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Effect of La doping on optical and electrical transport properties of nanocrystalline YCrO3
Accepted Manuscript Effect of La doping on optical and electrical transport properties of nanocrystalline YCrO3 R. Sinha, S. Kundu, S. Basu, A.K. Meikap PII:
S1293-2558(16)30554-4
DOI:
10.1016/j.solidstatesciences.2016.08.010
Reference:
SSSCIE 5363
To appear in:
Solid State Sciences
Received Date: 28 December 2015 Revised Date:
9 August 2016
Accepted Date: 11 August 2016
Please cite this article as: R. Sinha, S. Kundu, S. Basu, A.K. Meikap, Effect of La doping on optical and electrical transport properties of nanocrystalline YCrO3, Solid State Sciences (2016), doi: 10.1016/ j.solidstatesciences.2016.08.010. 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.
ACCEPTED MANUSCRIPT
Effect of La doping on Optical and electrical transport properties of nanocrystalline YCrO3 R.Sinhaa, S. Kunduc, S.Basub and A. K. Meikapb* a
SC
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Department of Physics, Asansol Engineering College,Kanyapur, Asansol-713305. West Bengal, India b Department of Physics, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur 713209,West Bengal, India c Department. of Physics, The University of Burdwan, Golapbag, Burdwan-713104, West Bengal, India, A relaxation peak has been noticed in M // ( f ) − f curve which is accompanied with the step like
transition in M / ( f ) − f which is shifting towards high frequency side with the increase of
0.007
298K 323K 373K 423K 473K 523K
0.004 0.003
/
M (f,T)
0.005
0.000
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0.002 0.001
-0.001 1 10
10
2
10
(b)
0.0015
AC C //
f(Hz)10
4
10
5
10
6
298K 323K 373K 423K 473K 523K
0.0020
M (f,T)
3
1% La
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0.0025
1% La
(a)
0.006
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temperature, reveals the hopping type conduction mechanism.
0.0010 0.0005 0.0000
10
2
10
3
10
4
f(Hz)
10
5
10
6
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Effect of La doping on Optical and electrical transport properties of nanocrystalline YCrO3 R.Sinhaa, S. Kunduc, S.Basub and A. K. Meikapb* a
SC
RI PT
Department of Physics, Asansol Engineering College,Kanyapur, Asansol-713305. West Bengal, India b Department of Physics, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur 713209,West Bengal, India c Department. of Physics, The University of Burdwan, Golapbag, Burdwan-713104, West Bengal, India,
Abstract
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In this work we have reported the synthesis and characterization of La doped YCrO3 nanoparticles following sol-gel method. The optical band gap of the investigated samples decreases with the increase of doping content. Photoluminescence spectra show distinct red light emission in the visible range around 630 nm. Dielectric permittivity is measured within the temperature range 298K-523K and in the frequency range 20 Hz - 1 MHz following the power law
, which
shows that the temperature exponent p increases with the decreasing frequency and its values varies
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from 11.4 to 17 for 1 MHz to 100 KHz frequency variation. The ac impedance analysis shows that grain boundary contribution is dominating over grain contribution. The dc conductivity of the investigated samples follows semiconductor behaviour. The analysis of both the dc and ac conductivity shows that the activation energy decreases and the conductivity increases with the
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increase of doping concentration which is very much important for its application as interconnect
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material in Solid Oxide Fuel Cells (SOFCs).
Keywords: A. Nanostructured materials; B. Sol-gel processes; C. Electric transport; C. Dielectric response, D. X-ray diffraction; D. Transmission electron microscopy.
*Corresponding author: Tel.: +91 343 2546808; fax: +91 343 2547375 (A.K. Meikap) *Email address: [email protected] (A. K. Meikap)
1
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1. Introduction Recently perovskites type oxides with ABO3 structure have been widely investigated as electrode materials in photo electrochemistry, solid-oxide fuel cells (SOFCs), sensors and catalysis [1-3]. It is very important to improve the features of the perovskites for obtaining a wide range of
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the application. Their utility can be enhanced by partial substitution of A-site or B-site or both sites, which can improve or change their physical and chemical properties and widen the application window. Solid oxides fuel cell (SOFC) is one of the most promising candidates for a new generation power system for its high energy conversion efficiency. It produces fewer pollutants and
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so have environmental advantages. But their operating temperature is very high (8000C-10000C). So the chemical and physical degradation of the building material at this temperature restricts the
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choice of the materials. YCrO3 possesses perovskites type structure with the general formula ABO3 and it is apparently stable to at least 1200K. It has promising utility for SOFCs ceramic interconnect material which is used to provide the conductive path for electrical current to flow between the electrode and the external circuit. LaCrO3 doped with alkaline metals (Ca, or Sr) have been extensively used as interconnect material for the present generation SOFCs [4-5]. Recently YCrO3 has drawn much more attention because it has many advantages over
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LaCrO3 for improved chemical and dimensional stability. But due to the low electrical conductivity its application is limited. In the recent years low temperature synthesis of Yittrium Chromite was done by Looby and Katz [6] following a molten salt technique at 9000C. Weber et al reported the electrical conductivity of YCrO3 [7-8]. Magneto and thermal dielectric properties of
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biferroicYCrO3 prepared by combustion synthesis was reported by Dura´n et al.[9]. Effect of cation substitution on electrical and thermal transport properties of YCrO3 and LaCrO3 was individually
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discussed by Weber et al.[8]. Later Krishnan et al.[10] reported the two-photon assisted excited state absorption in multiferroic YCrO3 nanoparticles. In the recent time for the purpose of obtaining higher electrical conductivity, Carnil II et al. fabricated (Y1-xCax)CrO3 (0.05
x
0.20)
ceramics by heating for 24 hours at 17500C and 10-10 atm. pressure and then annealing 15500C during 48 hours in air[11-12]. It was observed that the electrical conductivity increases with Ca doping. Considering the importance of doping in YCrO3 at A-site to improve its electrical properties, the present works reports the synthesis, characterization and electrical transport properties of La doped nanocrystalline YCrO3. To the best of our knowledge no experimental work yet have been reported to describing detail electrical transport mechanism so far which is essential 2
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for its device applications. We have achieved higher electrical conductivity with La doping which is very much important for the operation of SOFCs.
2. Experimental procedure Lanthanum doped YCrO3 [Y1-xLaxCrO3(YLaCrO) with x=0, 0.01, 0.05, 0.10] nanoparticles
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were prepared by sol-gel method. Dissolving weighted amount of Yttrium Nitrate Hexahydrate [Y(NO3)3 x 6H2O], Chromium Nitrate Nonahydrate [Cr(NO3)3 x 9H2O] and Lanthanum Nitrate Hexahydrate [La(NO3)3 x 6H2O], in 50 ml distilled water with continuous stirring precursor solution is prepared at first. Then 20 ml PVA solution of strength 12.5 gm/ liter is added to the
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above solution under continuous stirring for 1 hour. In the next step the solution is evaporated to get dry precursor powder. At last the grinded precursor powder is calcined in air at 8000C for 1
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hour duration. The X-Ray Powder Diffraction (XRD) patterns of various samples are recorded using a X’ pert Pro X-ray diffractometer with Ni-filtered CuKα radiation having λ=1.5414 Å with 2θ ranging from 200-800. The Transmission Electron Microscope (HRTEM, JEOL 2011) is used to study the morphology of the nanoparticles. Dispersing the very small part of the sample in Ethanol, one drop of dispersed solution is cast on a carbon coated copper grid for the HRTEM measurements. UV-Visible absorption spectroscopy is performed between 250-600 nm using a
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double beam spectro – photometer (Hitachi-3010). Photoluminescence spectroscopy study is performed with a Hitachi fluorescence spectrophotometer (F-2500). For the measurement of electrical properties the powder sample is taken in a steel mould of 1 cm diameter and compacted at a pressure of 7 tons/cm2. Silver paint electrodes are applied for both the opposite faces (Supplied by
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Acheson Colloiden B.V Holland). DC conductivity is measured by an Agilent 3458A multimeter. The temperature dependence of conductivity is measured by a furnace fitted with 8502 eurotherm
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controller. The ac measurements are performed using impedance analyzer (HP 4902 A).
