- Email: [email protected]
Impedance, AC conductivity and electric modulus analysis of l -leucine l -leucinium picrate
Accepted Manuscript Impedance, AC conductivity and electric modulus analysis of L-leucine L-leucinium picrate Sameh Guidara, Habib Feki, Younes Abid PII:
S0925-8388(15)31926-5
DOI:
10.1016/j.jallcom.2015.12.135
Reference:
JALCOM 36224
To appear in:
Journal of Alloys and Compounds
Received Date: 17 November 2015 Revised Date:
14 December 2015
Accepted Date: 17 December 2015
Please cite this article as: S. Guidara, H. Feki, Y. Abid, Impedance, AC conductivity and electric modulus analysis of L-leucine L-leucinium picrate, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2015.12.135. 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
AC C
EP
TE D
M AN U
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GRAPHICAL ABSTRACT
1
ACCEPTED MANUSCRIPT
Impedance, AC conductivity and electric modulus analysis of L-leucine L-leucinium picrate Sameh Guidara *, Habib Feki, Younes Abid
*
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Laboratoire de Physique Appliquée (LPA), Université de Sfax. Faculté des Sciences 3000, BP.1171, Sfax, Tunisie. Corresponding author Tel: +216 23865145, Email: [email protected]
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Abstract
In this paper, we report the measurements of impedance spectroscopy for the L-leucine L-
M AN U
leucinium picrate compound synthesized by slow evaporation technique at room temperature in the frequency and temperature ranges of 209Hz – 1MHz and 393 to 433K, respectively. The Nyquist plots exhibited single semi-circular arcs which were well fitted to an equivalent circuit. The frequency dependence of the AC conductivity has been investigated using the Jonscher universal power law: σ ω = σ + A ω . The exponent s remains constant in the investigated temperature range and is almost equal to 0.62. Single relaxation peak is observed
TE D
in the imaginary part of the electrical modulus, suggesting the response of grain. The close values of activation energies obtained from the analysis of hopping frequency, electric modulus and dc conductivity indicate that the transport in the title compound can be described
cations.
EP
through a simple hopping mechanism, dominated probably by the motion of L-leucinium
AC C
Keywords: Impedance spectroscopy, Electrical properties, AC conductivity, hopping mechanism. 1.
Introduction
Recently, there is a surge in interest to develop new organic materials suitable for electronic applications due to low cost and low processing temperature [1,2]. Picric acid is an interesting organic acid because of the presence of three electron withdrawing nitro groups which makes it as a good π acceptor for neutral carrier donor molecule. Moreover, it also acts as an acidic ligand to form salts through specific electrostatic or hydrogen bond interactions. The picrate anions provide the potential for a large range of optical properties such as efficient luminescence [3-5] and second-order non-linear optical activity due to the proton transfer [61
ACCEPTED MANUSCRIPT 8]. Moreover, picric acid complexes play a central role in bioelectrical and biological systems and exhibit good antibacterial and antifungal activities against various bacteria and fungi species [6]. Generally, picric acid derivatives are interesting candidates for the formation of salts with some amino acids which display special features such as molecular chirality, wide transparency in visible region and zwitterionic nature of the molecule such as L-alanine L-
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alaninium picrate monohydrate [9], DL-phenylalanine DL-phenylalaninium picrate [10], DLmethionine DL-methioninium picrate [11] and DL-valine DL-valinium picrate [12]. However, little work has been done on this kind of materials though other various interesting physical properties are expected. Our last published paper has been devoted to the powder X-ray
SC
diffraction and vibrational studies as well as DFT calculations of nonlinear optical properties of the organic compound L-Leucine L-Leucinium Picrate (LLLLP) [13]. Structural studies, characterization on second harmonic generation SHG and thermal analysis of LLLLP were
M AN U
carried out and reported earlier [14,15]. As an extension of our searches for exploring new properties concerning organic salts, we report in this paper, a detailed investigation of the electrical and electric modulus properties of the LLLLP compound using the impedance spectroscopy. This study may give valuable information on the electrical conductivity and can be also important for the improvement of practical applications. Experimental procedure
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2.
