Dielectric relaxations in multiferroic La2Ti2O7 ceramics

Accepted Manuscript Dielectric relaxations in multiferroic La2Ti2O7 ceramics N. Zhang, Q.J. Li, S.G. Huang, Y. Yu, J. Zheng, C. Cheng, C.C. Wang PII:

S0925-8388(15)30865-3

DOI:

10.1016/j.jallcom.2015.08.169

Reference:

JALCOM 35164

To appear in:

Journal of Alloys and Compounds

Received Date: 10 June 2015 Revised Date:

19 August 2015

Accepted Date: 20 August 2015

Please cite this article as: N. Zhang, Q.J. Li, S.G. Huang, Y. Yu, J. Zheng, C. Cheng, C.C. Wang, Dielectric relaxations in multiferroic La2Ti2O7 ceramics, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.08.169. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Dielectric relaxations in multiferroic La2Ti2O7 ceramics N. Zhang,1 Q.J. Li,1† S.G. Huang,1 Y. Yu,1 J. Zheng,2 C. Cheng,2 and C.C. Wang1† 1

Laboratory of Dielectric Functional Materials, School of Physics & Material Science,

2

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Anhui University, Hefei 230601, P.R. China Center of Modern Experimental Technology, Anhui University, Hefei 230039, P.R.

†

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China

Author to whom correspondence should be addressed;

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E-mail: [email protected](Li), [email protected](Wang). Tel.: +86 551 63861902; fax: +86 0551 65846849.

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ACCEPTED MANUSCRIPT Abstract La2Ti2O7 ceramic samples were prepared via conventional solid-state reaction route. The dielectric properties of La2Ti2O7 were investigated as functions of temperature

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(113–1073 K) and frequency (100 Hz–1 MHz). Our results revealed that La2Ti2O7 ceramics exhibit intrinsic dielectric response with a dielectric constant of ~57 in the temperature range below 250 K. Three thermally activated dielectric relaxations were

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observed when the temperature higher than 250 K. The low-temperature relaxations

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R1 with the activation energy of 0.38 eV is found to be a polaron relaxation results from hopping holes. The high-temperature relaxations R2 and R3 are related to the conduction progress associated with the singly and doubly ionized oxygen vacancies,

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respectively.

Key Words: Dielectric property; Electric modulus; Cationic vacancies; Conduction

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progress; Oxygen vacancies

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ACCEPTED MANUSCRIPT 1. Introduction The strong interaction between the magnetic and dielectric properties in multiferroic materials is expected to create unprecedented microelectronic and

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spintronic devices based on the magnetodielectric or magnetocapacitance effects [1−3]. From the application point of view, dielectric properties as one of the most important issues for multiferroic materials have been extensively investigated. Owing

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to the low magnetic-dielectric coupling temperatures for most of the multiferroic

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materials, the existing reports on dielectric properties were mainly performed in the low temperature range usually below room temperature [4−7]. Compared with the low-temperature

range,

dielectric

investigation

on

high-Curie

temperature

multiferroics like BiFeO3 revealed more dielectric relaxations [8,9]. This is because

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that both polarisability and conductivity have contribution to dielectric permittivity in the high-temperature range.

Rare-earth titanates with the general formula Re2Ti2O7 (Re = rare earth)

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characterized by the extremely high Curie temperatures TC > 1500 oC [10] have

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attracted considerable attention recently both in basic science and engineering [11−13].

Among them, La2Ti2O7 (LTO) has been studied extensively in the aspects

of piezoelectric, electro-optic, and photocatalytic properties [14−17]. LTO is also known as a layered perovskite structure showing monoclinic phase (P21) at room temperature, with lattice parameter a = 7.81 Å, b = 5.55 Å, c = 13.02 Å, and β = 98.43° [18]. At approximately 780 °C, the structure transforms into orthorhombic phase (Cmc21), and at 1500 °C it transforms into paraelectric phase (Cmcm). The layered 3 / 22

ACCEPTED MANUSCRIPT structure leads to LTO exhibiting high dielectric constant ( ε r = 42~62) at room temperature together with a low dielectric loss at microwave frequency [19,20]. The excellent temperature stability of the high-dielectric constant and low dielectric loss at

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microwave frequency make LTO a promising candidate for microwave applications, high temperature transducer material [19], and electro-optic devices [20−22]. It can be conveniently used above 1000 °C for controlling intelligent gas turbine engines. It is

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also considered as low temperature coefficient of capacitance (TCC) materials

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[23,24].