3. Results and Discussions The powder X-ray diffraction patterns (XRD) of Y1-xLaxCrO3 (with x=0, 0.01, 0.05, 0.10) are shown in Fig.1. XRD pattern shows that the samples are well crystalline in nature. All the prominent peaks in the figure are indexed to various (hkl) planes showing the formation of YCrO3 and it is also matched with “ICDD PDF No. 34-0365.” The average crystallite size of the nanoparticles are calculated with the help of Debye Scherer formula, D = 0.9λ/βFWcosθ, where D, λ and β FW are the average crystallite size, wavelength of X-ray and FWHM of diffraction peak. The 3
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estimated average crystallite size is varying from 35 nm- 45 nm. The lattice parameters and microstrain for Y1-xLaxCrO3 system are obtained from the Rietveld refinement method and tabulated in Table 1. Table 1 clearly reveals that the value of lattice parameters increases with the increasing value of La doping concentrations (x) and it suggests that the lattice volume (Å3)
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increases with the increase in x. This is due to the fact that the dopant La3+ ion has larger ionic radius (1.216Å) than the parent Y3+ ion (1.075Å) in 9 coordination system. Cell angle (β) decreases with the increase in x and it has a tendency of approaching towards 90°. Hence, it can be concluded from Table 1 that the structure of the Y1-xLaxCrO3 system becomes more stable
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(monoclinic→orthorhombic) with the increase in x. Rietveld refinement results also reveal that for Y1-xLaxCrO3 system lattice microstrain decreases with the increase in x which is shown in Table 1. Fig.2(a) shows the TEM image of Y0.99La0.01CrO3sample. Fig.2(b) shows the High
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Resolution TEM image of Y0.90La0.10CrO3. From HRTEM image the lattice spacing is calculated as 2.67 Å that represents (121) plane of YCrO3 indicating the formation of YCrO3. The selected area electron diffraction (SADE) pattern shown in Fig.2(c) is consisted of a set of concentric rings. Rings have been indexed which reconfirmed the formation of Y0.99La0.01CrO3 crystallites. EDAX image of Y0.90La0.10CrO3 sample in the Fig.2(d) confirms the formation of Y0.90La0.10CrO3
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nanomaterial. The average particle size is found to be 55 nm with standard deviation 1.2, estimated using log normal distribution function as shown in Fig.2(e). for Y0.99La0.01CrO3 sample. The UV- vis absorption spectra of the samples are recorded in the range 250-800 nm with an absorption peak observed at 320 nm and it is shown in the Fig.3. This is due to the direct band to
using the relation
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band transition in the material. The optical band gap Eg is calculated from the experimental data
αhν = A(hν − E g )m
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(1)
and A are the absorption coefficient, Planks constant, frequency and a constant respectively
where m can have values 0.5 and 2 depending on the mode of transition[13]. For direct transition the value of m is 0.5, while the constant A depends on the electron mobility. To calculate the band gap (αhν)2 is plotted against hν and is shown in the inset of Fig.3. It has been observed that band gap decreases with the increase of La doping concentration and this may be due to the creation of mid gap states. Band gap of all the samples lies between 3.39 eV to 2.76 eV which is summarized in table-2. The band gap of bulkYCrO3 is about 1.4 eV. The observed difference in the band gap is due to the nanocrystalline nature of the samples. 4
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Photo luminescence (PL) spectrum is important to distinguish defect related transitions. Luminicence properties depend on the electronic structure. So any change in the microstructure of the crystal with different morphologies, changes the electronic structure, all of which will influence the relaxation procedures of the carriers which is excited from the valence band .This results in the
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various luminescence spectrum [14]. Photoluminescence (PL) spectra of all the experimental samples are studied by exciting the samples at 250 nm and are presented in Fig.4. Emission peak is observed at 630 nm for all the samples, which lies within the visible region. No observable change in the peak positions is observed due to the change in doping concentration. This is due to the
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energy level of Cr+3 ions inYCrO3 which corresponds to the 2Eg–4A2g transition giving the red light emission in the visible region[10,15]. For all the samples PL emissions observed in the visible wave length region with different spectral intensities may give a sign of the effects of doping which
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causes the lattice distortion [14]. Observed red luminescence at room temperature can be used in opto-electronic devices.
To develop an idea about the effect of La doping in conduction mechanism, the temperature dependence of dc conductivity of the undoped and La doped YCrO3, have been measured within arrange 298 ≤ T ≤ 523 K. From the field electron scanning electron micrographic (FESEM) study
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we can have idea about the density of the pellets of the experimental samples. The volume fraction of Yttrium Chromite phase is 0.82 to 0.85 for all the sample pellets to the rest consisting of void space. So it can be concluded that Yttrium Chromite nanoparticles have volume fraction above the percolation threshold.
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It is observed from the measured data that the conductivity of all the samples increases by a significant amount with increasing doping concentration. The property of increasing conductivity with the increase of temperature establishes the semiconducting behavior of the pure and doped
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samples. The conductivity behavior can be explained in terms of Arrhenius model where the conductivity is explained as
E a k B T
σ dc (T ) = σ 0 exp −
Where,
is a constant,
(2)
and K are the activation energy of dc conduction and Boltzmann
constant respectively. The activation energy has been calculated from the slope of the straight line plot of ln
(T)] with 1000/T (not shown in the manuscript) and the values are listed in Table 2 5
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for different samples. When La is doped in YCrO3 compound then with the increase of the La concentration it is observed that the conductivity is increased and the activation energy is decreased for all the samples. Hence the transport process is increased with the expense of the activation energy, as the charge carriers require less energy to be thermally activated for the hopping process (T)] with 1000/T
indicates the
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for the transport mechanism. The linear variation of ln
dominance of simple hopping type charge transport mechanism for all the samples. For strong lattice particle interaction quasi particle polaron will generate. Small polaron model is applicable when the charge carriers are localized within one unit cell. It shows an activated behavior at higher
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temperature for the electronic transport process due to the charge carriers hopping from one side to another. To explain the conductivity mechanism both the adiabatic and non-adiabatic polaron
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models are introduced. The adiabatic model [16-17] states that the carriers settle quickly to the motion of the lattice and the energy is coincident between the neighboring sites. The dc conductivity can be written as
σ dc (T ) =
W 3ne 2 a 2 γ 0 exp − H 2k B T k BT
(3)
In the Eq.(3) ‘a’ denotes the hopping distance, n is the polaron density, WH is the hopping energy
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and ߛ0 is the optical phonon frequency. Hopping energy WH and the polaron binding energy WP are related to each other following the relation WH =
WP −J 2
(4) vs 1000/T is shown in
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Where J is the electronic orbital overlap integral. The plot of
Fig.5(a) which is a straight line graph. The value of WH is extracted from the slope for the different
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investigated samples and listed in Table 2. It is noticed that with the increase of the Lanthanum content the value of WH decreases. However according to non-adiabatic polaron model [18] the carriers move very slowly in comparison with the lattice distortion and relaxation. Hence the conductivity [19] can be expressed as
ne 2 a 2 J 2 2π σ dc (T ) = 2k B T 3 / 2 h WP k B
In the Fig.5(b)
1/ 2
W exp − P 2k B T
(5)
is plotted with 1000/T and from the slope of the straight line plot
polaron binding energy WP is calculated and given in Table 2. It is observed that like WH the value 6
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of WP is also decreasing with the increase of the Lanthanum content due to the increase of the electronic conduction process. Although the two models fit the experimental data reasonably but the applicability condition of both the models are different. In order to explain the applicability of the adiabatic and non-adiabatic models, Holstein [18] defined the critical value of the electronic
J max
W k T = P B π
1/ 4
hγ 0 π
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overlap integral Jmax given by the relation 1/ 2
(6)
When the electronic overlap integral is less than Jmax the non-adiabatic model is applicable but if
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the J is greater than Jmax the adiabatic model is valid. The values of ߛ0 are estimated by the fitting and it lies between 1.9 x 1011 Hz to 2.4 x1011 Hz for all the samples. The value of Jmax can be
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estimated from the Eq.(6) knowing the values of WP and ߛ0 for different samples for temperatures 298K and 523K and listed in Table 2. The value of J has been calculated knowing the value of WH and WP for different samples which lies between 15 meV to 20 meV. It has been observed that the value of Jmax is lower than the value of J for all the samples in the investigated temperature range 298K-523K. So adiabatic small polaron model is suitable for the explanation of the conductivity behavior of all the samples.
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The frequency dependence of conductivity for Y0.95La0.05CrO3 sample has been measured at different temperatures which are shown in Fig.6. It has been observed that the ac conductivity increases with the increase of frequency and temperature. At a fixed temperature it has been observed that initially the conductivity is frequency independent but its frequency dependence is
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observed with the increase of frequency. This is due to the dominance of dc contribution to ac contribution at lower frequency. On the other hand the total measured conductivity is the sum of ac and dc conductivity [20-22] and can be written as
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σ / ( f ) = σ dc + σ ac ( f ) = σ dc + β f
S
(7)
Where ߚ is the temperature dependent constant and ߪdc is the dc conductivity which generates at low frequency. If we subtract the dc contribution from the total estimated conductivity, we get the ac conductivity.
σ ac ( f ) = σ / ( f ) − σ dc = βf S So the plot of
(8)
with lnf will be a straight line having slope S. The frequency exponent S
measures the degree of interaction with the charge carriers with the lattice. The value of S 7
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decreases with the increase of temperature which is shown in the Fig.7 for the temperature range 298K-523K. For the explanation of ac conductivity in amorphous semiconductor and to explain the temperature behavior of S different theoretical models are proposed. These are classical hopping over barrier (HOB) [23], correlated barrier hopping (CBH) [23] and quantum mechanical tunneling
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(QMT). In QMT model [23-25] the carriers move through the tunneling between two localized states near the Fermi level. the charge carrier moves. As a result three types of charge carriers are distinguished such as electrons, small polarons and large polarons. In electron tunneling theory [24], S is independent of temperature but it varies with frequency. For small polaron theory [23] S
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increases with increasing temperature and for large polaron theory [23] S decreases with temperature first and then increases with the increasing temperature. In HOB model the barrier height is independent of the intersite separation so the value of S is unity. According to CBH model
frequency exponent can be written as
S = 1−
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[25] the charge carriers hop over the potential barrier between two charge defect states and the
6k B T
1 WM − k B T ln ωτ 0
(9)
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CBH model gives the decrease of S with the increase of temperature. Where WM , KB , ω and τ0 denotes the effective barrier height, Boltzmann constant, angular frequency and characteristic relaxation time respectively. The linear variation of frequency exponent(S) with temperature can be approximated [24, 26, 27] by the equation
6k B T WM
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S = 1−
(10)
The linear fitting of the experimental data following the Eq.(10) gives the values of WM which are
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presented in Table 2. Due to the introduction of Lanthanum electronic conduction increases which reduces the barrier height and hence conductivity increases. The decreasing nature of S with the increase of temperature shows that CBH model is applicable for all the samples. In the Fig.8(a) and Fig.8(b) the temperature variation of
is plotted with different
frequencies for 0% and 10% La doped samples respectively (figures for 1% and 5% are not given in the manuscript). It is clear from the Figure that the real part of complex conductivity increases with the increase of temperature and the loss peak is obtained. A loss peak is observed for pure and 1% La doped samples but it vanishes for the 5% La and 10% La doped samples due to the increase 8
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of the ac conductivity. Inset of Fig.8(a), and Fig.8(b), show the variation of dielectric loss with temperature. From all the figure it is clear that (i) a peak in dielectric loss is observed at a certain temperature and at a particular frequency(ii) the peak temperature is shifted towards the higher temperature region with the increasing frequency which reveals the strong dispersion of dielectric
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loss exists inside the material (iii) With the increase of frequency broadening of the peak is observed. As the peaks are shifted to the higher temperature region with the increase of the frequency, the number of charge carriers increase which describes the thermally activated nature of the samples [28].