Single crystals of LLLLP were grown by the slow evaporation technique. L-leucine and picric acid were dissolved respectively in water and acetone in the ratio 2:1 according to the following reaction:
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2 C6H13NO2 + C6H3N3O7 → C6H13NO2 · [C6H14NO2] +· [C6H2N3O7]– The resulting solution was then kept to evaporate at room temperature to finally lead to
AC C
yellow needle crystals.
The impedance spectroscopy was performed on a pellet of about 8 mm in diameter and 0.8 mm in thickness at frequency range from 209Hz to 1MHz with the TEGAM 3550 impedance analyzer. Measurements were carried out at temperatures from 393 to 433K. 3.
Results and discussion 3.1 Structural data
X-ray powder diffraction was used for the identification of the synthesized LLLLP crystal in our previous work [13]. Structural features at room temperature have been mentioned elsewhere [14]. In summary, the compound crystallizes in the triclinic system with the non 2
ACCEPTED MANUSCRIPT centrosymmetric space group P1 (Z=2) with a = 7.132(5)Å, b = 11.799(9)Å, c = 15.372(2)Å, α = 106.61 (6)°, β = 95.32 (6)° and γ = 90.97 (7)°. From the single crystal XRD data [14], the asymmetric unit of the title compound contains one unprotonated leucine residue, one protonated leucinium cation and one picrate anion. The structure is stabilized by an extensive network of O-H…O and N-H…O hydrogen bonds.
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3.2 Impedance Analysis
The complex impedance spectra of the explored compound for several temperatures are shown in Fig. 1 which present a single semicircular response corresponding to grain interior
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and no grain boundaries or electrode effects are involved in the patterns. Moreover, their centers are below the real axis, which indicates a non-Debye type of relaxation [16]. As temperature increases, the radius of semicircles decreases, indicating an activated thermal
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conduction mechanism in the studied temperature range. In order to study the electric properties of this compound, we have modeled the complex impedance spectra using Zview software and the best fit is obtained when employing an equivalent circuit formed by parallel combination of resistance R, capacitance C1, and fractal capacity CPE1 in series with capacitance C2 as shown in the inset in Fig. 1. The variation of simulated values of Z’ and
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-Z’’ are shown by solid lines in Fig. 1. The impedance of the capacitance and CPE are given, respectively by the relationships:
=
=
EP
(1)
(2)
where Q is a proportional factor, ω is the angular frequency and α is an empirical exponent
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with values between 0 and 1 which indicates the capacitive nature of the element. The obtained parameters for the circuit elements are summarized in Table 1. The angular frequency dependence of the experimental values of Z′ and -Z” for some temperatures with simulated ones using the extracted parameters of the equivalent circuit model are shown in Figs 2 and 3, respectively. The excellent agreement between experimental data and simulated lines indicates that the equivalent circuit describes well the electrical properties of this compound. The amplitude of Z’ decreases with the rise in temperature at low frequency due to the increase in ac conductivity and all curves merge at high-frequency region for all temperatures. This behavior suggests a possible release of space charge and a consequent lowering of the barrier properties in the materials [17]. With the increase of frequency, the 3
ACCEPTED MANUSCRIPT magnitude of -Z” increases at the beginning, reaches a peak (-Z”max) and then decreases with the rise in frequency at all measured temperatures. The broadening of peak and its shift towards higher frequency side with the increase of temperature demonstrate the presence of temperature dependent electrical relaxation phenomenon, and the relaxation time decreases with increasing temperature [18]. At higher frequency side all the curves are merged for all
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temperatures which might be caused by the reduction in space charge polarization at this frequency region. 3.3 Conductivity study 3.3.1 DC conductivity
σ =
×
(3)
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Dc conductivity σdc can be calculated using the values of the extracted circuit parameters as:
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where e, S and R represent, the thickness, the area of the pellet and the resistance, respectively. The temperature dependence of the conductivity Ln(σdc.T) versus (1000/T) in the studied temperature range is given in Fig. 4. The linearity of the obtained experimental points shows firstly that this compound does not have any phase transition in the studied temperature range which confirms the results of DSC measurements carried out by G. Bhagavannarayana et al [15], and secondly the σdc.T exhibits an Arrhenius type behavior
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described by the following expression:
σ T = Bexp −
!"