However, the low-frequency dielectric properties of LTO were barely reported. In the present work, we performed detailed investigations on the low-frequency (100 Hz–1 MHz) dielectric properties of LTO over a wide temperature range from 113 to

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1073 K. We found that LTO exhibits a dielectric plateau resulting from intrinsic dielectric response below 250 K, while above 250 K, LTO shows three relaxations related to oxygen vacancies.

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2. Experimental details

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Single-phase LTO ceramics were prepared by standard solid state reaction method using high purity (99.99%) starting powders of La2O3 and TiO2. Stoichiometric amount of powders (1:2) were thoroughly mixed using a mortar and calcined at 1000 °C for 24 h followed by furnace cooling. Then, the mixture was reground and pressed into pellets with size of 12 mm in diameter and about 1 mm in thickness under a pressure of 20 MPa, and finally sintered at 1350 °C for 10 h at a heating rate of 3 °C/min followed by furnace cooling. Phase purity of the sintered 4 / 22

ACCEPTED MANUSCRIPT pellets was characterized by X-ray diffraction (XRD) performed on a MXP18AHF diffractometer (MARK, Japan) with Cu Kα radiation. The morphology and microstructure of the sample were examined using a field emission scanning electron

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microscope (SEM, Model S-4800, Hitachi Co., Tokyo, Japan). Element content analysis was measured by X-ray fluorescence (XRF, Model XRF-1800, Shimadzu corporation, Japan) with an Rh anode X-ray tube. The dielectric properties were

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measured on a Wayne Kerr 6500B precise impedance analyzer with the sample

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mounted in a holder placed inside a PST-2000HL dielectric measuring system. The temperature variations were controlled by a Stanford temperature controller. The amplitude of ac measuring signal was 100 mV. Electrodes were made by printing platinum paste on both sides of the disk-type samples. Annealing treatment were

for 2 h.

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performed in flowing (200 ml/min) O2 and N2 (both with purity > 99.999%) at 800°C

3. Result and Discussion

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3.1. Sample Characterization

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The XRD pattern of the as-prepared LTO ceramic sample at room temperature is shown in Fig. 1. The pattern was analyzed using Jade 5 power diffraction data analysis software. It was found that the pattern can be indexed to a monoclinic structure with the lattice constant a = 7.7450(2) Å, b = 12.938(7) Å, and c = 5.5539(7) Å, which are fairly consistent with those reported in literature [18]. A typical SEM micrograph of LTO ceramic is shown in the inset of Fig. 1, which reveals that the pellet is dense and compact with mean grain size of ~1.8–2.5 μm. 5 / 22

ACCEPTED MANUSCRIPT 3.2. Dielectric Properties Figure 2(a) and 2(b) present, respectively, the temperature dependence of dielectric constant ε '(T ) (the real part of the complex permittivity ε * ) and loss tangent

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tan δ (T ) ( tan δ = ε '' ε ' , where ε '' is the imaginary part of the complex permittivity) of LTO measured with various frequencies. In the high temperature range (250–1073 K), ε '(T ) shows two stepwise increases occurring around 500 and 750 K. Two sets

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of humps in the curves of tan δ (T ) can be seen in the temperature range where the

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two stepwise increases occur. The position of both humps shift to high temperature with increasing frequency, indicating that the two relaxations follow a thermally activated behavior. In the temperature range of 300–500 K, ε '(T ) seems to behave as a dielectric plateau independent of frequency and temperature. However, a careful

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examination reveals an additional relaxation in this temperature range. The inset of Fig. 2(a) displays the enlarged view of this temperature range, from which an additional stepwise increase can be clearly seen. Correspondingly, a set of thermally

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activated relaxation peaks can be identified in the curves of tan δ (T ) . This finding

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indicates that there are three relaxations in the sample. To discuss each relaxation clearly, the relaxations are designated as R1, R2, and R3 in the order of ascending temperature as indicated by the arrows in Fig. 2(b). In the inset of Fig. 2(b), it can be seen that ε ' for LTO are almost independent of

frequency and temperature in the low temperature range from 113 to 250 K. The dielectric constant plateau shows a value of ~57. This behavior indicates that LTO