and imaginary part (
of
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Fig. 9(a) and 9(b) describe the variation of real part (
dielectric constant with frequency for Y0.99La0.01CrO3 sample at different temperatures following
ε /(f )=
C ( f )d ε0A
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the relations
ε // ( f ) = ε / ( f )D( f )
(11) (12)
Where C(f) and D(f) are the capacitance and dissipation factor respectively. ε0 is the permittivity of the free space, A is the area of the electrode and d is the thickness of the investigated sample. It is and (
fall sharply with the increasing frequency but it
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clear from the figure that
shows weak variation at higher frequency which is consistent with the previous report [29-30]. The reason can be explained as, at lower frequency electronic, ionic, interfacial and orientational polarizations contribute to the dielectric constant [31-33]. Due to the rapid variation of frequency
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the dipoles can’t follow the applied electric field and due to which orientational polarization is stopped when the frequency is high. As a result the dipole oscillation lags behind the applied field
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and a weak variation of real and imaginary part of dielectric constant are observed at higher frequency due to the contribution of interfacial and space charge polarization only of the samples. For three different frequencies the real part of dielectric permittivity plotted with temperature is shown in Fig.10 for Y0.90La0.10CrO3 sample which follows the power law
ε /(f )∝T p
(13)
Where p is the temperature exponent of dielectric permittivity which is strongly dependent on frequency. The theoretical fitting of the power law is indicated by the solid line in Fig.10 and the points indicate the experimental data. The value of p is calculated from the power law fitting and it 9
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is observed that for Y0.90La0.10CrO3 sample at 1 MHz the value of p is 11.4, for 500 KHz it is 15 and for 100 KHz its value is 17. This suggests that the value of p depends strongly on frequency. For all samples the value of p decreases with the increase of frequency implying the larger variation of
with temperature at low frequency compared to the high frequency. This is because at
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low frequency the structural inhomogeneities may trap the hopping electrons and as a result interfacial polarization is exhibited, which enhances the dielectric permittivity.
According to Maxwell-Wegner capacitor model [34-36] the complex impedance can be modeled by an equivalent circuit consisting of resistance and capacitance due to grain and
written as 1 = Z / ( f ) − jZ // ( f ) j 2πfC 0 ε ( f )
Z/(f )=
Z
//
(f )=
Rg
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Z=
1 + (2πfR g C g ) 2πfR g2 C g
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interfacial grain boundary contribution [37-38]. The complex impedance of such a system can be
2
1 + (2πfR g C g ) 2
+
+
R gb
1 + (2πfR gb C gb )
2
2 2πfR gb C gb
1 + (2πfR gb C gb )
2
(14)
(15)
(16)
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Where Rg and Rgb are the grain and grain boundary resistance respectively. Cg and Cgb are the grain and grain boundary capacitance respectively. C0 is the free space capacitance and ߱=2ߨf. The real part of complex impedance for all the samples has been estimated using the relation
[{
ε // ( f )
} {
}]
2πfC 0 ε / ( f ) + ε // ( f )
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Z/(f )=
2
2
(17)
The frequency variation of the real part of impedance at room temperature is plotted in Fig.11 for
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all the samples. It is observed that with the increase of frequency the value of
decreases which
suggests the increase of ac conductivity with frequency. In the Fig.11 the points indicate the experimental data and the theoretical best fit of Eq.(15) are indicated by the solid line. The values of grain boundary resistance and capacitance are calculated from the fitting and shown in the Table 2 for different samples. From the values of extracted parameters it can be concluded that the grain boundary contribution dominates over the grain contribution. It may be due to disordered atomic arrangement in grain boundary region enhancing the electron scattering. The Cole-Cole diagram of Y0.99La0.01CrO3 sample for different temperatures are shown in the Fig.12 where imaginary part of 10
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impedance is plotted with real part of impedance. The semicircles shown in the figure corresponds to the universal dielectric behavior of the sample. We have investigated the dielectric relaxation spectra in terms of dielectric modulus as a function of frequency. The complex dielectric modulus has been defined as [39-40]
Where
and
1
ε
*
(f )
= M / ( f ) + jM // ( f )
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M *( f ) =
(18)
are the real and imaginary part of dielectric modulus respectively and can be
M // ( f ) =
ε /(f )
{ε ( f )} + {ε ( f )} 2
/
//
2
ε // ( f )
{ε ( f )} + {ε ( f )} 2
/
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M /(f )=
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written as
//
2
While studying the variation of the real part of electric modulus
(19)
(20)
, with frequency at different
temperatures no characteristic peak has been observed as shown in the Fig.13(a) for Y0.95La0.05CrO3 sample. It is also valid for the other samples. A loss peak has been observed when with frequency for all the samples. In the Fig.13(b) the variation of
is plotted
with frequency is shown for
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Y0.99La0.01CrO3sample at different temperatures. When the free charges are blocked at the surface of the nanocrystals, due to the dielectric difference between grain and grain boundaries interfacial polarization is produced. In the Fig.13(b) it is clear that the position of the relaxation peaks is
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shifted to the higher frequency side with the increase of the temperature. This is because with the increase of the temperature the free charges accumulate at the interface and due to the increase in the charge carrier mobility the relaxation time decreases. Therefore the loss peak is shifted to the
of
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higher frequency region with the increase of the temperature. The normalized plot with
are shown in the Fig.14(a) for all the samples at 323K fixed
temperature and in the Fig.14(b) it is shown for different temperatures for Y0.99La0.01CrO3 sample. It is observed that the spectra of
for different temperatures merge on a single master
curve. It concludes that all the dynamic process possessing at different temperature have similar activation energy. From the temperature variation of relaxation time (߬) following Arrhenius model the average activation energy(Ear) for the charge carrier hopping corresponding to the relaxation behavior can be estimated. The Arrhenius equation can be written as 11
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E ar k T B
τ = τ 0 exp
(21)
Where τ0 is the pre-exponential factor, Ear is the average activation energy. The values of ߬ [41] corresponding to maxima in Fig.13(b) can be estimated following the relation
Where the maximum value of relaxation time. In the Fig.15
occurs at the frequency
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(22)
and ߬ is the corresponding
is plotted with inverse of temperature (1000/T). In the plot the
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points indicate the experimental data and the theoretical best fit of Eq.(21) is indicated by the solid line. In the plot
verses 1000/T for each sample a single slope is obtained which indicates the
presence of single activation energy ranging from 0.45 eV to 0.35 eV for different samples. It is
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observed that the value of effective barrier height WM is greater than the value of activation energy for all the samples. The lower value of activation energy [42-44] indicates that a strong charge transfer occurs due to the delocalization of charge carriers accumulated on the interface. The value of the pre exponential factor ߬0 is estimated from the fitting using Eq.(21) in the Fig.15. It lies in the range 4.48 x 10-11 s to 5.1x10-12 s.
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4. Conclusions
The La doped YCrO3 nanoparticles have been prepared by sol-gel method. The samples are characterized by XRD, HRTEM, UV-Vis spectrometry and PL spectroscopy. XRD and TEM analysis confirms the formation the nanoparticles. The band gap of the different samples measured
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from the UV-Vis absorption spectra shows that the band gap of the different samples decreases with the increase of the doping concentration and this can be explained by the creation of mid gap states. From the PL analysis it is observed that all the samples shows distinct red light emission in
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the visible range around 630 nm which is attributed to the energy level of Cr+3 ions in YCrO3 corresponding to the
2
Eg–4A2g transition. The dc conductivity of the samples increases with the
increase of temperature showing semiconducting behavior and it also increases with La content following a simple hopping type conduction mechanism. Dc conductivity behavior is explained by adiabatic small polaron hopping model. The frequency exponent S, estimated from ac conductivity data, decreases with the increase of temperature obeying the CBH model. As the relaxation time decreases with the increase of the charge carrier mobility, the relaxation peaks in imaginary part of electric modulus shift to the higher frequency side with the increasing temperature. The variation of 12
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dielectric constant with the frequency and temperature are also studied. Analyzing the real part of impedance it is proved that the grain boundary resistance dominates over the grain resistance. The activation energy of all the samples are estimated from both the dc and ac data and in both cases the value decreases with the increase of the Lanthanum content. The increasing nature of the
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conductivity with the increase of doping concentration enhances the application of La doped YCrO3 as the interconnect material in SOFCs.
Acknowledgments
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Authors are very much thankful to DAE-BRNS, Government of India for financial support during the work. The authors gratefully acknowledge the support getting from the Centre of Excellence,
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TEQIP-II, NIT Durgapur.
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References [1] F. Meng, Z. Hong, J. Arndt, Nano Research 5 (2012) 213. [2] S. Tao, J. T. S. Irvine, Chemistry of Materials 16 (2004) 4116.
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[3] S. McIntosh, M. V. D. Bossche, Solid State Ionics 192 (2011) 453. [4] A. Chakraborty, R. N. Basu, H. S. Maiti, J. Mater. Lett. 45 (2000) 162.