#
(4)
where B is a pre-exponential factor, kB the Boltzmann’s constant and Ea is the thermal
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activation energy of ion migration. The activation energy value obtained from a linear fit of data points is E = 1.7 eV.
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3.3.2 AC conductivity
The frequency variation of AC conductivity at different temperatures for the title compound is shown in Fig. 5. It is seen that the conductivity pattern can be categorically divided into two parts. At lower frequencies, the ac conductivity remains almost constant which corresponds to D.C. conductivity while it shows dispersion with increasing angular frequency, which is a characteristic of ωs. Angular frequency-dependent conductivity is well described by the Jonscher universal power law [19]:
σ ω = σ + Aω
4
(5)
ACCEPTED MANUSCRIPT where σdc characterizes the direct conductivity occurring at low frequency, A is a factor dependent on temperature and s is an exponent function of temperature (0
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temperature range and are almost in the vicinity of (0.62 ± 0.07). For each temperature, the crossover frequency from dc to the dispersive region of the ac conductivity is characterized by a change in slope at a certain value of frequency which is known as hopping frequency ωh and it can be calculated directly from ac conductivity data using the following formula [20]: ,
#
(6)
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σ*+ /
ω) =
The hopping frequency is temperature dependent and it obeys the Arrhenius equation
#
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ω) = ω. exp −
!"
(7)
Where ω. is the pre-exponential of hopping frequency and E the activation energy for the hopping frequency [20]. The variation of Ln(ωh) with temperature is shown in Fig. 6. The activation energy calculated from linear fit of experimental data is 1.68 eV. It is worthwhile to note that the calculated activation energy determined from D.C. conductivity and that
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estimated from hopping frequency are very close, suggesting that the mobility of the charge carriers is due to a hopping mechanism. In order to determine the ions responsible for the conduction in this compound we refer to the material structure. To have a look into the structure arrangement, we have presented in Fig. 7 the atomic arrangement projection along
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the b axis of the title compound. From the single crystal XRD data, both amino groups of the leucinium residues connect to carboxyl O atoms of symmetry-related leucine residues, thus
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forming infinite chains along the a axis [14]. From the structure disposition and since the leucinium cation is 1.72 times lighter than the picrate anion, we can deduce that the conductivity in this sample is may be assured by the contribution of the movements of cationic parts along the a-axis. Ac conductivity can be expressed by a function with a non-dimensional frequency through Ghosh model [21]: σ + ω σ*+
= F # ω ω
0
(8)
In this scaling model, the ac conductivity axis is scaled by σdc and the frequency axis by ωh, σdc is a parameter obtained from the fit of the conductivity as a function of angular frequency. 5
ACCEPTED MANUSCRIPT Scaling conductivity plots of the LLLLP compound at different temperatures are shown in Fig. 8 such that all normalized curves overlap each other forming a single curve which implies that the relaxation dynamics of charge carriers is independent of temperature. 3.4 Modulus analysis
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Electric modulus formalism is used in order to study the electrical relaxation process and it is suitable to extract the electrode polarization. The complex electric modulus M is defined by the reciprocal of the complex permittivity ε* (M* = 1/ε*) and can be calculated from the
M ∗ = M 3 + jM′′
M 3 = ωC. Z′′
(10) (11)
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M 33 = ωC. Z′
(9)
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following equations:
where M’, M”, Z’ and Z” are the real and imaginary parts of the complex modulus M* and electric impedance Z*, respectively. C0=ε0S/e is the vacuum capacitance of the measuring cell. In order to determine some characteristic parameters of the charge carriers such as their activation energy and relaxation frequency, numerical simulations of the modulus spectra are interesting. We have fitted the imaginary part of the electric modulus for different temperature
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with an approximate frequency representation of the Kohlrausch–Williams–Watts (KWW) function, proposed by Bergman [22]:
9": ;
β β ω ω <β= #@β : ; #= # A >?β ω ω: ;
(12)
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M"ω =
