~105) in a wide frequency and temperature range. The high dielectric materials or named as giant dielectric materials have drawn increasing attention ever since [1-8]. Lots of models have been proposed to
Corresponding author. a) [email protected] (S. Huang). b) [email protected] (D. X. Huo). 1
ACCEPTED MANUSCRIPT explain the abnormal permittivity, including the Cu deficiency, nanoscale disorder, internal domain and internal barrier layer capacitor (IBLC) models [9-12]. The actual mechanism is still not fully understood so far, and further modeling and theoretical calculations are needed. It is widely accepted that the giant dielectric response originates from extrinsic effects. The Maxwell-Wagner polarization at the grain boundaries is believed to be the primary source of the large dielectric permittivity in CaCu3Ti4O12 ceramics [12]. Although there exist potential application prospects, unfortunately the giant dielectric materials usually possess large values of dielectric loss. Thus, reduction of dielectric loss in CaCu3Ti4O12 or exploring new materials with large dielectric permittivity and low loss are very important. Rare earth iron garnets are ferrimagnetic insulators with general formula of Re3Fe5O12 (Re = rare earth). The spins of Fe at the tetrahedral sites are antiparallel to those at the octahedral sites, and the moments of rare earth (except Y and Lu) at the dodecahedral sites are aligned antiparallel to the net moments of Fe. The spins of Fe in the iron garnets are ordered in a ferromagnetic arrangement along the [111] directions [13]. They are basic materials for high-tech applications, such as memory devices, oscillators, waveguide optical isolators and phase shifters [14]. Similar to CaCu3Ti4O12 ceramics, large permittivity and low dielectric loss with thermal and frequency stability are available in these kinds of materials [14,15]. More importantly, large magnetodielectric effect can also be obtained in several rare earth iron garnets [13,16-18]. For instance, the magnetodielectric coupling takes place even at a remarkably low magnetic field (H < 2 kOe) in Tb3Fe5O12 single crystals [16]. Therefore, the dielectric behaviors should be further explored for purpose of applications by using cross coupling interaction, which may lead to the dynamical control of electric polarization (magnetization) by varying magnetic (electric) field. In this work, the dielectric behaviors are investigated from 100 Hz to 10 MHz in the temperature range from 300 K to 700 K in R3Fe5O12 (R = Eu, Gd) ceramics prepared by sol-gel method. The samples display large dielectric permittivity and low dielectric loss at room temperature. The dielectric behaviors are composed of two relaxations and the formation mechanism is discussed. The experimental observations of complex impedance spectra are well described by the IBLC model. 2. Experiment 2
ACCEPTED MANUSCRIPT Polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics were synthesized by a sol-gel method. First, citric acid was dissolved in distilled water. R(NO3)3·6H2O and Fe(NO3)3·9H2O in 3:5 ratio were added into the solution, and the molar ratio of metal nitrates and citric acid was maintained at 1:1. The pH value of the solution was adjusted to ~7 by adding ammonia. Then, the solution was evaporated at 100 °C in a water bath, and the obtained gel was decomposed and burned at 150 °C in an oven. After that, the obtained precursory powder was carefully ground and calcined at 550 °C for 5 h. Finally, the calcined powder was pressed into pellets (13 mm in diameter and 1 mm in thickness) and sintered at 1100 °C for 10 h in air. The possible decomposition and crystallization of the precursors were determined by using a simultaneous thermal analyzer (Mettler Toledo, TGA/DSC 1) from room temperature to 1000 °C. The structure of the end products was identified using an X-ray diffractometer (XRD, PANalytical, Empyrean) with Cu-Kα radiation. Surface morphology, grain size distribution and chemical composition were investigated using a scanning electron microscope (SEM, FEI, Sirion 200) equipped with an energy dispersive X-ray (EDX) detector. The state of Fe 2p electrons was examined by an X-ray photoelectron spectrometer (XPS, Kratos, Axis-Ultra DLD-600W). The dielectric property and complex impedance spectra were measured in the frequency range from 100 Hz to 10 MHz at various temperatures between 300 K and 700 K using a precision impedance analyzer (Wayne Kerr, 6500B). Prior to the electrical characterization, both surfaces of the sintered pellets were polished, and then coated with silver paint. 3. Result and discussion The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves for the precursors of Eu3Fe5O12 and Gd3Fe5O12 are shown in Fig. 1(a) and (b). It should be note that the precursors had already burned in the process of keeping warm at 150 °C for several hours, thus there is no sharp exothermic peak or large weight loss in the TGA and DSC curves. The weight loss (~2 %) below ~660 °C could be attributed to the removal of water and the slow burning of residual organics. Above ~660 °C, tiny thermal effects (less than ~0.5 %) can be observed, which are considered as the formation of the R3Fe5O12 (R = Eu, Gd) phases. Room temperature XRD patterns of Eu3Fe5O12 and Gd3Fe5O12 ceramics are shown in Fig. 2(a) 3