[5] L. P. Rivas-Vazquez, J. C. Rendon-Angeles, J. L. Rodriguez-Galicia, C. A. GutierrezChavarria, K. J. Zhu, K. Yanagisawa, J. Eur. Ceram. Soc. 26 (2006) 81. [6] J. T. Looby, L. Katz, J. Am. Chem. Soc. 76 (1954) 6029.
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[8] W. J. Weber, C. W. Griffin, J. L. Bates, J. Am. Ceram. Soc. 70 (1987) 265. [9] A. Duran, A. M. Arevalo-Lopez, E. Castillo-Martınez, M. Garcıa - Guaderrama, E. Moran, M. P. Cruz, F. Fernandez, M. A. Alario-Franco J. Solid State Chem. 183 (2010) 1863. [10] S. Krishnan, C. S. S Sandeep, R. Philip, N. Kalarikkal, Chem. Phys. Lett. 529 (2012) 59. [11] G. F. Carini II, H. U. Anderson, D. M. Sparlin, M. M. Nasrallah, Solid State Ionics 49 (1991) 233.
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[12] G. F. Carini II, H. U. Anderson, M. M. Nasrallah, D. M. Sparlin, J. Solid State Chem. 94
[13] Md. Shkir, H. Abbas, Siddhartha, Z. R. Khan, J. Phys. Chem. Solids 73 (2012) 1309. [14] S. M. El-Sheikh, M. M. Rashad, J. Alloys Compd. 496 (2010) 723.
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[15] S. Sugano, K. Aoyagi, K. Tsushima, J. Phys. Soc. Jpn. 31 (1971) 706. [16] D. Emin, T. Holstein, Ann. Phys. 53 (1960) 439.
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[17] A. Karmakar, S. Majumdar, S. Giri, J. Phys. Condens. Matter 23 (2011) 495902. [18] T. Holstein, Ann. Phys. 8 (1959) 343. [19] S. Saha, A. Nandy, A. K. Meikap, S. K. Pradhan, Mater. Res. Bull. 68 (2015) 66. [20] D. P. Almond, C. C. Hunter, A. R. Weast, J. Mater. Sci. 19 (1984) 3236. [21] A. K. Jonscher, Nature 267 (1977) 673. [22] A. K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Group, London, 1983. [23] A. R. Long, Adv. Phys. 31 (1982) 553. [24] S. R. Elliott, Adv. Phys. 36 (1987)135. [25] P. Extrance, S. R. Elliott, E. A. Davis, Phys. Rev. B 32 (1985) 8148. 14
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[26] G. E. Pike, Phys. Rev. B, 6 (1972) 1572. [27] S. Sinha, S. K. Chatterjee, J. Ghosh and A. K. Meikap, J. Phys. D: Appl. Phys. 47 (2014) 257301. [28] P. Kumar, B. P. Singh, T. P. Sinha, N. K. Singh, Solid State Sci. 13 (2011) 2060.
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[29] P. H. Bell, W. P. Davey, J . Chem. Phys. 9 (1941) 441.
[30] H. Frohlieh, Theory of Dielectrics, Oxford University Press, London, 1958.
[31] K. K. Srivastava, A. Kumar, O. S. Panwar, L. N. Lakshminarayan, J. Non-Cryst. Solids 33 (1979) 205.
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[32] B. Tareev, Physics of Dielectric Materials, Mir Publishers, Moscow, 1975. [33] R. Ertugrul, A. Tataroghi, Chin. Phys. Lett. 29 (2012) 077304.
[34] J. C. Maxwell, A treatise on Electricity and Magnetism, Oxford University Press, Oxford,
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1988.
[35] K. W. Wagner, Ann. Phys . 40 (1913) 817.
[36] V. Hippel, Dielectrics and Waves, Wiley, New York, 1954. [37] R. Sinha, S. Basu, A. K. Meikap, Physica E 69 (2015) 47.
[38] L. Raniero, E. Fortunato, I. Ferreira, R. Martins, J. Non-Cryst. Solids 352 (2006) 1880.
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[39] N. G. McCrum, B. E. Read, G. Williams, An Elastic and Dielectric Effects in Polymeric solids, John Wiley and Sons, New York, 1967.
[40] L. D. Tung, V. Kolesnichenko, G. Caruntu, Y. Remond, V. O. Gloub, C. J. O’Connor, L. Spinu, Physica B 319 (2002) 116.
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[41] R. Singh, V. Arora, R. P. Tandon, A. Mansingh, S. Chandra, Synth. Met. 104 (1999) 137. [42] W. Cao, R. Gerhardt, Solid State Ionics 42 (1990) 213. [43] R. Gerhardt, J. Phys. Chem. Solids 55 (1994) 1491.
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[44] S. Komine, Physica B, 392 (2007) 348.
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Table 1 List of lattice parameters and microstrain obtained from the Rietveld refinement method of Y1-xLaxCrO3 samples.
Y1La0CrO3
Y0.99La0.01CrO3
Y0.95La0.05CrO3
Y0.90La0.10CrO3
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Properties
7.6301
7.6371
7.6487
b (Å)
7.5777
7.5964
7.6145
c (Å)
7.5713
7.5719
7.5727
7.5853
β (Degree)
93.06
92.96
92.88
92.73
Microstrain
1.325 X 10-3
1.066 X 10-3
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1.146 X 10-3
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a (Å)
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7.6556
7.6175
0.938 X 10-3
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Table 2 Values of relevant physical parameters, conductivity at 320K (ߪ(320K)), band gap
Y0.99La0.01CrO3
Y0.95La0.05CrO3
Y0.90La0.10CrO3
σ(320K)(Ω-1m-1)
9.35 X 10-6
1.54 X 10-4
4.09 X 10-4
7.06 X 10-4
Eg(eV)
3.39
2.92
2.85
2.76
Ea(dc)(eV)
0.44
0.39
0.34
0.30
Ear(ac)(eV)
0.45
0.43
0.38
0.35
WP(eV)
0.99
0.88
0.78
0.70
WH(eV)
0.48
0.42
0.37
0.33
WM(eV)
0.60
0.53
0.50
0.48
γ0(Hz)
1.90 X 1011
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Y1La0CrO3
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Properties
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energy (Eg), dc activation energy (Ea(dc)), ac activation energy (Ear(ac)), polaron binding energy (WP), hopping energy (WH), effective barrier height (WM), optical phonon frequency (ߛ0),electronic orbital overlap integral (J), critical value of electronic orbital overlap integral (Jmax), grain resistance(Rg), grain boundary resistance(Rgb), grain capacitance (Cg) and grain boundary capacitance (Cgb)
2.30 X 1011
2.20 X 1011
2.40 X 1011
20.00
20.00
20.00
15.00
Jmax(298K) (meV)
10.45
5.13
4.84
4.81
Jmax(523K) (meV)
12.03
5.90
5.57
5.54
Rg(MΩ)
0.82
0.39
0.28
0.20
Rgb(MΩ)
3.59
1.11
0.31
0.26
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Cgb(F)
6.80 X 10-11
4.70 X 10-11
4.00 X 10-11
3.60 X 10-11
1.24 X 10-10
1.53 X 10-10
3.84 X 10-10
5.00 X 10-11
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J(meV)
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FIGURE CAPTION Fig.1 X-ray diffraction pattern (XRD) of YCrO3 and La doped YCrO3 samples.
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Fig.2 (a) The TEM image of Y0.99La0.01CrO3 sample. (b) The High Resolution TEM image of Y0.90La0.10CrO3 sample. (c) The SADE pattern of Y0.99La0.01CrO3 sample consisted of a set of concentric rings. (d) The EDAX image of Y0.90La0.10CrO3 sample. (e) Log normal distribution of Fig.2(a) for Y0.99La0.01CrO3 sample. Fig.3 The UV-vis absorption spectra of the samples. The optical band gap of Y0.99La0.01CrO3 sample is shown in the inset of Fig.3
vs 1000/T for all samples. (b)
is plotted
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Fig.5 (a)The plotting of with 1000/T for all the samples.
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Fig.4 Photoluminescence (PL) spectra of all the experimental samples.
Fig.6 The frequency dependence of ac conductivity for Y0.95La0.05CrO3 sample at different temperatures. Fig.7 The temperature variation of the frequency exponent S for all the samples.
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Fig. 8 The temperature variation of ac conductivity is plotted with different frequencies for (a) pureYCrO3 and (b) 10% La doped samples respectively. Fig.9 Variation of (a) real part ( and (b) imaginary part ( frequency for Y0.99La0.01CrO3 sample at different temperatures.
of dielectric constant with
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Fig.10 Real part of dielectric permittivity for three different frequencies is plotted with temperature forY0.90La0.10CrO3 sample.
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Fig.11 The frequency variation of the real part of impedance at room temperature for all the samples. Fig.12 The Cole-Cole diagram of Y0.99La0.01CrO3 sample for different temperatures. Fig.13 The frequency variation of (a) real part of electric modulus and (b) imaginary part of electric modulus ( ) for Y0.95La0.05CrO3 sample at different temperatures. Fig.14 (a)The normalized plot of temperature. (b)The normalized plot of for Y0.99La0.01CrO3 sample. Fig.15
with with
plotted with inverse of temperature (1000/T).
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for all the samples at 323K fixed for different temperatures
(242)
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5%La
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40
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0%La 50
60
2θ(Degree) Fig.1
10%La
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(331)
(321) (240) (123)
(200) (121) (002) (210) (031) (220) (022) (131) (202) (040) (230) (212) (311)
(101) (020) (111)
4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0
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70
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0%La 1%La 5%La 10%La
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Highlights The dc conductivity of the samples increases with increasing La concentration.
•
A linear variation has been observed in S-T curve.
•
Band gap decreases with the increase of La concentration due to creation of mid gap.