where M”max and ωmax is the peak maximum and peak angular frequency of imaginary part of
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the modulus, respectively. β is the well-known Kohlrausch parameter. This parameter characterizes the degree of non-Debye behavior which decreases with the increase in the relaxation time distribution (0 ≤ β ≤ 1). The variation of the experimental and simulated data of the imaginary part of the modulus complex as a function of frequency at several temperatures is shown in Fig. 9. The M” spectra for the studied compound clearly show one relaxation peak observed for each temperature representing the bulk grain behavior. Moreover, the peak corresponding to M”max shifts to higher frequencies with elevation of temperature, which indicates a thermally activated dielectric relaxation process in which the hopping mechanism of charge carriers dominates intrinsically [23]. The relaxation frequency fp corresponds to M”max expressed as:
6
ACCEPTED MANUSCRIPT BC =
ωDEF GH
(13)
The variation of the frequency fp with temperature follows the Arrhenius relation and the calculated activation energy from a linear fit to the data points is 1.69 eV (Fig. 6). This result is in good agreement with the activation energies determined from σ . T and ω) . This close resemblance indicates that the mobility of the charge carriers in the title compound can be
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described through a simple hopping mechanism, dominated by the motion of leucinium cations along the “a” direction. Fig. 10 shows the normalized plots of M”/M”max at different temperature for LLLLP compound. All the peaks merge into one master curve and almost perfectly overlap at different temperature. This result is in good agreement with the β value
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which remains constant at different temperature (Fig. 10 inset). It suggests that the distribution of the relaxation time is independent of temperature [24]. At high frequency, the
M AN U
variation of M”/M”max vs log(f/fmax) represents the range of frequencies in which the charge carriers are confined to their potential wells, and therefore they can make only localized motions inside the well. While, at a frequency range below M”max peak, the charge carriers are mobile over long distances. The peak indicates the transition from the long-range to the shortrange mobility with increasing frequency [25]. Fig. 11 shows the imaginary part of the electrical modulus M" as a function of the real part M′ at different temperatures. In the studied
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frequency range, one relaxation peak is observed which is attributed to the grain effect. The well superposition of the curves at several temperatures shows that the Kohlrausch parameter β is temperature independent which is in concordance with the previous result and indicating that the temperature does not affect the relaxation dynamics. The combined plot of M” and (-
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Z”) versus frequency can distinguish whether the short range or long range movement of charge carries is dominant in a relaxation process [24]. Thus, Fig. 12 presents the plots of
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M”/M”max and –Z” at T= 415K. This figure unambiguously illustrates that the peak maximum of the two curves do not occur in the same frequency indicating that the relaxation process is dominated by the short range movement of charge carriers and the departure from the ideal Debye type behavior [26]. Conclusions: In summary, the electric and dielectric properties of L-leucine L-leucinium picrate compound were investigated as a function of frequency (209Hz - 1MHz) and temperature (393 - 433K). The complex impedance plots have revealed the presence of only one semicircular arc ascribed to grain effect which is well fitted to an equivalent circuit model. The ac conductivity spectra are found to obey the Jonscher’s universal power law at different temperatures. 7
ACCEPTED MANUSCRIPT Moreover, the superposition of scaling conductivity spectra into one master curve using the Ghosh model at several temperatures implies that the relaxation dynamics of charge carriers is independent of temperature. The dielectric data have been analyzed in modulus formalism indicating the presence of a single relaxation peak. The variation of the bulk relaxation frequency fp with temperature is governed by a typical Arrhenius behavior showing one
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straight line region with calculated activation energy E = 1.69 eV. Since the activation energy determined from the electric modulus study was in concordance with those obtained from conductivity, we can conclude that the transport is due to a hopping mechanism and probably
AC C
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dominated by the motion of cationic parts “L-Leucinium” along the “a” direction.