ACCEPTED MANUSCRIPT and (b), in which the black curves represent the observed data, the open circles represent the calculated patterns obtained by Rietveld refinement, the blue curves give the differences between the observed and calculated data, and the green bars indicate the Bragg position. The reliability factors of the refinement are wRp = 5.67 %, Rp = 4.07 %, χ2 = 0.9632 for Eu3Fe5O12 and wRp = 5.44 %, Rp = 4.10 %, χ2 = 1.020 for Gd3Fe5O12. The good agreement between the observed and calculated data is indicated by the small reliability factors. The Rietveld analysis suggests that the samples possess garnet phase with space group Ia3d, and no impurity is found within the resolution of the equipment. The calculated lattice parameters (a = b = c) are ~12.50369(8) Å and ~12.47521(8) Å for Eu3Fe5O12 and Gd3Fe5O12, which are close to the previous reported values [19,20]. Fig. 3(a) and (b) show the SEM micrographs of the surface morphology for Eu3Fe5O12 and Gd3Fe5O12 ceramics, respectively. The grains are unequal in shape and the size varies from ~100 nm to ~2 μm. Distinct grain boundaries and considerable porosities are evident for both Eu3Fe5O12 and Gd3Fe5O12 samples sintered at 1100 °C for 10 h. From the results of EDX analysis (the data are not shown here), the atomic ratios of R and Fe for Eu3Fe5O12 and Gd3Fe5O12 are 16.67:25.06 and 20.37:31.65, which are close to 3:5, indicating compositional homogeneity. The XPS spectra are used to evaluate the valence state of Fe ions. It is know that the binding energy of Fe 2p3/2 is 709.3 eV for Fe2+ and 710.7 eV for Fe3+ [14]. By Gaussian-Lorentzian curve fitting, the peaks can be separated into two distinctive peaks [Fig. 4(a) and (b)], which indicates that the oxidation of Fe ions in the prepared R3Fe5O12 (R = Eu, Gd) ceramics is the coexistence of Fe2+ and Fe3+. The dielectric behaviors of the end products have been studied in the frequency range from 100 Hz to 10 MHz above room temperature. Dielectric properties are often represented in terms of complex dielectric permittivity given by ε* = ε′ - iε″, where ε′ and ε″ are the real part and imaginary part of the complex permittivity. The tangent value of dielectric loss is described as tanδ = ε″/ε′. Fig. 5(a)-(c) and Fig. 5(d)-(f) show the frequency dependence of ε′, ε″ and tanδ for Eu3Fe5O12 and Gd3Fe5O12 at different temperatures, respectively. The R3Fe5O12 (R = Eu, Gd) ceramics exhibit relatively large dielectric permittivity in the low frequency range, and the low frequency dielectric permittivity is weakly temperature dependent at lower temperatures. With decreasing frequency and increasing temperature, the ε′ increases gradually. At higher frequency, the rapidly decrease of ε′ is 4
ACCEPTED MANUSCRIPT observed because that the variation of fields is too fast for the dipoles to align to the direction of the fields [21]. There are a series of broad peaks in ε″ and tanδ curves in the high frequency range, corresponding to the sharp drop in ε′. With the increase of temperature, the peaks shift to higher frequency range, which is a hint of thermally excited relaxation process [1]. In order to prove this point, the variations of logarithmic frequency vs. reciprocal temperature at the peaks for the high frequency relaxations are given in Fig. 6. The obtained data for both relaxations of Eu3Fe5O12 and Gd3Fe5O12 samples fall perfectly on straight lines, indicating that the dielectric behaviors of the relaxations follow the Arrhenius law:
f f 0 exp( E a /k B T ) ,
(1)
where f0 is the pre-exponential factor, Ea is the activation energy, kB represents the Boltzmann constant. The values of Ea for the relaxations can be calculated from the slopes of Lnf vs. 1000/T. As shown in Fig. 6, the calculated Ea for Eu3Fe5O12 and Gd3Fe5O12 are 0.23 eV and 0.22 eV, which are quite close to the values that obtained from the electrons hopping between Fe2+ and Fe3+ in Re3Fe5O12 and BaFe0.5Nb0.5O3 [16,22-26]. In stoichiometric compounds, the oxygen vacancies could be created by the loss of oxygen during sintering at high temperature,
OO VO 1/ 2O2 ,
(2)
VO VO e ,
(3)
VO VO e ,
(4)