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The activation energies decrease with increasing doping percentage.
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A relaxation peak has been observed in M // ( f ) − T curve.
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S1293-2558(16)30554-4
DOI:
10.1016/j.solidstatesciences.2016.08.010
Reference:
SSSCIE 5363
To appear in:
Solid State Sciences
Received Date: 28 December 2015 Revised Date:
9 August 2016
Accepted Date: 11 August 2016
Please cite this article as: R. Sinha, S. Kundu, S. Basu, A.K. Meikap, Effect of La doping on optical and electrical transport properties of nanocrystalline YCrO3, Solid State Sciences (2016), doi: 10.1016/ j.solidstatesciences.2016.08.010. 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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Effect of La doping on Optical and electrical transport properties of nanocrystalline YCrO3 R.Sinhaa, S. Kunduc, S.Basub and A. K. Meikapb* a
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Department of Physics, Asansol Engineering College,Kanyapur, Asansol-713305. West Bengal, India b Department of Physics, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur 713209,West Bengal, India c Department. of Physics, The University of Burdwan, Golapbag, Burdwan-713104, West Bengal, India, A relaxation peak has been noticed in M // ( f ) − f curve which is accompanied with the step like
transition in M / ( f ) − f which is shifting towards high frequency side with the increase of
0.007
298K 323K 373K 423K 473K 523K
0.004 0.003
/
M (f,T)
0.005
0.000
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0.002 0.001
-0.001 1 10
10
2
10
(b)
0.0015
AC C //
f(Hz)10
4
10
5
10
6
298K 323K 373K 423K 473K 523K
0.0020
M (f,T)
3
1% La
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0.0025
1% La
(a)
0.006
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temperature, reveals the hopping type conduction mechanism.
0.0010 0.0005 0.0000
10
2
10
3
10
4
f(Hz)
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5
10
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Effect of La doping on Optical and electrical transport properties of nanocrystalline YCrO3 R.Sinhaa, S. Kunduc, S.Basub and A. K. Meikapb* a
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Department of Physics, Asansol Engineering College,Kanyapur, Asansol-713305. West Bengal, India b Department of Physics, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur 713209,West Bengal, India c Department. of Physics, The University of Burdwan, Golapbag, Burdwan-713104, West Bengal, India,
Abstract
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In this work we have reported the synthesis and characterization of La doped YCrO3 nanoparticles following sol-gel method. The optical band gap of the investigated samples decreases with the increase of doping content. Photoluminescence spectra show distinct red light emission in the visible range around 630 nm. Dielectric permittivity is measured within the temperature range 298K-523K and in the frequency range 20 Hz - 1 MHz following the power law
, which
shows that the temperature exponent p increases with the decreasing frequency and its values varies
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from 11.4 to 17 for 1 MHz to 100 KHz frequency variation. The ac impedance analysis shows that grain boundary contribution is dominating over grain contribution. The dc conductivity of the investigated samples follows semiconductor behaviour. The analysis of both the dc and ac conductivity shows that the activation energy decreases and the conductivity increases with the
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increase of doping concentration which is very much important for its application as interconnect
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material in Solid Oxide Fuel Cells (SOFCs).
Keywords: A. Nanostructured materials; B. Sol-gel processes; C. Electric transport; C. Dielectric response, D. X-ray diffraction; D. Transmission electron microscopy.
*Corresponding author: Tel.: +91 343 2546808; fax: +91 343 2547375 (A.K. Meikap) *Email address: [email protected] (A. K. Meikap)
1
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1. Introduction Recently perovskites type oxides with ABO3 structure have been widely investigated as electrode materials in photo electrochemistry, solid-oxide fuel cells (SOFCs), sensors and catalysis [1-3]. It is very important to improve the features of the perovskites for obtaining a wide range of
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the application. Their utility can be enhanced by partial substitution of A-site or B-site or both sites, which can improve or change their physical and chemical properties and widen the application window. Solid oxides fuel cell (SOFC) is one of the most promising candidates for a new generation power system for its high energy conversion efficiency. It produces fewer pollutants and
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so have environmental advantages. But their operating temperature is very high (8000C-10000C). So the chemical and physical degradation of the building material at this temperature restricts the
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choice of the materials. YCrO3 possesses perovskites type structure with the general formula ABO3 and it is apparently stable to at least 1200K. It has promising utility for SOFCs ceramic interconnect material which is used to provide the conductive path for electrical current to flow between the electrode and the external circuit. LaCrO3 doped with alkaline metals (Ca, or Sr) have been extensively used as interconnect material for the present generation SOFCs [4-5]. Recently YCrO3 has drawn much more attention because it has many advantages over
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LaCrO3 for improved chemical and dimensional stability. But due to the low electrical conductivity its application is limited. In the recent years low temperature synthesis of Yittrium Chromite was done by Looby and Katz [6] following a molten salt technique at 9000C. Weber et al reported the electrical conductivity of YCrO3 [7-8]. Magneto and thermal dielectric properties of
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biferroicYCrO3 prepared by combustion synthesis was reported by Dura´n et al.[9]. Effect of cation substitution on electrical and thermal transport properties of YCrO3 and LaCrO3 was individually
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discussed by Weber et al.[8]. Later Krishnan et al.[10] reported the two-photon assisted excited state absorption in multiferroic YCrO3 nanoparticles. In the recent time for the purpose of obtaining higher electrical conductivity, Carnil II et al. fabricated (Y1-xCax)CrO3 (0.05
x
0.20)
ceramics by heating for 24 hours at 17500C and 10-10 atm. pressure and then annealing 15500C during 48 hours in air[11-12]. It was observed that the electrical conductivity increases with Ca doping. Considering the importance of doping in YCrO3 at A-site to improve its electrical properties, the present works reports the synthesis, characterization and electrical transport properties of La doped nanocrystalline YCrO3. To the best of our knowledge no experimental work yet have been reported to describing detail electrical transport mechanism so far which is essential 2
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for its device applications. We have achieved higher electrical conductivity with La doping which is very much important for the operation of SOFCs.
2. Experimental procedure Lanthanum doped YCrO3 [Y1-xLaxCrO3(YLaCrO) with x=0, 0.01, 0.05, 0.10] nanoparticles
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were prepared by sol-gel method. Dissolving weighted amount of Yttrium Nitrate Hexahydrate [Y(NO3)3 x 6H2O], Chromium Nitrate Nonahydrate [Cr(NO3)3 x 9H2O] and Lanthanum Nitrate Hexahydrate [La(NO3)3 x 6H2O], in 50 ml distilled water with continuous stirring precursor solution is prepared at first. Then 20 ml PVA solution of strength 12.5 gm/ liter is added to the
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above solution under continuous stirring for 1 hour. In the next step the solution is evaporated to get dry precursor powder. At last the grinded precursor powder is calcined in air at 8000C for 1
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hour duration. The X-Ray Powder Diffraction (XRD) patterns of various samples are recorded using a X’ pert Pro X-ray diffractometer with Ni-filtered CuKα radiation having λ=1.5414 Å with 2θ ranging from 200-800. The Transmission Electron Microscope (HRTEM, JEOL 2011) is used to study the morphology of the nanoparticles. Dispersing the very small part of the sample in Ethanol, one drop of dispersed solution is cast on a carbon coated copper grid for the HRTEM measurements. UV-Visible absorption spectroscopy is performed between 250-600 nm using a
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double beam spectro – photometer (Hitachi-3010). Photoluminescence spectroscopy study is performed with a Hitachi fluorescence spectrophotometer (F-2500). For the measurement of electrical properties the powder sample is taken in a steel mould of 1 cm diameter and compacted at a pressure of 7 tons/cm2. Silver paint electrodes are applied for both the opposite faces (Supplied by
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Acheson Colloiden B.V Holland). DC conductivity is measured by an Agilent 3458A multimeter. The temperature dependence of conductivity is measured by a furnace fitted with 8502 eurotherm
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controller. The ac measurements are performed using impedance analyzer (HP 4902 A).