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ACCEPTED MANUSCRIPT shows intrinsic dielectric response resulting from the electronic and/or ionic polarization in the low temperature range. From Fig. 2(b) one notes that the high-temperature relaxations (R2 and R3) behave

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as humps because of the increasing background. The remarkable background in high-temperature is usually caused by conductivity [4]. In this case, we applied the electric modulus, which is a powerful function in revealing the background obscured

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relaxation [25]. Figure 3(a) shows the imaginary part of electric modulus as a function

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of temperature at various frequencies. Thanks to the absence of background, two pronounced relaxations (R2 and R3) are observed. The enlarged view of the low-temperature region is shown in the inset of Fig. 3(a), which clearly shows the low-temperature relaxation R1. The relaxation parameters can be obtained in terms of

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Arrhenius law f = f 0 exp(− Ea kBTP ) ,

(1)

where f 0 is the pre-exponential factor, Ea is the activation energy, k B is the

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Boltzmann constant, and TP is the temperature where the maximum M ''(T ) occurs.

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The peak positions of R1 and R2 can be extracted easily. However, the M '' peak of R3 behaves as a hump superimposed on the M '' peak of R2. To extract the peak position of R3 accurately, two Gaussian peaks were used to fit the high-temperature data. As a representative example, Fig. 3(b) displays the fitting result for the experimental data measured at 100 kHz. It is seen that perfect fitting result is achieved. The resultant fitting peaks of R3 at different measuring frequencies were pictured in Fig. 3(c). Figure 3(d) displays the Arrhenius plots for R1, R2, and R3. It is seen that 7 / 22

ACCEPTED MANUSCRIPT R1 and R3 follow the Arrhenius law very well. But for R2, a distinct deviation from the Arrhenius relation leading to two linear segments can be clearly seen. The inflection temperature was found to be ~ 640 K. The relaxation parameters for R1, R2

these relaxations separately.

3.3. The low-temperature dielectric relaxation (R1)

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and R3 of LTO were summarized in the Table. 1. In the following, we will discuss

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To get further information about R1, the same sample used in Fig. 2 was annealed

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first in N2 and then in O2. After each treatment, dielectric properties were measured as a function of temperature. Figure 4(a) presents the comparison of the temperature dependence of M '' obtained at 100 kHz. It is clearly seen that R1 disappears after N2 annealing treatment, while re-appears by O2 annealing treatment. This feature is

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similar to the dielectric behavior caused by surface-layer effect as reported in CaCu3Ti4O12 [26] and Sr3CuNb2O9 [27]. The surface layer is caused by the inhomogeneous distribution of oxygen vacancies in the outmost layer of the sample

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and can be eliminated or greatly weakened by grinding off the surface of the sample.

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To further clarify whether or not the R1 arises from the surface layer effect, the pellet used in Fig. 4(a) with a thickness of 1.30 mm was polished evenly from both sides down to a thickness of 0.78 mm. Figure 4(b) compares the imaginary part of electric modulus as a function of temperature at 100 kHz before and after the polishing treatment. One notes that the thickness reduction of 40% has no obvious influence on R1. It was reported that a thickness reduction of 23% can obviously weaken the surface-layer effect and the reduction of 45% leads to the complete vanishing of the 8 / 22

ACCEPTED MANUSCRIPT effect [26]. This result indicates that a mechanism other than the surface-layer-effect underlies R1. Since the surface-layer-effect is an interfacial effect, which also can depressed by dc bias. The above result implies that R1 might be a bulk effect rather

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than an interfacial effect. To clarify this inference, we conducted dielectric measurements in the frequency domain under different dc biases. Figure 3(c) illustrates the relaxation of R1 in the frequency domain by plotting M '' as a function

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of frequency at 393 K and under different dc biases. It is clearly seen that R1 is

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independent of dc bias, confirming that the response stems from the bulk response. According to the reported literature, cationic vacancies in many oxides, such as Co vacancies in CoTiO3 [28], LaCoO3 [29], and Ba2CoNbO6 [30], Zn vacancies in ZnO [31], etc., lead to hole doping of theses samples. The holes interact with their

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surroundings give rise to polarions. The hoping motion of the hole-polaron can make contribution to both conduction and dielectric response. The activation energy for this kind of relaxation/conduction is about 0.35 eV, which is very close to 0.38 eV for R1

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in LTO. Moreover, R1 features the similar annealing behavior as the relaxation