8
ACCEPTED MANUSCRIPT References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99–117. [2] B.J. Leever, C.A. Bailey, T.J. Marks, M.C. Hersam, M. Durstock, Adv. Energy Mater. 2 (2012) 120–128.
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[3] M. Obulichetty, D. Saravanabharathi, Spectrochim. Acta A 118 (2014) 861–866. [4] J. Jayabharathi, V. Thanikachalam, M. Padmavathy, N. Srinivasan, Spectrochim. Acta A 81 (2011) 380–389.
[5] S. Anandhi, T.S. Shyju, R. Gopalakrishnan, J. Cryst. Growth 312 (2010) 3292–3299.
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[6] T. Dhanabal, G. Amirthaganesan, M. Dhandapani, Samar K. Das, J. Mol. Struct. 1035 (2013) 483–492.
[7] S.A. Martin Britto Dhas, G. Bhagavannarayana, S. Natarajan, J. Cryst. Growth 310 (2008)
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3535– 3539.
[8] A. Antony Joseph, I. John David Ebenezar, C. Ramachandra Raja, Optik 123 (2012) 1436–1439.
[9] V.V. Ghazaryan, M. Fleck, A.M. Petrosyan, J. Mol. Struct. 1015 (2012) 51–55. [10] K. Anitha, R.K. Rajaram, Acta Crystallogr. E61 (2005) o589–o591.
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[11] K. Anitha, S. Athimoolam, R.K. Rajaram, Acta Crystallogr. E62 (2006) o8–o10. [12] K. Anitha, S. Annavenus, B. Sridhar, R.K. Rajaram, Acta Crystallogr. E60 (2004) o1722–o1724.
[13] S. Guidara, H. Feki, Y Abid, Spectrochim. Acta A 115 (2013) 437–444.
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[14] K. Anitha, S. Athimoolam, R. K. Rajaram, Acta Crystallogr. E61 (2005) o1604–o1606. [15] G. Bhagavannarayana, B. Riscob, Mohd. Shakir, Mater. Chem. Phys. 126 (2011) 20–23. [16] J. R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987 (Chapter 4).
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[17] A. Kumar, B.P. Singh, R.N.P. Choudhary, Awalendra K. Thakur, Mater. Chem. Phys. 99 (2006) 150–159.
[18] M. Ram, Solid State Sci. 12 (2010) 350–354. [19] A.K. Jonscher, Nature 267 (1977) 673–679. [20] D.P. Almond , A.R. West, Solid State Ionics 9 & 10 (1983) 277–282. [21] A. Ghosh, A. Pan, Phys. Rev. Lett. 84 (2000) 2188–2190. [22] R. Bergman, J. Appl. Phys. 88 (2000) 1356–1365. [23] B. Behera, P. Nayak, R.N.P. Choudhary, Mater. Res. Bull. 43 (2008) 401–410. [24] L. Ying, Y. Haibo, L. Miao, Z. Ge, Mater. Res. Bull. 51 (2014) 44–48.
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ACCEPTED MANUSCRIPT [25] P.S. Das, P.K. Chakraborty, B. Behera, R.N.P. Choudhary, Physica B 395 (2007) 98– 103.
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[26] M. Pant, D. K. Kanchan, N. Gondaliya, Mater. Chem. Phys. 115 (2009) 98–104.
10
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Table 1: The equivalent circuit parameters for the LLLLP
C1(pF)
Q(nF)
α
C2(pF)
151.00 85.00 50.40 39.60 26.40 19.50 13.00 10.70 6.23 4.32 2.73 2.14
2.51 2.51 2.52 2.52 2.53 2.54 2.57 2.57 2.61 2.60 2.68 2.68
0.20 0.27 0.44 0.43 0.56 0.72 1.07 1.19 1.87 2.56 4.21 4.51
0.40 0.40 0.39 0.40 0.40 0.40 0.38 0.39 0.37 0.37 0.34 0.34
1.97 2.80 2.51 3.02 3.26 3.22 3.22 3.34 3.39 3.46 3.57 3.65
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R (MΩ)
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T(K) 393 398 403 406 408 411 413 415 418 423 428 433
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Fig. 1: The Nyquist plots for LLLLP at different temperatures.