3. Results and Discussions The powder X-ray diffraction patterns (XRD) of Y1-xLaxCrO3 (with x=0, 0.01, 0.05, 0.10) are shown in Fig.1. XRD pattern shows that the samples are well crystalline in nature. All the prominent peaks in the figure are indexed to various (hkl) planes showing the formation of YCrO3 and it is also matched with “ICDD PDF No. 34-0365.” The average crystallite size of the nanoparticles are calculated with the help of Debye Scherer formula, D = 0.9λ/βFWcosθ, where D, λ and β FW are the average crystallite size, wavelength of X-ray and FWHM of diffraction peak. The 3
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estimated average crystallite size is varying from 35 nm- 45 nm. The lattice parameters and microstrain for Y1-xLaxCrO3 system are obtained from the Rietveld refinement method and tabulated in Table 1. Table 1 clearly reveals that the value of lattice parameters increases with the increasing value of La doping concentrations (x) and it suggests that the lattice volume (Å3)
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increases with the increase in x. This is due to the fact that the dopant La3+ ion has larger ionic radius (1.216Å) than the parent Y3+ ion (1.075Å) in 9 coordination system. Cell angle (β) decreases with the increase in x and it has a tendency of approaching towards 90°. Hence, it can be concluded from Table 1 that the structure of the Y1-xLaxCrO3 system becomes more stable
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(monoclinic→orthorhombic) with the increase in x. Rietveld refinement results also reveal that for Y1-xLaxCrO3 system lattice microstrain decreases with the increase in x which is shown in Table 1. Fig.2(a) shows the TEM image of Y0.99La0.01CrO3sample. Fig.2(b) shows the High
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Resolution TEM image of Y0.90La0.10CrO3. From HRTEM image the lattice spacing is calculated as 2.67 Å that represents (121) plane of YCrO3 indicating the formation of YCrO3. The selected area electron diffraction (SADE) pattern shown in Fig.2(c) is consisted of a set of concentric rings. Rings have been indexed which reconfirmed the formation of Y0.99La0.01CrO3 crystallites. EDAX image of Y0.90La0.10CrO3 sample in the Fig.2(d) confirms the formation of Y0.90La0.10CrO3
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nanomaterial. The average particle size is found to be 55 nm with standard deviation 1.2, estimated using log normal distribution function as shown in Fig.2(e). for Y0.99La0.01CrO3 sample. The UV- vis absorption spectra of the samples are recorded in the range 250-800 nm with an absorption peak observed at 320 nm and it is shown in the Fig.3. This is due to the direct band to
using the relation
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band transition in the material. The optical band gap Eg is calculated from the experimental data
αhν = A(hν − E g )m
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(1)
and A are the absorption coefficient, Planks constant, frequency and a constant respectively
where m can have values 0.5 and 2 depending on the mode of transition[13]. For direct transition the value of m is 0.5, while the constant A depends on the electron mobility. To calculate the band gap (αhν)2 is plotted against hν and is shown in the inset of Fig.3. It has been observed that band gap decreases with the increase of La doping concentration and this may be due to the creation of mid gap states. Band gap of all the samples lies between 3.39 eV to 2.76 eV which is summarized in table-2. The band gap of bulkYCrO3 is about 1.4 eV. The observed difference in the band gap is due to the nanocrystalline nature of the samples. 4
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Photo luminescence (PL) spectrum is important to distinguish defect related transitions. Luminicence properties depend on the electronic structure. So any change in the microstructure of the crystal with different morphologies, changes the electronic structure, all of which will influence the relaxation procedures of the carriers which is excited from the valence band .This results in the
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various luminescence spectrum [14]. Photoluminescence (PL) spectra of all the experimental samples are studied by exciting the samples at 250 nm and are presented in Fig.4. Emission peak is observed at 630 nm for all the samples, which lies within the visible region. No observable change in the peak positions is observed due to the change in doping concentration. This is due to the
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energy level of Cr+3 ions inYCrO3 which corresponds to the 2Eg–4A2g transition giving the red light emission in the visible region[10,15]. For all the samples PL emissions observed in the visible wave length region with different spectral intensities may give a sign of the effects of doping which
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causes the lattice distortion [14]. Observed red luminescence at room temperature can be used in opto-electronic devices.
To develop an idea about the effect of La doping in conduction mechanism, the temperature dependence of dc conductivity of the undoped and La doped YCrO3, have been measured within arrange 298 ≤ T ≤ 523 K. From the field electron scanning electron micrographic (FESEM) study
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we can have idea about the density of the pellets of the experimental samples. The volume fraction of Yttrium Chromite phase is 0.82 to 0.85 for all the sample pellets to the rest consisting of void space. So it can be concluded that Yttrium Chromite nanoparticles have volume fraction above the percolation threshold.
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It is observed from the measured data that the conductivity of all the samples increases by a significant amount with increasing doping concentration. The property of increasing conductivity with the increase of temperature establishes the semiconducting behavior of the pure and doped
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samples. The conductivity behavior can be explained in terms of Arrhenius model where the conductivity is explained as
E a k B T
σ dc (T ) = σ 0 exp −
Where,
is a constant,
(2)
and K are the activation energy of dc conduction and Boltzmann
constant respectively. The activation energy has been calculated from the slope of the straight line plot of ln
(T)] with 1000/T (not shown in the manuscript) and the values are listed in Table 2 5
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for different samples. When La is doped in YCrO3 compound then with the increase of the La concentration it is observed that the conductivity is increased and the activation energy is decreased for all the samples. Hence the transport process is increased with the expense of the activation energy, as the charge carriers require less energy to be thermally activated for the hopping process (T)] with 1000/T
indicates the
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for the transport mechanism. The linear variation of ln
dominance of simple hopping type charge transport mechanism for all the samples. For strong lattice particle interaction quasi particle polaron will generate. Small polaron model is applicable when the charge carriers are localized within one unit cell. It shows an activated behavior at higher
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temperature for the electronic transport process due to the charge carriers hopping from one side to another. To explain the conductivity mechanism both the adiabatic and non-adiabatic polaron
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models are introduced. The adiabatic model [16-17] states that the carriers settle quickly to the motion of the lattice and the energy is coincident between the neighboring sites. The dc conductivity can be written as
σ dc (T ) =
W 3ne 2 a 2 γ 0 exp − H 2k B T k BT
(3)
In the Eq.(3) ‘a’ denotes the hopping distance, n is the polaron density, WH is the hopping energy
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and ߛ0 is the optical phonon frequency. Hopping energy WH and the polaron binding energy WP are related to each other following the relation WH =
WP −J 2
(4) vs 1000/T is shown in
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Where J is the electronic orbital overlap integral. The plot of
Fig.5(a) which is a straight line graph. The value of WH is extracted from the slope for the different
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investigated samples and listed in Table 2. It is noticed that with the increase of the Lanthanum content the value of WH decreases. However according to non-adiabatic polaron model [18] the carriers move very slowly in comparison with the lattice distortion and relaxation. Hence the conductivity [19] can be expressed as
ne 2 a 2 J 2 2π σ dc (T ) = 2k B T 3 / 2 h WP k B
In the Fig.5(b)
1/ 2
W exp − P 2k B T
(5)
is plotted with 1000/T and from the slope of the straight line plot
polaron binding energy WP is calculated and given in Table 2. It is observed that like WH the value 6
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of WP is also decreasing with the increase of the Lanthanum content due to the increase of the electronic conduction process. Although the two models fit the experimental data reasonably but the applicability condition of both the models are different. In order to explain the applicability of the adiabatic and non-adiabatic models, Holstein [18] defined the critical value of the electronic
J max
W k T = P B π
1/ 4
hγ 0 π
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overlap integral Jmax given by the relation 1/ 2
(6)
When the electronic overlap integral is less than Jmax the non-adiabatic model is applicable but if
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the J is greater than Jmax the adiabatic model is valid. The values of ߛ0 are estimated by the fitting and it lies between 1.9 x 1011 Hz to 2.4 x1011 Hz for all the samples. The value of Jmax can be
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estimated from the Eq.(6) knowing the values of WP and ߛ0 for different samples for temperatures 298K and 523K and listed in Table 2. The value of J has been calculated knowing the value of WH and WP for different samples which lies between 15 meV to 20 meV. It has been observed that the value of Jmax is lower than the value of J for all the samples in the investigated temperature range 298K-523K. So adiabatic small polaron model is suitable for the explanation of the conductivity behavior of all the samples.
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The frequency dependence of conductivity for Y0.95La0.05CrO3 sample has been measured at different temperatures which are shown in Fig.6. It has been observed that the ac conductivity increases with the increase of frequency and temperature. At a fixed temperature it has been observed that initially the conductivity is frequency independent but its frequency dependence is
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observed with the increase of frequency. This is due to the dominance of dc contribution to ac contribution at lower frequency. On the other hand the total measured conductivity is the sum of ac and dc conductivity [20-22] and can be written as
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σ / ( f ) = σ dc + σ ac ( f ) = σ dc + β f
S
(7)
Where ߚ is the temperature dependent constant and ߪdc is the dc conductivity which generates at low frequency. If we subtract the dc contribution from the total estimated conductivity, we get the ac conductivity.
σ ac ( f ) = σ / ( f ) − σ dc = βf S So the plot of
(8)
with lnf will be a straight line having slope S. The frequency exponent S
measures the degree of interaction with the charge carriers with the lattice. The value of S 7
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decreases with the increase of temperature which is shown in the Fig.7 for the temperature range 298K-523K. For the explanation of ac conductivity in amorphous semiconductor and to explain the temperature behavior of S different theoretical models are proposed. These are classical hopping over barrier (HOB) [23], correlated barrier hopping (CBH) [23] and quantum mechanical tunneling
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(QMT). In QMT model [23-25] the carriers move through the tunneling between two localized states near the Fermi level. the charge carrier moves. As a result three types of charge carriers are distinguished such as electrons, small polarons and large polarons. In electron tunneling theory [24], S is independent of temperature but it varies with frequency. For small polaron theory [23] S
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increases with increasing temperature and for large polaron theory [23] S decreases with temperature first and then increases with the increasing temperature. In HOB model the barrier height is independent of the intersite separation so the value of S is unity. According to CBH model
frequency exponent can be written as
S = 1−
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[25] the charge carriers hop over the potential barrier between two charge defect states and the
6k B T
1 WM − k B T ln ωτ 0
(9)
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CBH model gives the decrease of S with the increase of temperature. Where WM , KB , ω and τ0 denotes the effective barrier height, Boltzmann constant, angular frequency and characteristic relaxation time respectively. The linear variation of frequency exponent(S) with temperature can be approximated [24, 26, 27] by the equation
6k B T WM
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S = 1−
(10)
The linear fitting of the experimental data following the Eq.(10) gives the values of WM which are
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presented in Table 2. Due to the introduction of Lanthanum electronic conduction increases which reduces the barrier height and hence conductivity increases. The decreasing nature of S with the increase of temperature shows that CBH model is applicable for all the samples. In the Fig.8(a) and Fig.8(b) the temperature variation of
is plotted with different
frequencies for 0% and 10% La doped samples respectively (figures for 1% and 5% are not given in the manuscript). It is clear from the Figure that the real part of complex conductivity increases with the increase of temperature and the loss peak is obtained. A loss peak is observed for pure and 1% La doped samples but it vanishes for the 5% La and 10% La doped samples due to the increase 8
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of the ac conductivity. Inset of Fig.8(a), and Fig.8(b), show the variation of dielectric loss with temperature. From all the figure it is clear that (i) a peak in dielectric loss is observed at a certain temperature and at a particular frequency(ii) the peak temperature is shifted towards the higher temperature region with the increasing frequency which reveals the strong dispersion of dielectric
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loss exists inside the material (iii) With the increase of frequency broadening of the peak is observed. As the peaks are shifted to the higher temperature region with the increase of the frequency, the number of charge carriers increase which describes the thermally activated nature of the samples [28].