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associated with hole-polaron reported in CoTiO3 [28], therein the holes were introduced into the sample due to the vaporization of Co. Based on these results, the relaxation R1 features the nature of a polaron relaxation due to hopping holes. The hole-polaron is most probable associated with La vacancies, which can be created by either La deficiency or O excess. The latter case can be excluded because the following results will reveal that the sample is O deficiency instead of excess. In our present sample, although La is not a volatile element, the starting powder La2O3 is apt 9 / 22

ACCEPTED MANUSCRIPT to absorb carbon dioxide and water when exposed to air. Before weighting, without the process of pre-calcining La2O3 powder at high enough temperature (usually 900 o

C) for sufficient time (~ 2 h) in order to dehydrate, it will lead to La deficiency in the

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starting powders. Our sample is just this case. In an attempt to analysis the ratio of La and Ti in LTO, we performed XRF measurement on the as-prepared LTO powder and the result was displayed in Fig. 5. The XRF-FP software uses a fundamental

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parameters method to calculate the concentrations of the elements in the sample.

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According to the spectra lines of La-Lα and Ti-Kα, the contents of the major elements La and Ti are 52.8810 wt% and 20.9560 wt%, respectively. The molar ratio of La : Ti is calculated to be 0.87 : 1. As expected, the content of La is less than the stoichiometric value. Meanwhile, a little amount of C was detected, which might

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result from carbon dioxide introduced by La2O3. Some trace elements of Si, P, and Al were also detected, but their contents are too low to have significant influence on the dielectric properties. The La deficiency yields La vacancies in the sample, and then

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the ionization of La vacancies creates holes. Therefore, R1 can be reasonably ascribed

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to the polaron relaxation caused by hopping holes. Since the hoping motion of charged carriers not only produce dielectric response

but also give to conductivity described the power law known as universal dielectric response (UDR) [32], to further confirm the above point we conducted ac conductivity analysis. The ac conductivity σ (ω , T ) is given by

σ (ω , T ) = ωε 0ε " (ω , T )

(2)

where ε 0 is the dielectric permittivity of free space. Figure 6 displays the ac 10 / 22

ACCEPTED MANUSCRIPT conductivity as a function of frequency at a series of temperatures. It is seen that, for

T = 303 K, the plot shows a low-frequency plateau followed by a rapid increase at high frequencies. This is the typical feature for UDR behavior described by [32]

σ (ω , T ) = σ dc + Aω s

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(3)

where σ dc is the dc conductivity, A and s ( 0 < s ≤ 1 ) are temperature-dependent constants. The plateau represents the value of σ dc , which was found to decrease

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initially with increasing temperature, reaching a minimum at T ~393 K, and then

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increases for further increasing temperature. This finding can be well understood based on the fact that oxygen vacancies as an intrinsic defect in oxides is unavoidable in the present sample. Therefore, both positively (oxygen vacancies) and negatively (lanthanum vacancies) charged vacancies, leading to electrons and holes coexist in the

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sample. The dielectric and conductive properties of the sample are determined by the competition between the electrons and holes. The above results indicate that, in the low-temperature range, the contribution to the polarization (conduction) from the

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holes is dominating. However, oxygen emission from sample occurs in the heating

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measurement run. The ionization of the oxygen vacancies creates the conducting electrons [33]

OO ↔ VO + 1 2 O2

(2)

VO ↔ VO• + e

(3)

VO• ↔ VO•• + e

(4)

where the Kröger-Vink notation of defects is adopted. VO• and VO•• represents the oxygen vacancy carrying one and two excess positive charges, respectively. The 11 / 22

ACCEPTED MANUSCRIPT released electron will recombine with holes leading to the reduction in dc conductivity until at a critical temperature where the contribution to the polarization (conduction) from the electrons takes over the contribution from the holes. Thereafter, oxygen

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vacancies dominate the polarization (conduction) and the conductivity will increases with further increasing temperature. Therefore, there exists a critical temperature where the dc conductivity shows minimum value. This temperature was found to be

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is related to the hopping motion of holes.