where VO is the loss of lattice oxygen, VO and VO are the single-ionized and doubly ionized oxygen vacancy, and e is the released or captured electron [25]. The electron released from the reaction may be captured by Fe3+ to generate Fe2+ [27]. It should be note that there is another relaxation in the low frequency range, as seen in Fig. 5(c) and (f), which is less obvious than the high frequency relaxation. To further explore the relaxations processes, temperature dependence of dielectric properties for Eu3Fe5O12 and Gd3Fe5O12 are measured. The data of ε′ and tanδ are shown in Fig. 7(a)-(d), respectively. From Fig. 7(a) and (b), it can be seen that the ε′ curves increase rapidly with the increase of temperature, showing two stepwise increases around 300 K and 500 K. Corresponding to 5
ACCEPTED MANUSCRIPT the stepwise increases, two sets of dielectric loss peaks appear and the positions of the peaks shift to higher temperature direction with temperature increasing. The high frequency dielectric loss continuously decreases with the increase of temperature after reaching the maximum values at the peaks. However, the tanδ curves for the low frequency relaxations behave a rapid increasing background, as seen in Fig. 7(c) and (d). This background is usually caused by the conductivity, which shadows the relaxation [28]. Fig. 7(e) and (f) represent the linear fitting of the high frequency and low frequency relaxations for R3Fe5O12 (R = Eu, Gd) ceramics. Both of the relaxation processes follow the Arrhenius law. For the high frequency relaxations, the calculated Ea of Eu3Fe5O12 and Gd3Fe5O12 are 0.23 eV and 0.23 eV. These values are in accordance with the Ea calculated from the frequency dependent permittivity data (Fig. 6). For the low frequency relaxations, the Ea are 0.96 eV and 0.92 eV for Eu3Fe5O12 and Gd3Fe5O12. In order to study the low frequency relaxations, the conductivity (σ) is measured. The frequency dependence of real parts of ac conductivity (σ′) for Eu3Fe5O12 and Gd3Fe5O12 are shown in Fig. 8(a) and (b). The σ′ monotonously decreases with decreasing frequency and becomes nearly independence of frequency above a certain temperature. The grain boundary induced insulting behavior is quite similar to those observed in CaCu3Ti4O12 and BaFe0.5Nb0.5O3 [25,27,29]. The behavior could be described by using a power law with an exponent s (0 < s ≤ 1). This power law is usually represented as “universal dielectric response” (UDR), which can be described as:
σ ac σ dc σ 0 f s ,
(5)
where σdc is the dc bulk conductivity, σ0 and s are the temperature dependent adjusting constants. The dc σ can be obtained by extrapolating the curves toward low frequency [25]. The increase of dc σ with increasing temperature could be explained by the thermally activated defect carries, such as the localized charge carries or oxygen vacancies [25,30]. Fig. 8(c) and (d) show the variations of dc σ with corresponding reciprocal temperature. It obeys the Arrhenius law,
0 exp( E a /k BT ) ,
(6)
where σ0 is the pre-exponential term, Ea is the activation energy, and kB is the Boltzmann constant. By means of linear fitting, the Ea of 0.94 eV and 1.10 eV for Eu3Fe5O12 and Gd3Fe5O12 are obtained, which are quite close to the Ea of the low frequency relaxations. It is reported that the oxygen 6
ACCEPTED MANUSCRIPT vacancies exist in the single-ionized state with activation values in the range 0.3-0.5 eV and 0.6-1.2 eV for doubly ionized oxygen vacancies [31-34]. Compared the Ea derived from the dielectric permittivity-temperature data and the frequency dependence of ac σʹ with the Ea for doubly ionized oxygen vacancies, it can be concluded that the low frequency dielectric relaxations in the high temperature range mainly arise from the doubly ionized oxygen vacancies. The complex impedance analysis provides information to the relaxation process of grain, grain boundary and electrode-specimen interfaces of the polycrystalline materials. For investigating the grain and grain boundary contributions to the electrical conductivity in R3Fe5O12 (R = Eu, Gd) ceramics, the complex impedance at different temperatures are measured and the plots of Z′ vs. Z″ are shown in Fig. 9. The insets of Fig. 9(a) and (c) show the magnifications of the curves near the zero point for Eu3Fe5O12 and Gd3Fe5O12. From Fig. 9(a) and (c), it is clear that there are two semicircular arcs in the complex impedance planes at 300 K in the frequency range from 100 Hz to 10 MHz, representing individually different electrical mechanisms. Similarly with CaCu3Ti4O12, the large arc at low frequency can be attributed to the contribution of the grain boundaries and the other one in the high frequency arises from the grains [12]. Therefore, the dielectric behaviors can be explained by the IBLC model using an equivalent circuit consisting of two parallel RC elements connected in series. In this model, one RC element represents the semiconductive bulk grain contribution and the other one is the grain boundary response. The grain and grain boundary contributions to the impedance are reflected by the deviation to the alternating fields in the complex impedance spectra. With the increase of temperature, the centers of the semicircular arcs shift towards the origin, indicating that the conductivity of the samples increases sharply. The small arcs at the high frequency vanish at higher temperatures due to the limitation of the frequency. When the resistance of grains is much lower, the resistance in the equivalent circuit would be dominated by the grain boundaries resistance. No signal from the electrode is observed in the measured frequency and temperature range. Based on the complex impedance spectra, the high dielectric property of R3Fe5O12 (R = Eu, Gd) ceramics can be ascribed to the external effects of the grains and grain boundaries. 4. Conclusion 7