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AC C
EP
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Fig. 2: The angular frequency dependence of Z’ at some measurement temperatures of LLLLP.
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AC C
EP
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Fig. 3: The angular frequency dependence of -Z” at some measurement temperatures of LLLLP.
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Fig. 4: Temperature dependence of Ln (σdc.T) versus reciprocal temperature of LLLLP.
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Fig. 5: Angular frequency dependence of the AC conductivity at different temperatures of
AC C
EP
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LLLLP.
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AC C
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Fig.6: Temperature dependence of the relaxation and hopping frequency of LLLLP.
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Fig.7: Projection along the b axis of the atomic arrangement of LLLLP [14].
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AC C
EP
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Fig. 8: Plot of (σac/σdc) versus (ω/ωh) at different temperatures of LLLLP.
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Fig. 9: Angular frequency dependence of the imaginary part of electric modulus at several temperatures of LLLLP.
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Fig. 10: Modulus scaling behavior of M”/M”max versus Log(f/fmax) of LLLLP compound.
AC C
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(inset) Temperature dependence of the Kohlrausch parameter β.
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Fig.11: Variation of M″ as a function of the real part M′ of the electrical modulus of LLLLP.
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Fig. 12: The M”/M”max and (-Z”) spectra at T= 415 K of LLLLP.
ACCEPTED MANUSCRIPT Highlights ► The electrical properties were studied using the impedance measurements. ► The ac conductivity of L-Leucine L-leucinium picrate follows the Jonscher’s law.
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► The mobility of the charge carriers is due to a hopping mechanism.
S0925-8388(15)31926-5
DOI:
10.1016/j.jallcom.2015.12.135
Reference:
JALCOM 36224
To appear in:
Journal of Alloys and Compounds
Received Date: 17 November 2015 Revised Date:
14 December 2015
Accepted Date: 17 December 2015
Please cite this article as: S. Guidara, H. Feki, Y. Abid, Impedance, AC conductivity and electric modulus analysis of L-leucine L-leucinium picrate, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2015.12.135. 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
AC C
EP
TE D
M AN U
SC
RI PT
GRAPHICAL ABSTRACT
1
ACCEPTED MANUSCRIPT
Impedance, AC conductivity and electric modulus analysis of L-leucine L-leucinium picrate Sameh Guidara *, Habib Feki, Younes Abid
*
RI PT
Laboratoire de Physique Appliquée (LPA), Université de Sfax. Faculté des Sciences 3000, BP.1171, Sfax, Tunisie. Corresponding author Tel: +216 23865145, Email: [email protected]
SC
Abstract
In this paper, we report the measurements of impedance spectroscopy for the L-leucine L-
M AN U
leucinium picrate compound synthesized by slow evaporation technique at room temperature in the frequency and temperature ranges of 209Hz – 1MHz and 393 to 433K, respectively. The Nyquist plots exhibited single semi-circular arcs which were well fitted to an equivalent circuit. The frequency dependence of the AC conductivity has been investigated using the Jonscher universal power law: σ ω = σ + A ω . The exponent s remains constant in the investigated temperature range and is almost equal to 0.62. Single relaxation peak is observed
TE D
in the imaginary part of the electrical modulus, suggesting the response of grain. The close values of activation energies obtained from the analysis of hopping frequency, electric modulus and dc conductivity indicate that the transport in the title compound can be described
cations.
EP
through a simple hopping mechanism, dominated probably by the motion of L-leucinium
AC C
Keywords: Impedance spectroscopy, Electrical properties, AC conductivity, hopping mechanism. 1.