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around 373 K. The above evidences convincingly demonstrate that the relaxation R1

3.4. The high-temperature relaxations (R2 and R3)

We now turn our attention to the high-temperature relaxations R2 and R3. As already mentioned that oxygen vacancies dominate the polarization in the

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high-temperature range, these relaxations might be related to the oxygen vacancies. Activation energy analysis is favorable for better understanding the relaxation mechanism. We, therefore, performed detailed dielectric measurements in the

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frequency domain. Figure 6 (a), 6(b), and 6(c) display, respectively, the frequency

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dependence of M '' at a series of temperatures ranging from 473 to 873 K for LTO pellet in the original (as-prepared), O2- and N2-annealed cases. It is seen that, thanks to the absence of remarkable background, the isotherms show pronounced peaks following the thermally activated behavior. Theoretically, a single relaxation peak should be symmetric in shape. After a careful examination, we note that the M ''( f ) curves, especially for those recorded at higher temperatures are asymmetric. This fact can be clearly seen in the representation showing the normalized spectroscopic plot. 12 / 22

ACCEPTED MANUSCRIPT For example, Fig. 8 (a) displays the normalized electric modulus ( M " / M "max ) recorded at 753 K for the as-prepared case versus the logarithmic scale of the normalized frequency ( f f P ), with f p being the peak position and M "max being the

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value of M ''( f ) at the peak position. The width at half-maximum of the peak for the low-frequency wing was found to be 1.086, which is much higher than 0.698 of the high-frequency width. This finding evidences the existence of another weak peak in

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the low-frequency range. To obtain the accurate peak positions, least-squares fittings

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by using two Gauss-type peaks were performed to fit the M ''( f ) curves. As an illustration, the fitting curve as well as the resulting peaks (solid lines) to the experimental data (open circles) of M ''( f ) displayed in Fig. 8(a) were shown in Fig. 8 (b). Perfect fitting result was achieved. Since for a thermally activated relaxation,

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the higher frequency it appears in the frequency spectrum, the lower temperature it occurs in the temperature spectrum, the low- and high-frequency relaxations can be identified to be R3 and R2, respectively. Based on the fitting results, the peak

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positions of R2 and R3 can be obtained and the relaxation parameters can be deduced

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in terms of the Arrhenius law. Fig. 6(a′), 6(b′), and 6(c′) are the Arrhenius plots of R2 and R3 for the original, O2-, and N2-annealed cases, respectively. The calculated values of Ea and f 0 were tabulated in Table 2. The activation energy value for R2 and R3 locates in the range of 0.5-1.2 eV for the relaxation caused by oxygen vacancies [34-37], confirming that these relaxations are related to oxygen vacancies. It is well-known that oxygen vacancies can make contribution to polarization (conduction) in the forms of singly and doubly charged states in lower and higher 13 / 22

ACCEPTED MANUSCRIPT temperature ranges, respectively. In our previous work [38], the activation energy for the relaxation associated with doubly ionized oxygen vacancies in NiTiO3 was found to be 1.17 eV. Paladino et al. [39] reported the activation energy for diffusion of VO••

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in SrTiO3 crystal to be 0.98–1.13 eV. Therefore, the relaxing species for the high-temperature relaxation (R3) with an activation energy around 1.0 eV in LTO can be reasonably ascribed to the doubly charged oxygen vacancies. If R3 was related to

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the electromigration of oxygen vacancies, the polarization and conduction processes

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should show close values of activation energy [40]. To further confirm this point, we performed analysis on the temperature-dependent conductivity as shown in Fig. 9. For the sake of clarity, only one conductivity curve measured at 1 kHz was plotted according to the following Arrhenius relation:

(6)

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σ =σ 0 exp(− Econd k BT )

where σ 0 is the pre-exponent constant and Econd is the activation energy of conductivity. Two main features can be extracted from Fig. 9: (1) With increasing

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temperature, the conductivity decreases at first and then increases after passing a

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minimum value of 7.22×10-9 Ω-1 cm-1 at about 383 K. The temperature matches well with the critical temperature (373 K) where the contribution to the polarization (conduction) from the electrons takes over the contribution from the holes. (2) When temperature is higher than 430 K, the conductivity curve shows two linear sections with a transition region connecting them. The activation energy Econd for the lowand high-T sections was calculated to be 0.80 and 0.97 eV, respectively. It is seen that the value of Econd =0.97 eV is very close to the value of Ea =0.98 eV, confirming that 14 / 22

ACCEPTED MANUSCRIPT R3 is dominated by a hopping progress of the doubly charged oxygen vacancies. The transition temperature where oxygen vacancies change from singly charged state to doubly charged state were reported to be ~ 800 K in SrTiO3 [41] and ~700 K

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in NiTiO3 [38]. The conductivity curve achieves the high-T linear section at the temperature ~730 K. This indicates that of the hopping process was dominated by the singly charged oxygen vacancies in the temperature between 430 and 730 K.