ACCEPTED MANUSCRIPT In summary, the structure, morphology and dielectric properties of polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics are investigated. By using the sol-gel method, R3Fe5O12 (R = Eu, Gd) ceramics with space group Ia3d are prepared. The X-ray diffraction patterns show that the ceramics are single phase without impurity. The dielectric behaviors and impedance spectra in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 to 700 K. The materials possess large dielectric permittivity and low dielectric loss in a wide temperature and frequency range, which makes them as an attractive candidate for high dielectric applications. The high frequency dielectric relaxations with Ea = 0.23 eV for Eu3Fe5O12 and Ea = 0.22 eV for Gd3Fe5O12 originate from the dipolar effect associated with the electrons hopping between Fe2+ and Fe3+, while the low frequency dielectric relaxations in the high temperature range with Ea = 0.93 eV for Eu3Fe5O12 and Ea = 0.95 eV for Gd3Fe5O12 mainly arise from the doubly ionized oxygen vacancies. According to the complex impedance spectra, the ceramics are electrically heterogeneous consisting semiconducting grains and insulating grain boundaries, and the IBLC model can be used to explain the origin of the large dielectric permittivity. Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No. 11474111, 11574066 and 51372058) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51601049, 11604067). We would like to thank the staffs of Analysis Center of HUST for their assistance in various measurements. Reference [1] J. B. Wu, C. W. Nan, Y. H. Lin, and Y. Deng, Giant dielectric permittivity observed in Li and Ti doped NiO, Phys. Rev. Lett. 89 (2002) 217601. [2] C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673-676. [3] R. K. Pandey, W. A. Stapleton, J. Tate, A. K. Bandyopadhyay, I. Sutanto, S. Sprissler, and S. Lin, Applications of CCTO supercapacitor in energy storage and electronics, AIP Advances 3 (2013) 062126. [4] M. T. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Z. Zhao, M. 8
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ACCEPTED MANUSCRIPT Figure caption Fig. 1. The TGA and DSC curves for the precursors powders of (a) Eu3Fe5O12 and (b) Gd3Fe5O12. Fig. 2. Powder XRD patterns for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics. Fig. 3. The SEM photographs for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics sintered at 1100 °C for 10 h. Fig. 4. The XPS spectra of the Fe 2p core level binding energy for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics. Fig. 5. Frequency dependence of (a) and (d) ε′, (b) and (e) ε″, (c) and (f) tanδ at different temperatures for R3Fe5O12 (R = Eu, Gd) ceramics. Fig. 6. The variations of logarithmic frequency vs. reciprocal temperatures at the relaxation peaks for R3Fe5O12 (R = Eu, Gd) ceramics. Fig. 7. Temperature dependence of (a) and (b) ε′, (c) and (d) tanδ at different frequencies for R3Fe5O12 (R = Eu, Gd) ceramics. (e) and (f) show the Arrhenius fitting based on the high frequency and low frequency relaxations for Eu3Fe5O12 and Gd3Fe5O12, respectively. Fig. 8. The frequency dependence of ac σʹ at different temperatures for (a) Eu3Fe5O12 and (b) Eu3Fe5O12. The short dashes are the fitting results according to equation (5). (c) and (d) show the variations of dc σ vs. corresponding temperatures. Fig. 9. Complex impedance spectra for (a) and (b) Eu3Fe5O12, (c) and (d) Gd3Fe5O12. The insets of (a) and (c) show the magnifications of the curves measured at 300 K near the origin point.
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S. Huang, K.P. Su, H.O. Wang, S.L. Yuan, D.X. Huo PII:
S0254-0584(17)30377-2
DOI:
10.1016/j.matchemphys.2017.05.016
Reference:
MAC 19689
To appear in:
Materials Chemistry and Physics
Received Date:
22 December 2016
Revised Date:
28 April 2017
Accepted Date:
11 May 2017
Please cite this article as: S. Huang, K.P. Su, H.O. Wang, S.L. Yuan, D.X. Huo, High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.05.016
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ACCEPTED MANUSCRIPT 1. R3Fe5O12 (R = Eu, Gd) ceramics are fabricated via sol-gel method. 2. High dielectric property is observed in R3Fe5O12 (R = Eu, Gd) ceramics. 3. Two types of dielectric relaxations are observed.