and imaginary part (
of
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Fig. 9(a) and 9(b) describe the variation of real part (
dielectric constant with frequency for Y0.99La0.01CrO3 sample at different temperatures following
ε /(f )=
C ( f )d ε0A
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the relations
ε // ( f ) = ε / ( f )D( f )
(11) (12)
Where C(f) and D(f) are the capacitance and dissipation factor respectively. ε0 is the permittivity of the free space, A is the area of the electrode and d is the thickness of the investigated sample. It is and (
fall sharply with the increasing frequency but it
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clear from the figure that
shows weak variation at higher frequency which is consistent with the previous report [29-30]. The reason can be explained as, at lower frequency electronic, ionic, interfacial and orientational polarizations contribute to the dielectric constant [31-33]. Due to the rapid variation of frequency
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the dipoles can’t follow the applied electric field and due to which orientational polarization is stopped when the frequency is high. As a result the dipole oscillation lags behind the applied field
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and a weak variation of real and imaginary part of dielectric constant are observed at higher frequency due to the contribution of interfacial and space charge polarization only of the samples. For three different frequencies the real part of dielectric permittivity plotted with temperature is shown in Fig.10 for Y0.90La0.10CrO3 sample which follows the power law
ε /(f )∝T p
(13)
Where p is the temperature exponent of dielectric permittivity which is strongly dependent on frequency. The theoretical fitting of the power law is indicated by the solid line in Fig.10 and the points indicate the experimental data. The value of p is calculated from the power law fitting and it 9
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is observed that for Y0.90La0.10CrO3 sample at 1 MHz the value of p is 11.4, for 500 KHz it is 15 and for 100 KHz its value is 17. This suggests that the value of p depends strongly on frequency. For all samples the value of p decreases with the increase of frequency implying the larger variation of
with temperature at low frequency compared to the high frequency. This is because at
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low frequency the structural inhomogeneities may trap the hopping electrons and as a result interfacial polarization is exhibited, which enhances the dielectric permittivity.
According to Maxwell-Wegner capacitor model [34-36] the complex impedance can be modeled by an equivalent circuit consisting of resistance and capacitance due to grain and
written as 1 = Z / ( f ) − jZ // ( f ) j 2πfC 0 ε ( f )
Z/(f )=
Z
//
(f )=
Rg
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Z=
1 + (2πfR g C g ) 2πfR g2 C g
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interfacial grain boundary contribution [37-38]. The complex impedance of such a system can be
2
1 + (2πfR g C g ) 2
+
+
R gb
1 + (2πfR gb C gb )
2
2 2πfR gb C gb
1 + (2πfR gb C gb )
2
(14)
(15)
(16)
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Where Rg and Rgb are the grain and grain boundary resistance respectively. Cg and Cgb are the grain and grain boundary capacitance respectively. C0 is the free space capacitance and ߱=2ߨf. The real part of complex impedance for all the samples has been estimated using the relation
[{
ε // ( f )
} {
}]
2πfC 0 ε / ( f ) + ε // ( f )
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Z/(f )=
2
2
(17)
The frequency variation of the real part of impedance at room temperature is plotted in Fig.11 for
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all the samples. It is observed that with the increase of frequency the value of
decreases which
suggests the increase of ac conductivity with frequency. In the Fig.11 the points indicate the experimental data and the theoretical best fit of Eq.(15) are indicated by the solid line. The values of grain boundary resistance and capacitance are calculated from the fitting and shown in the Table 2 for different samples. From the values of extracted parameters it can be concluded that the grain boundary contribution dominates over the grain contribution. It may be due to disordered atomic arrangement in grain boundary region enhancing the electron scattering. The Cole-Cole diagram of Y0.99La0.01CrO3 sample for different temperatures are shown in the Fig.12 where imaginary part of 10
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impedance is plotted with real part of impedance. The semicircles shown in the figure corresponds to the universal dielectric behavior of the sample. We have investigated the dielectric relaxation spectra in terms of dielectric modulus as a function of frequency. The complex dielectric modulus has been defined as [39-40]
Where
and
1
ε
*
(f )
= M / ( f ) + jM // ( f )
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M *( f ) =
(18)
are the real and imaginary part of dielectric modulus respectively and can be
M // ( f ) =
ε /(f )
{ε ( f )} + {ε ( f )} 2
/
//
2
ε // ( f )
{ε ( f )} + {ε ( f )} 2
/
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M /(f )=
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written as
//
2
While studying the variation of the real part of electric modulus
(19)
(20)
, with frequency at different
temperatures no characteristic peak has been observed as shown in the Fig.13(a) for Y0.95La0.05CrO3 sample. It is also valid for the other samples. A loss peak has been observed when with frequency for all the samples. In the Fig.13(b) the variation of
is plotted
with frequency is shown for
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Y0.99La0.01CrO3sample at different temperatures. When the free charges are blocked at the surface of the nanocrystals, due to the dielectric difference between grain and grain boundaries interfacial polarization is produced. In the Fig.13(b) it is clear that the position of the relaxation peaks is
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shifted to the higher frequency side with the increase of the temperature. This is because with the increase of the temperature the free charges accumulate at the interface and due to the increase in the charge carrier mobility the relaxation time decreases. Therefore the loss peak is shifted to the
of
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higher frequency region with the increase of the temperature. The normalized plot with
are shown in the Fig.14(a) for all the samples at 323K fixed
temperature and in the Fig.14(b) it is shown for different temperatures for Y0.99La0.01CrO3 sample. It is observed that the spectra of
for different temperatures merge on a single master
curve. It concludes that all the dynamic process possessing at different temperature have similar activation energy. From the temperature variation of relaxation time (߬) following Arrhenius model the average activation energy(Ear) for the charge carrier hopping corresponding to the relaxation behavior can be estimated. The Arrhenius equation can be written as 11
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E ar k T B
τ = τ 0 exp
(21)
Where τ0 is the pre-exponential factor, Ear is the average activation energy. The values of ߬ [41] corresponding to maxima in Fig.13(b) can be estimated following the relation
Where the maximum value of relaxation time. In the Fig.15
occurs at the frequency
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(22)
and ߬ is the corresponding
is plotted with inverse of temperature (1000/T). In the plot the
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points indicate the experimental data and the theoretical best fit of Eq.(21) is indicated by the solid line. In the plot
verses 1000/T for each sample a single slope is obtained which indicates the
presence of single activation energy ranging from 0.45 eV to 0.35 eV for different samples. It is
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observed that the value of effective barrier height WM is greater than the value of activation energy for all the samples. The lower value of activation energy [42-44] indicates that a strong charge transfer occurs due to the delocalization of charge carriers accumulated on the interface. The value of the pre exponential factor ߬0 is estimated from the fitting using Eq.(21) in the Fig.15. It lies in the range 4.48 x 10-11 s to 5.1x10-12 s.
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4. Conclusions
The La doped YCrO3 nanoparticles have been prepared by sol-gel method. The samples are characterized by XRD, HRTEM, UV-Vis spectrometry and PL spectroscopy. XRD and TEM analysis confirms the formation the nanoparticles. The band gap of the different samples measured
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from the UV-Vis absorption spectra shows that the band gap of the different samples decreases with the increase of the doping concentration and this can be explained by the creation of mid gap states. From the PL analysis it is observed that all the samples shows distinct red light emission in
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the visible range around 630 nm which is attributed to the energy level of Cr+3 ions in YCrO3 corresponding to the
2
Eg–4A2g transition. The dc conductivity of the samples increases with the
increase of temperature showing semiconducting behavior and it also increases with La content following a simple hopping type conduction mechanism. Dc conductivity behavior is explained by adiabatic small polaron hopping model. The frequency exponent S, estimated from ac conductivity data, decreases with the increase of temperature obeying the CBH model. As the relaxation time decreases with the increase of the charge carrier mobility, the relaxation peaks in imaginary part of electric modulus shift to the higher frequency side with the increasing temperature. The variation of 12
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dielectric constant with the frequency and temperature are also studied. Analyzing the real part of impedance it is proved that the grain boundary resistance dominates over the grain resistance. The activation energy of all the samples are estimated from both the dc and ac data and in both cases the value decreases with the increase of the Lanthanum content. The increasing nature of the
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conductivity with the increase of doping concentration enhances the application of La doped YCrO3 as the interconnect material in SOFCs.
Acknowledgments
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Authors are very much thankful to DAE-BRNS, Government of India for financial support during the work. The authors gratefully acknowledge the support getting from the Centre of Excellence,
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TEQIP-II, NIT Durgapur.
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[44] S. Komine, Physica B, 392 (2007) 348.
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Table 1 List of lattice parameters and microstrain obtained from the Rietveld refinement method of Y1-xLaxCrO3 samples.