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Therefore, the relaxing species for the relaxation R2 can be reasonably ascribed to the

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singly charged oxygen vacancies. Which is also supported by the fact that Econd =0.80 eV is close to the value of Ea =0.83 eV for the low-T segment of R2 as seen in Fig. 7. Based on the above findings, the experimental results can be explained immediately. For the original case, the Arrhenius plot of R2 exhibits as low- and

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high-T segments with an inflection point is ~610 K. This is because that in the low-T segment, the hopping motion of the singly charged oxygen vacancies is hard because of the Coulombic interaction with the holes. However, in the high temperature region,

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more oxygen vacancies are created as well as more electrons. This leads to the

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reduction in the Coulombic interaction due to the electron-hole recombination. Therefore, the hopping motion of the vacancies becomes easier and the activation energy of high-T segment is smaller than that of the low-T segment. Annealing in O2 decreases the concentration of oxygen vacancies. As a result, the activation energy values for both segments are somewhat larger than those in the original case. This agrees well with the fact that the activation energy for an oxygen-vacancy-related relaxation increases (decreases) with the decreasing (increasing) concentration of 15 / 22

ACCEPTED MANUSCRIPT oxygen vacancies [25]. On the other hand, a higher inflection point (~ 630 K) is observed because a higher temperature is needed to create sufficient electrons to recombine with the holes. Annealing in N2 increases the concentration of oxygen

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vacancies. Consequently, the activation energy for both relaxations (R2 and R3) in the N2-annealed case decreases notably. Meanwhile, thanks to the great annihilation of the holes, the two-Arrhenius segments of R2 disappear. These results convincingly

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confirm R2 is related to the hopping motion of singly charged oxygen vacancies.

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4. Conclusions

In summary, the dielectric properties of LTO ceramics were investigated in the frequency range from 100 Hz to 1 MHz and the temperature range from 113 to 1073 K. We found that LTO shows an intrinsic dielectric response over the temperature

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range below 250 K. Three thermally activated relaxations (R1, R2, and R3) were observed in the temperature range of 250-1073 K. The low-temperature relaxation R1 was suggested to be a polaron relaxation caused by hopping holes. The

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high-temperature relaxations R2 and R3 were argued to be related to the hopping

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motions of singly and doubly charged oxygen vacancies, respectively.

Acknowledgements The authors thank financial support from National Natural Science Foundation

of China (Grant Nos. 11404002, 11404003, and 51402001) and Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration of Anhui University (Grant No. 01001795). This work was supported in 16 / 22

ACCEPTED MANUSCRIPT part by the China Postdoctoral Science Foundation (Grant No. 2014M561805), Anhui Province Postdoctoral Science Foundation (Grant No. 2014B007), and Zhejiang Provincial Natural Science Foundations of China (Grant Nos. LY12F02014 and

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LY13F010006).

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[26] C.C. Wang, L.W. Zhang, Appl. Phys. Lett. 88 (2006) 042906. [27] L.N. Liu, C.C. Wang, X.H. Sun, G.J. Wang, C.M. Lei, T. Li, J. Alloys Comp. 552 (2013) 279–282.

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[28] C.C. Wang, L.W. Zhang, J. Phys. D: Appl. Phys. 40 (2007) 6834–6838.

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[29] E. Iguchi, K. Ueda, W.H. Jung, Phys. Rev. B 54 (1996) 17431-17437. [30] G.J. Wang, C.C. Wang, S.G. Huang, C.M. Lei, X.H. Sun, T. Li, L.N. Liu, J. Am. Ceram. Soc. 96 (2013) 2203–2210.

[31] C.C. Wang, L.N. Liu, Q.J. Li, S.G. Huang, J. Zhang, J. Zheng, C. Cheng, J. Appl.

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Phys. 116 (2014) 124101.

[32] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics, London, 1983.

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[33] C.C. Wang, L.W. Zhang, Phys. Rev. B 74, (2006) 024106.