ACCEPTED MANUSCRIPT High temperature dielectric response in R3Fe5O12 (R = Eu, Gd) ceramics S. Huang1,2,a), K. P. Su1, H. O. Wang1, S. L. Yuan2, and D. X. Huo1,b) 1Institute 2School
of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, P. R. China
of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
Polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics are synthesized by a sol-gel method. The X-ray diffraction patterns show that both materials exhibit the same cubic structure with space group Ia3d. The dielectric behaviors and impedance spectra in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 K to 700 K. The materials exhibit relatively large dielectric permittivity and low dielectric loss in a wide temperature and frequency range. Two dielectric relaxation processes are observed in the measurements. The high frequency relaxation is shown to originate from the electrons hopping between Fe2+ and Fe3+, while the low frequency relaxation most likely arises from the doubly ionized oxygen vacancies. The impedance spectra indicate that the ceramics are electrically heterogeneous consisting of semiconducting grains and insulating grain boundaries (corresponding to the high and low frequency electrical responses, respectively), and the heterogeneous structures play important roles in the dielectric properties. 1. Introduction Materials with considerable high dielectric property have been playing significant roles in microelectronics due to the important application values in devices, such as the miniaturization of capacitive electronic elements and the designing of high energy density storage components [1-3]. The large permittivity is traditionally found in classic ferroelectric materials because of the presence of permanent dipole moments during ferroelectric phase transition [4,5]. However, the high dielectric property can be achieved only in a narrow temperature range around the ferroelectric transition temperature. Recently, Subramanian et al. [6] have found that the non-ferroelectric ceramics CaCu3Ti4O12 and the relevant compounds can also exhibit anomalously large permittivity (~105) in a wide frequency and temperature range. The high dielectric materials or named as giant dielectric materials have drawn increasing attention ever since [1-8]. Lots of models have been proposed to
Corresponding author. a) [email protected] (S. Huang). b) [email protected] (D. X. Huo). 1
ACCEPTED MANUSCRIPT explain the abnormal permittivity, including the Cu deficiency, nanoscale disorder, internal domain and internal barrier layer capacitor (IBLC) models [9-12]. The actual mechanism is still not fully understood so far, and further modeling and theoretical calculations are needed. It is widely accepted that the giant dielectric response originates from extrinsic effects. The Maxwell-Wagner polarization at the grain boundaries is believed to be the primary source of the large dielectric permittivity in CaCu3Ti4O12 ceramics [12]. Although there exist potential application prospects, unfortunately the giant dielectric materials usually possess large values of dielectric loss. Thus, reduction of dielectric loss in CaCu3Ti4O12 or exploring new materials with large dielectric permittivity and low loss are very important. Rare earth iron garnets are ferrimagnetic insulators with general formula of Re3Fe5O12 (Re = rare earth). The spins of Fe at the tetrahedral sites are antiparallel to those at the octahedral sites, and the moments of rare earth (except Y and Lu) at the dodecahedral sites are aligned antiparallel to the net moments of Fe. The spins of Fe in the iron garnets are ordered in a ferromagnetic arrangement along the [111] directions [13]. They are basic materials for high-tech applications, such as memory devices, oscillators, waveguide optical isolators and phase shifters [14]. Similar to CaCu3Ti4O12 ceramics, large permittivity and low dielectric loss with thermal and frequency stability are available in these kinds of materials [14,15]. More importantly, large magnetodielectric effect can also be obtained in several rare earth iron garnets [13,16-18]. For instance, the magnetodielectric coupling takes place even at a remarkably low magnetic field (H < 2 kOe) in Tb3Fe5O12 single crystals [16]. Therefore, the dielectric behaviors should be further explored for purpose of applications by using cross coupling interaction, which may lead to the dynamical control of electric polarization (magnetization) by varying magnetic (electric) field. In this work, the dielectric behaviors are investigated from 100 Hz to 10 MHz in the temperature range from 300 K to 700 K in R3Fe5O12 (R = Eu, Gd) ceramics prepared by sol-gel method. The samples display large dielectric permittivity and low dielectric loss at room temperature. The dielectric behaviors are composed of two relaxations and the formation mechanism is discussed. The experimental observations of complex impedance spectra are well described by the IBLC model. 2. Experiment 2