Y1La0CrO3
Y0.99La0.01CrO3
Y0.95La0.05CrO3
Y0.90La0.10CrO3
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Properties
7.6301
7.6371
7.6487
b (Å)
7.5777
7.5964
7.6145
c (Å)
7.5713
7.5719
7.5727
7.5853
β (Degree)
93.06
92.96
92.88
92.73
Microstrain
1.325 X 10-3
1.066 X 10-3
AC C
EP
TE D
M AN U
1.146 X 10-3
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a (Å)
16
7.6556
7.6175
0.938 X 10-3
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Table 2 Values of relevant physical parameters, conductivity at 320K (ߪ(320K)), band gap
Y0.99La0.01CrO3
Y0.95La0.05CrO3
Y0.90La0.10CrO3
σ(320K)(Ω-1m-1)
9.35 X 10-6
1.54 X 10-4
4.09 X 10-4
7.06 X 10-4
Eg(eV)
3.39
2.92
2.85
2.76
Ea(dc)(eV)
0.44
0.39
0.34
0.30
Ear(ac)(eV)
0.45
0.43
0.38
0.35
WP(eV)
0.99
0.88
0.78
0.70
WH(eV)
0.48
0.42
0.37
0.33
WM(eV)
0.60
0.53
0.50
0.48
γ0(Hz)
1.90 X 1011
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Y1La0CrO3
M AN U
Properties
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energy (Eg), dc activation energy (Ea(dc)), ac activation energy (Ear(ac)), polaron binding energy (WP), hopping energy (WH), effective barrier height (WM), optical phonon frequency (ߛ0),electronic orbital overlap integral (J), critical value of electronic orbital overlap integral (Jmax), grain resistance(Rg), grain boundary resistance(Rgb), grain capacitance (Cg) and grain boundary capacitance (Cgb)
2.30 X 1011
2.20 X 1011
2.40 X 1011
20.00
20.00
20.00
15.00
Jmax(298K) (meV)
10.45
5.13
4.84
4.81
Jmax(523K) (meV)
12.03
5.90
5.57
5.54
Rg(MΩ)
0.82
0.39
0.28
0.20
Rgb(MΩ)
3.59
1.11
0.31
0.26
EP
Cgb(F)
6.80 X 10-11
4.70 X 10-11
4.00 X 10-11
3.60 X 10-11
1.24 X 10-10
1.53 X 10-10
3.84 X 10-10
5.00 X 10-11
AC C
Cg(F)
TE D
J(meV)
17
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FIGURE CAPTION Fig.1 X-ray diffraction pattern (XRD) of YCrO3 and La doped YCrO3 samples.
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Fig.2 (a) The TEM image of Y0.99La0.01CrO3 sample. (b) The High Resolution TEM image of Y0.90La0.10CrO3 sample. (c) The SADE pattern of Y0.99La0.01CrO3 sample consisted of a set of concentric rings. (d) The EDAX image of Y0.90La0.10CrO3 sample. (e) Log normal distribution of Fig.2(a) for Y0.99La0.01CrO3 sample. Fig.3 The UV-vis absorption spectra of the samples. The optical band gap of Y0.99La0.01CrO3 sample is shown in the inset of Fig.3
vs 1000/T for all samples. (b)
is plotted
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Fig.5 (a)The plotting of with 1000/T for all the samples.
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Fig.4 Photoluminescence (PL) spectra of all the experimental samples.
Fig.6 The frequency dependence of ac conductivity for Y0.95La0.05CrO3 sample at different temperatures. Fig.7 The temperature variation of the frequency exponent S for all the samples.
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Fig. 8 The temperature variation of ac conductivity is plotted with different frequencies for (a) pureYCrO3 and (b) 10% La doped samples respectively. Fig.9 Variation of (a) real part ( and (b) imaginary part ( frequency for Y0.99La0.01CrO3 sample at different temperatures.
of dielectric constant with
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Fig.10 Real part of dielectric permittivity for three different frequencies is plotted with temperature forY0.90La0.10CrO3 sample.
AC C
Fig.11 The frequency variation of the real part of impedance at room temperature for all the samples. Fig.12 The Cole-Cole diagram of Y0.99La0.01CrO3 sample for different temperatures. Fig.13 The frequency variation of (a) real part of electric modulus and (b) imaginary part of electric modulus ( ) for Y0.95La0.05CrO3 sample at different temperatures. Fig.14 (a)The normalized plot of temperature. (b)The normalized plot of for Y0.99La0.01CrO3 sample. Fig.15
with with
plotted with inverse of temperature (1000/T).
18
for all the samples at 323K fixed for different temperatures
(242)
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5%La
M AN U 30
AC C
EP
20
40
1%La
0%La 50
60
2θ(Degree) Fig.1
10%La
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(331)
(321) (240) (123)
(200) (121) (002) (210) (031) (220) (022) (131) (202) (040) (230) (212) (311)
(101) (020) (111)
4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0
TE D
Intensity(a.u)
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70
80
AC C
EP
TE D
M AN U
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Fig.2
1.1
8
0%La 1%La 5%La 10%La
6
0.8
4
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0.7
Band gap =2.90 ev
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0.9
1%La
(hυα )2
1.0
2
0.6
0 2
0.5 0.4
3
hυ
4
5
700
800
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Absorbance(a.u)
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0.3
EP
0.2
300
AC C
0.1 200
400
500
600
Wave length (nm) Fig.3
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500
0% La 1% La 5% La 10% La
SC
450
M AN U
350 300 250 200
TE D
Pl Intensity
400
150
50
EP
100
AC C
0 550
575
600
625
650
675
700
Wave Length(nm) Fig.4
725
6
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(b )
4 2
-2 -4 1 .8
2 .0
2 .2
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0% L a 1% L a 5% L a 10% La
0
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3/2
-1
-1
3/2
ln[T σdc(T)/Ω m K ]
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2 .4
2 .6
2 .8
3 .0
3 .2
-1
1 0 0 0 /T ( K )
0 -2 -4 -6
(a )
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0% La 1% La 5% La 10% L a
EP
-1
-1
ln[ Tσ(T)/Ω m K]
4
2 .0
AC C
1 .8
2 .2
2 .4
2 .6 -1
1 0 0 0 /T ( K )
Fig.5
2 .8
3 .0
3 .2
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-6 -7 -8
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-9 -10
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ln[σ (f,T)(Ω m )]
298K 323K 373K 423K 473K 523K
-4
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5% La
-3
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ln[f(Hz)] Fig.6
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0.6
0% La 1% La 5% La 10% La
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SC
0.5
S
0.4
300
AC C
0.1
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0.2
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0.3
350
400
450
T(K) Fig.7
500
550
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(a)
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1 .1 1 .0
100 KH z
0 .9 0 .8 0 .7
500KH z
0 .6 0 .5 0 .4 0 .2 0 .1 0 .0 300
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35 0
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55 0
6 00
T (K )
0.000 300
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1 MHz
0 .3
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/
1 .2
0.006
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1 MHz 500 KHz 100 KHz
0% La
1 .3
tanδ
-1
-1
σ (f,T)(Ω m )
0.008
350
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T(K)
0.05 4.5 4.0 3.5
0.03 tan δ
3.0 2.0 1.5 1.0
1 MHz 500 KHz 100 KHz
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0.02
2.5
TE D
0.04
1 MHz 500 KHz 100 KHz
10% La
0.5
0.01
0.0
300
AC C
/
-1
-1
σ (f,T)(Ω m )
(b)
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450
500
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T(K)
0.00
300
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400
T(K) Fig.8
450
500
550
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21000
(b)
298K 323K 373K 423K 473K 523K
1% La
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18000
//
ε (f)
15000 12000 9000
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6000
0 2
3
10
10
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3000
10
4
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f(Hz)
(a)
3500
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3000
2000
/
ε (f)
2500
AC C
500
298K 323K 373K 423K 473K 523K
EP
1500 1000
1% La
0 2 10
3
10
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f(Hz) Fig.9
10
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2000
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10% La
1800 1600 1400
1000 800 600
200 0
TE D
400
350
AC C
EP
300
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T
M AN U
/
ε (f,T)
1200
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1 MHz 500 KHz 100 KHz
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T
11.4
T
400
450
T(K)
Fig.10
500
550
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2.0x10
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10% La
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4.0x10
5
2.0x10
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4.0x10
5% La
0.0 6 1.2x10 8.0x10
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/
Z (f)(Ω)
0.05 6.0x10
5 5
4.0x10 0.0
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EP
10
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6
1.8x10 6 1.2x10 5 6.0x10 0.0
3
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1% La
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4
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f(Hz)
Fig.11
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45000 1% La
40000
SC
35000
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Z (Ω)
30000
//
323K 373K 423K 473K 523K
25000 20000
10000 5000 0
20000
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40000 /
Z (Ω )
Fig.12
60000
80000
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0.007
1% La
(a)
0.006
0.002 0.001
SC
0.003
/
M (f,T)
0.004
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298K 323K 373K 423K 473K 523K
0.005
-0.001 1 10 0.0025
3
10
f(Hz)
10
5
10
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1% La 298K 323K 373K 423K 473K 523K
0.0015
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TE D
//
M (f,T)
2
(b)
0.0020
0.0010
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0.0005 0.0000
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0.000
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10
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f(Hz) Fig.13
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1.0
(a)
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T = 323K
0% La %1% La %5% La %10% La
0.6 0.4
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M''/M''max
0.8
0.0 -3
-2
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-1
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(b)
0.8
1% La
TE D
1.0
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0.2
298K 323K 373K
EP
M''/M''max
f/fMAX
0.0
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-3
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f/fMAX
Fig.14
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ln[τ(s)]
-8
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0% La 1% La 5% La 10% La
-7
-9 -10 -11
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-14
2.2
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2.0
2.4
2.6
2.8
3.0 3.2 -1
1000/T(K ) Fig.15
3.4
3.6
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Highlights The dc conductivity of the samples increases with increasing La concentration.
•
A linear variation has been observed in S-T curve.
•
Band gap decreases with the increase of La concentration due to creation of mid gap.
•
The activation energies decrease with increasing doping percentage.
•
A relaxation peak has been observed in M // ( f ) − T curve.
AC C
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•