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[34] J.F. Scott, Ferroelectric Memories, Springer, Berlin, 2000. [35] C. Ang, Z. Yu, L.E. Cross, Phys. Rev. B 62 (2000) 228. [36] O. Bidault, P. Goux, M. Kchikech, M. Belkaoumi, M. Maglione, Phys. Rev. B 49 (1994) 7868.

[37] H.S. Shulman, D. Damjanovic, N. Setter, J. Am. Ceram. Soc. 83 (2000) 528. [38] T. Li, C.C. Wang, C.M. Lei, X.H. Sun, G.J. Wang, L.N. Liu, Curr. Appl. Phys. 13 (2013) 1728-1731. 19 / 22

ACCEPTED MANUSCRIPT [39] A.E. Paladino, J. Am. Ceram. Soc. 48 (1965) 476-478. [40] G.C. Deng, G.R. Li, A.L. Ding, Q.R. Yin, Appl. Phys. Lett. 87 (2005) 192905.

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[41] C. Lee, J. Destry, J. Brebnerc, Phys. Rev. B, 11 (1975) 2299–2310

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ACCEPTED MANUSCRIPT Table titles and figure captions Table. 1. Relaxation parameters for R1, R2 and R3 of LTO ceramic obtained from the temperature domain.

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Table. 2. Relaxation parameters for R2 and R3 of LTO ceramic obtained from the frequency domain.

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Fig. 1. The XRD pattern and SEM micrograph (inset) obtained at room temperature

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of the LTO ceramic.

Fig. 2. Temperature dependence of ε (a) and tanδ (b) of LTO measured at various frequencies. Inset in Fig. 2(a) is the enlarged view of the rectangle region. Inset in Fig. 2(b) is the temperature dependence of ε in the temperature range below 300 K.

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Fig. 3. (a) Temperature dependence of  measured at various frequencies. The inset show is the enlarged view of the rectangle region. (b) Comparison between the experimental curve of  measured at 100 kHz and the fitting result. (c) The

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resultant fitting peaks of R3 at various frequencies. (d) The Arrhenius plots of the

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three relaxations. The straight lines are linear fitting results.

Fig. 4. (a) Temperature dependence of  at 100 kHz for LTO sample after different thermal treatments: as-prepared, annealed in N2 and O2. (b) Temperature dependence of  at 100 kHz for a LTO pellet before and after polishing treatment. (c) Frequency dependence of  under different dc biases recorded at 393 K for the as-prepared LTO sample.

Fig. 5. X-ray fluorescence spectra of the as-prepared LTO powder. The table shows 21 / 22

ACCEPTED MANUSCRIPT the elements analysis results.

Fig. 6. Frequency dependence of ac conductivity for the as-prepared LTO sample at different temperatures.

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Fig. 7. Frequency dependence of M '' for LTO sample at different temperatures (left panels) and the corresponding Arrhenius plots for R2 and R3 (right panels) in the as-prepared, O2- and N2-annealed cases.

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Fig. 8. (a) The normalized electric modulus recorded 753 K for the as-prepared case

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versus the logarithmic scale of the normalized frequency. (b)The fitting curve as well as the resulting peaks (solid lines) to the experimental data (open circles) of M ''( f ) measured in 753K.

Fig. 9. The Arrhenius plot of ac conductivity measured at 1 kHz for LTO pellet. The

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straight lines are linear fitting results according to the Arrhenius relation.

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ACCEPTED MANUSCRIPT Table. 1. R2

Relaxation R1

R3

parameters

Low-T

High-T

0.38

0.86

0.62

0.98

f0 (Hz)

1.44×1010

5.01×1010

6.92×108

2.48×1010

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Table. 2. R2

R3

High-T

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Low-T

Measuring case

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Ea (eV)

Ea (eV)

f0 (Hz)

Ea (eV)

f0 (Hz)

Ea (eV)

f0 (Hz)

As-prepared

0.83

8.52×1010

0.54

1.87×109

0.98

9.81×1010

O2-annealed

0.89

8.33×1010

0.73

4.20×109

1.09

2.02×109

0.58

7.78×108

0.62

4.35×107

AC C

EP

TE D

N2-annealed

AC C

EP

TE D

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4

EP

10

AC C

ε′

48

3

10

46

400

ƒi

500

T (K) 2

10

(a)

ε′

50

nc

re

as

in

g

(b)

ε′

60 1

10

55

tanδ

200

R3

300

T (K) R2

-1

10

Frequency (Hz) 100 1k 10k 50k 100k 200k 500k

R1

400

600

T (K)