ACCEPTED MANUSCRIPT Polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics were synthesized by a sol-gel method. First, citric acid was dissolved in distilled water. R(NO3)3·6H2O and Fe(NO3)3·9H2O in 3:5 ratio were added into the solution, and the molar ratio of metal nitrates and citric acid was maintained at 1:1. The pH value of the solution was adjusted to ~7 by adding ammonia. Then, the solution was evaporated at 100 °C in a water bath, and the obtained gel was decomposed and burned at 150 °C in an oven. After that, the obtained precursory powder was carefully ground and calcined at 550 °C for 5 h. Finally, the calcined powder was pressed into pellets (13 mm in diameter and 1 mm in thickness) and sintered at 1100 °C for 10 h in air. The possible decomposition and crystallization of the precursors were determined by using a simultaneous thermal analyzer (Mettler Toledo, TGA/DSC 1) from room temperature to 1000 °C. The structure of the end products was identified using an X-ray diffractometer (XRD, PANalytical, Empyrean) with Cu-Kα radiation. Surface morphology, grain size distribution and chemical composition were investigated using a scanning electron microscope (SEM, FEI, Sirion 200) equipped with an energy dispersive X-ray (EDX) detector. The state of Fe 2p electrons was examined by an X-ray photoelectron spectrometer (XPS, Kratos, Axis-Ultra DLD-600W). The dielectric property and complex impedance spectra were measured in the frequency range from 100 Hz to 10 MHz at various temperatures between 300 K and 700 K using a precision impedance analyzer (Wayne Kerr, 6500B). Prior to the electrical characterization, both surfaces of the sintered pellets were polished, and then coated with silver paint. 3. Result and discussion The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves for the precursors of Eu3Fe5O12 and Gd3Fe5O12 are shown in Fig. 1(a) and (b). It should be note that the precursors had already burned in the process of keeping warm at 150 °C for several hours, thus there is no sharp exothermic peak or large weight loss in the TGA and DSC curves. The weight loss (~2 %) below ~660 °C could be attributed to the removal of water and the slow burning of residual organics. Above ~660 °C, tiny thermal effects (less than ~0.5 %) can be observed, which are considered as the formation of the R3Fe5O12 (R = Eu, Gd) phases. Room temperature XRD patterns of Eu3Fe5O12 and Gd3Fe5O12 ceramics are shown in Fig. 2(a) 3
ACCEPTED MANUSCRIPT and (b), in which the black curves represent the observed data, the open circles represent the calculated patterns obtained by Rietveld refinement, the blue curves give the differences between the observed and calculated data, and the green bars indicate the Bragg position. The reliability factors of the refinement are wRp = 5.67 %, Rp = 4.07 %, χ2 = 0.9632 for Eu3Fe5O12 and wRp = 5.44 %, Rp = 4.10 %, χ2 = 1.020 for Gd3Fe5O12. The good agreement between the observed and calculated data is indicated by the small reliability factors. The Rietveld analysis suggests that the samples possess garnet phase with space group Ia3d, and no impurity is found within the resolution of the equipment. The calculated lattice parameters (a = b = c) are ~12.50369(8) Å and ~12.47521(8) Å for Eu3Fe5O12 and Gd3Fe5O12, which are close to the previous reported values [19,20]. Fig. 3(a) and (b) show the SEM micrographs of the surface morphology for Eu3Fe5O12 and Gd3Fe5O12 ceramics, respectively. The grains are unequal in shape and the size varies from ~100 nm to ~2 μm. Distinct grain boundaries and considerable porosities are evident for both Eu3Fe5O12 and Gd3Fe5O12 samples sintered at 1100 °C for 10 h. From the results of EDX analysis (the data are not shown here), the atomic ratios of R and Fe for Eu3Fe5O12 and Gd3Fe5O12 are 16.67:25.06 and 20.37:31.65, which are close to 3:5, indicating compositional homogeneity. The XPS spectra are used to evaluate the valence state of Fe ions. It is know that the binding energy of Fe 2p3/2 is 709.3 eV for Fe2+ and 710.7 eV for Fe3+ [14]. By Gaussian-Lorentzian curve fitting, the peaks can be separated into two distinctive peaks [Fig. 4(a) and (b)], which indicates that the oxidation of Fe ions in the prepared R3Fe5O12 (R = Eu, Gd) ceramics is the coexistence of Fe2+ and Fe3+. The dielectric behaviors of the end products have been studied in the frequency range from 100 Hz to 10 MHz above room temperature. Dielectric properties are often represented in terms of complex dielectric permittivity given by ε* = ε′ - iε″, where ε′ and ε″ are the real part and imaginary part of the complex permittivity. The tangent value of dielectric loss is described as tanδ = ε″/ε′. Fig. 5(a)-(c) and Fig. 5(d)-(f) show the frequency dependence of ε′, ε″ and tanδ for Eu3Fe5O12 and Gd3Fe5O12 at different temperatures, respectively. The R3Fe5O12 (R = Eu, Gd) ceramics exhibit relatively large dielectric permittivity in the low frequency range, and the low frequency dielectric permittivity is weakly temperature dependent at lower temperatures. With decreasing frequency and increasing temperature, the ε′ increases gradually. At higher frequency, the rapidly decrease of ε′ is 4