800

1000

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T (K)

(a)

8

0

-3

400

500

T (K)

600

4

ƒ

re in c

as

1000

(b)

4

in g

2

0

-3

M’’ (×10 )

6

6

0

4

2

6

(d)

(c) R3

Ea=0.62eV

Frequency (Hz) 100 1k 10k 50k 100k 200k 500k

5

641K

Ea=0.38eV 4

Ea=0.86eV Ea=0.98eV

0 400

600

800

T (K)

1000

1.0

1.5

3 R1 R2 2 R3

2.0

2.5 -1

1000/Tp (K )

3.0

logƒp (Hz)

M’’ (×10 )

2

800

100kHz total fitting curve R2 R3

-3

600

-4

4

1000

M’’ (×10 )

R1

T (K)

800

AC C

12

600

M’’ (×10 )

400

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300

350

400

3 d=1.30 mm 2

as-prepared N2 annealing O2 annealing

1 3 -4

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100 kHz

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2

1

-4

(b)

1.30mm 0.78mm

(c)

393 K

M’’ (×10 )

500

(a)

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-4

M’’ (×10 )

450

R1

100 kHz

M’’ (×10 )

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T (K)

2

dc bias (V) 0 5 10 15 20

1 3

10

10

4

5

ƒ (Hz)

10

6

10

250

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EP

TE D

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XRF- FP analysis results 52.8810%

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20.9560%

O

O Ka

TAP

FPC

25.3135%

C

C Ka

SX-98

FPC

0.7934%

Si

Si Ka

PET

FPC

0.0302%

P

P Ka

Ge

FPC

0.0080%

Al

Al Ka

PET

FPC

0.0089%

LaLb2

50

LaLg1

100

TiKa

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LiF

LaLa

LiF

Ti Ka

TiKb

La La

Ti

LaLb1

150

La

LaLg4 LaLg2

Intensity (kcps)

200

detector value wt(%)

LaLb3

element X-ray line crystal

0 40

50

60

70

2θ (Deg.)

80

90

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-6

-7

10

-1

-1

σ (Ω m )

10

T(K) 303 353 393 433

-8

10

100

1000

ƒ (Hz)

333 373 413 453

10000

2

6 610K

4

473 493 513 533 553 573 593 613 633 653 673 693 713 733 753 773 793 813 833 853 873

-3

M’’ (×10 )

(b) O2 annealed

4 2

-3

M’’ (×10 )

0 6

3

Ea=0.98eV

0

(c) N2 annealed

4 2

R3

(b’) 630K

3

10

4

10

ƒ (Hz)

5

10

6

10

5 4

Ea=0.89eV 3

Ea=1.09eV

R3

R2

(c’)

2 6 5

Ea=0.58eV

4

R2

R3

0 2

2 6

R2

Ea=0.73eV

Ea=0.62eV 10

5

Ea=0.83eV

T (K)

6

(a’)

Ea=0.54eV

1.2

1.4

1.6

1.8 -1

1000/T (K )

logƒp (Hz)

4

R2

logƒp (Hz)

T increasing

(a) original

2.0

3 2 2.2

logƒp (Hz)

-3

M’’ (×10 )

6

AC C

EP

TE D

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(a)

1.0

(b)

753K Peak1 Peak2 PeakSum

753K

5 4

0.5 1.086

2 0.698

R3

1 0

0.0 -3

-2

-1

0

log(ƒ/ƒp)

1

2

2

3

4

5

logƒ (Hz)

6

-3

3

M" (×10 )

M"/M"max

R2

AC C

EP

TE D

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-3

ƒ = 1 kHz

10

Econ= 0.97eV

-4

-1

-1

σ (Ω m )

10

-5

10

730K

-6

10

Econ= 0.80eV

-7

10

430K

383K

-8

10

1.0

1.5

2.0

2.5 -1

1000/T (K )

3.0

ACCEPTED MANUSCRIPT


The La2Ti2O7 ceramics were prepared by conventional solid-state reaction method.
The LTO shows an intrinsic dielectric response ( ε r ~57)

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under 250 K.
The sample successively shows three relaxations from 250 K to 1073 K.

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The low-temperature relaxation is a polaron relaxation caused by

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hopping holes.


The other two relaxations are conduction relaxation due to

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hopping motions of OVs.