ACCEPTED MANUSCRIPT observed because that the variation of fields is too fast for the dipoles to align to the direction of the fields [21]. There are a series of broad peaks in ε″ and tanδ curves in the high frequency range, corresponding to the sharp drop in ε′. With the increase of temperature, the peaks shift to higher frequency range, which is a hint of thermally excited relaxation process [1]. In order to prove this point, the variations of logarithmic frequency vs. reciprocal temperature at the peaks for the high frequency relaxations are given in Fig. 6. The obtained data for both relaxations of Eu3Fe5O12 and Gd3Fe5O12 samples fall perfectly on straight lines, indicating that the dielectric behaviors of the relaxations follow the Arrhenius law:
f f 0 exp( E a /k B T ) ,
(1)
where f0 is the pre-exponential factor, Ea is the activation energy, kB represents the Boltzmann constant. The values of Ea for the relaxations can be calculated from the slopes of Lnf vs. 1000/T. As shown in Fig. 6, the calculated Ea for Eu3Fe5O12 and Gd3Fe5O12 are 0.23 eV and 0.22 eV, which are quite close to the values that obtained from the electrons hopping between Fe2+ and Fe3+ in Re3Fe5O12 and BaFe0.5Nb0.5O3 [16,22-26]. In stoichiometric compounds, the oxygen vacancies could be created by the loss of oxygen during sintering at high temperature,
OO VO 1/ 2O2 ,
(2)
VO VO e ,
(3)
VO VO e ,
(4)
where VO is the loss of lattice oxygen, VO and VO are the single-ionized and doubly ionized oxygen vacancy, and e is the released or captured electron [25]. The electron released from the reaction may be captured by Fe3+ to generate Fe2+ [27]. It should be note that there is another relaxation in the low frequency range, as seen in Fig. 5(c) and (f), which is less obvious than the high frequency relaxation. To further explore the relaxations processes, temperature dependence of dielectric properties for Eu3Fe5O12 and Gd3Fe5O12 are measured. The data of ε′ and tanδ are shown in Fig. 7(a)-(d), respectively. From Fig. 7(a) and (b), it can be seen that the ε′ curves increase rapidly with the increase of temperature, showing two stepwise increases around 300 K and 500 K. Corresponding to 5
ACCEPTED MANUSCRIPT the stepwise increases, two sets of dielectric loss peaks appear and the positions of the peaks shift to higher temperature direction with temperature increasing. The high frequency dielectric loss continuously decreases with the increase of temperature after reaching the maximum values at the peaks. However, the tanδ curves for the low frequency relaxations behave a rapid increasing background, as seen in Fig. 7(c) and (d). This background is usually caused by the conductivity, which shadows the relaxation [28]. Fig. 7(e) and (f) represent the linear fitting of the high frequency and low frequency relaxations for R3Fe5O12 (R = Eu, Gd) ceramics. Both of the relaxation processes follow the Arrhenius law. For the high frequency relaxations, the calculated Ea of Eu3Fe5O12 and Gd3Fe5O12 are 0.23 eV and 0.23 eV. These values are in accordance with the Ea calculated from the frequency dependent permittivity data (Fig. 6). For the low frequency relaxations, the Ea are 0.96 eV and 0.92 eV for Eu3Fe5O12 and Gd3Fe5O12. In order to study the low frequency relaxations, the conductivity (σ) is measured. The frequency dependence of real parts of ac conductivity (σ′) for Eu3Fe5O12 and Gd3Fe5O12 are shown in Fig. 8(a) and (b). The σ′ monotonously decreases with decreasing frequency and becomes nearly independence of frequency above a certain temperature. The grain boundary induced insulting behavior is quite similar to those observed in CaCu3Ti4O12 and BaFe0.5Nb0.5O3 [25,27,29]. The behavior could be described by using a power law with an exponent s (0 < s ≤ 1). This power law is usually represented as “universal dielectric response” (UDR), which can be described as:
σ ac σ dc σ 0 f s ,
(5)
where σdc is the dc bulk conductivity, σ0 and s are the temperature dependent adjusting constants. The dc σ can be obtained by extrapolating the curves toward low frequency [25]. The increase of dc σ with increasing temperature could be explained by the thermally activated defect carries, such as the localized charge carries or oxygen vacancies [25,30]. Fig. 8(c) and (d) show the variations of dc σ with corresponding reciprocal temperature. It obeys the Arrhenius law,
0 exp( E a /k BT ) ,
(6)
where σ0 is the pre-exponential term, Ea is the activation energy, and kB is the Boltzmann constant. By means of linear fitting, the Ea of 0.94 eV and 1.10 eV for Eu3Fe5O12 and Gd3Fe5O12 are obtained, which are quite close to the Ea of the low frequency relaxations. It is reported that the oxygen 6
ACCEPTED MANUSCRIPT vacancies exist in the single-ionized state with activation values in the range 0.3-0.5 eV and 0.6-1.2 eV for doubly ionized oxygen vacancies [31-34]. Compared the Ea derived from the dielectric permittivity-temperature data and the frequency dependence of ac σʹ with the Ea for doubly ionized oxygen vacancies, it can be concluded that the low frequency dielectric relaxations in the high temperature range mainly arise from the doubly ionized oxygen vacancies. The complex impedance analysis provides information to the relaxation process of grain, grain boundary and electrode-specimen interfaces of the polycrystalline materials. For investigating the grain and grain boundary contributions to the electrical conductivity in R3Fe5O12 (R = Eu, Gd) ceramics, the complex impedance at different temperatures are measured and the plots of Z′ vs. Z″ are shown in Fig. 9. The insets of Fig. 9(a) and (c) show the magnifications of the curves near the zero point for Eu3Fe5O12 and Gd3Fe5O12. From Fig. 9(a) and (c), it is clear that there are two semicircular arcs in the complex impedance planes at 300 K in the frequency range from 100 Hz to 10 MHz, representing individually different electrical mechanisms. Similarly with CaCu3Ti4O12, the large arc at low frequency can be attributed to the contribution of the grain boundaries and the other one in the high frequency arises from the grains [12]. Therefore, the dielectric behaviors can be explained by the IBLC model using an equivalent circuit consisting of two parallel RC elements connected in series. In this model, one RC element represents the semiconductive bulk grain contribution and the other one is the grain boundary response. The grain and grain boundary contributions to the impedance are reflected by the deviation to the alternating fields in the complex impedance spectra. With the increase of temperature, the centers of the semicircular arcs shift towards the origin, indicating that the conductivity of the samples increases sharply. The small arcs at the high frequency vanish at higher temperatures due to the limitation of the frequency. When the resistance of grains is much lower, the resistance in the equivalent circuit would be dominated by the grain boundaries resistance. No signal from the electrode is observed in the measured frequency and temperature range. Based on the complex impedance spectra, the high dielectric property of R3Fe5O12 (R = Eu, Gd) ceramics can be ascribed to the external effects of the grains and grain boundaries. 4. Conclusion 7
ACCEPTED MANUSCRIPT In summary, the structure, morphology and dielectric properties of polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics are investigated. By using the sol-gel method, R3Fe5O12 (R = Eu, Gd) ceramics with space group Ia3d are prepared. The X-ray diffraction patterns show that the ceramics are single phase without impurity. The dielectric behaviors and impedance spectra in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 to 700 K. The materials possess large dielectric permittivity and low dielectric loss in a wide temperature and frequency range, which makes them as an attractive candidate for high dielectric applications. The high frequency dielectric relaxations with Ea = 0.23 eV for Eu3Fe5O12 and Ea = 0.22 eV for Gd3Fe5O12 originate from the dipolar effect associated with the electrons hopping between Fe2+ and Fe3+, while the low frequency dielectric relaxations in the high temperature range with Ea = 0.93 eV for Eu3Fe5O12 and Ea = 0.95 eV for Gd3Fe5O12 mainly arise from the doubly ionized oxygen vacancies. According to the complex impedance spectra, the ceramics are electrically heterogeneous consisting semiconducting grains and insulating grain boundaries, and the IBLC model can be used to explain the origin of the large dielectric permittivity. Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No. 11474111, 11574066 and 51372058) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51601049, 11604067). We would like to thank the staffs of Analysis Center of HUST for their assistance in various measurements. Reference [1] J. B. Wu, C. W. Nan, Y. H. Lin, and Y. Deng, Giant dielectric permittivity observed in Li and Ti doped NiO, Phys. Rev. Lett. 89 (2002) 217601. [2] C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673-676. [3] R. K. Pandey, W. A. Stapleton, J. Tate, A. K. Bandyopadhyay, I. Sutanto, S. Sprissler, and S. Lin, Applications of CCTO supercapacitor in energy storage and electronics, AIP Advances 3 (2013) 062126. [4] M. T. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Z. Zhao, M. 8
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ACCEPTED MANUSCRIPT Figure caption Fig. 1. The TGA and DSC curves for the precursors powders of (a) Eu3Fe5O12 and (b) Gd3Fe5O12. Fig. 2. Powder XRD patterns for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics. Fig. 3. The SEM photographs for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics sintered at 1100 °C for 10 h. Fig. 4. The XPS spectra of the Fe 2p core level binding energy for (a) Eu3Fe5O12 and (b) Gd3Fe5O12 ceramics. Fig. 5. Frequency dependence of (a) and (d) ε′, (b) and (e) ε″, (c) and (f) tanδ at different temperatures for R3Fe5O12 (R = Eu, Gd) ceramics. Fig. 6. The variations of logarithmic frequency vs. reciprocal temperatures at the relaxation peaks for R3Fe5O12 (R = Eu, Gd) ceramics. Fig. 7. Temperature dependence of (a) and (b) ε′, (c) and (d) tanδ at different frequencies for R3Fe5O12 (R = Eu, Gd) ceramics. (e) and (f) show the Arrhenius fitting based on the high frequency and low frequency relaxations for Eu3Fe5O12 and Gd3Fe5O12, respectively. Fig. 8. The frequency dependence of ac σʹ at different temperatures for (a) Eu3Fe5O12 and (b) Eu3Fe5O12. The short dashes are the fitting results according to equation (5). (c) and (d) show the variations of dc σ vs. corresponding temperatures. Fig. 9. Complex impedance spectra for (a) and (b) Eu3Fe5O12, (c) and (d) Gd3Fe5O12. The insets of (a) and (c) show the magnifications of the curves measured at 300 K near the origin point.
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