Introduction
Recently, there is a surge in interest to develop new organic materials suitable for electronic applications due to low cost and low processing temperature [1,2]. Picric acid is an interesting organic acid because of the presence of three electron withdrawing nitro groups which makes it as a good π acceptor for neutral carrier donor molecule. Moreover, it also acts as an acidic ligand to form salts through specific electrostatic or hydrogen bond interactions. The picrate anions provide the potential for a large range of optical properties such as efficient luminescence [3-5] and second-order non-linear optical activity due to the proton transfer [61
ACCEPTED MANUSCRIPT 8]. Moreover, picric acid complexes play a central role in bioelectrical and biological systems and exhibit good antibacterial and antifungal activities against various bacteria and fungi species [6]. Generally, picric acid derivatives are interesting candidates for the formation of salts with some amino acids which display special features such as molecular chirality, wide transparency in visible region and zwitterionic nature of the molecule such as L-alanine L-
RI PT
alaninium picrate monohydrate [9], DL-phenylalanine DL-phenylalaninium picrate [10], DLmethionine DL-methioninium picrate [11] and DL-valine DL-valinium picrate [12]. However, little work has been done on this kind of materials though other various interesting physical properties are expected. Our last published paper has been devoted to the powder X-ray
SC
diffraction and vibrational studies as well as DFT calculations of nonlinear optical properties of the organic compound L-Leucine L-Leucinium Picrate (LLLLP) [13]. Structural studies, characterization on second harmonic generation SHG and thermal analysis of LLLLP were
M AN U
carried out and reported earlier [14,15]. As an extension of our searches for exploring new properties concerning organic salts, we report in this paper, a detailed investigation of the electrical and electric modulus properties of the LLLLP compound using the impedance spectroscopy. This study may give valuable information on the electrical conductivity and can be also important for the improvement of practical applications. Experimental procedure
TE D
2.
Single crystals of LLLLP were grown by the slow evaporation technique. L-leucine and picric acid were dissolved respectively in water and acetone in the ratio 2:1 according to the following reaction:
EP
2 C6H13NO2 + C6H3N3O7 → C6H13NO2 · [C6H14NO2] +· [C6H2N3O7]– The resulting solution was then kept to evaporate at room temperature to finally lead to
AC C
yellow needle crystals.
The impedance spectroscopy was performed on a pellet of about 8 mm in diameter and 0.8 mm in thickness at frequency range from 209Hz to 1MHz with the TEGAM 3550 impedance analyzer. Measurements were carried out at temperatures from 393 to 433K. 3.
Results and discussion 3.1 Structural data
X-ray powder diffraction was used for the identification of the synthesized LLLLP crystal in our previous work [13]. Structural features at room temperature have been mentioned elsewhere [14]. In summary, the compound crystallizes in the triclinic system with the non 2
ACCEPTED MANUSCRIPT centrosymmetric space group P1 (Z=2) with a = 7.132(5)Å, b = 11.799(9)Å, c = 15.372(2)Å, α = 106.61 (6)°, β = 95.32 (6)° and γ = 90.97 (7)°. From the single crystal XRD data [14], the asymmetric unit of the title compound contains one unprotonated leucine residue, one protonated leucinium cation and one picrate anion. The structure is stabilized by an extensive network of O-H…O and N-H…O hydrogen bonds.
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3.2 Impedance Analysis
The complex impedance spectra of the explored compound for several temperatures are shown in Fig. 1 which present a single semicircular response corresponding to grain interior
SC
and no grain boundaries or electrode effects are involved in the patterns. Moreover, their centers are below the real axis, which indicates a non-Debye type of relaxation [16]. As temperature increases, the radius of semicircles decreases, indicating an activated thermal
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conduction mechanism in the studied temperature range. In order to study the electric properties of this compound, we have modeled the complex impedance spectra using Zview software and the best fit is obtained when employing an equivalent circuit formed by parallel combination of resistance R, capacitance C1, and fractal capacity CPE1 in series with capacitance C2 as shown in the inset in Fig. 1. The variation of simulated values of Z’ and
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-Z’’ are shown by solid lines in Fig. 1. The impedance of the capacitance and CPE are given, respectively by the relationships:
=
=
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(1)
(2)
where Q is a proportional factor, ω is the angular frequency and α is an empirical exponent
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with values between 0 and 1 which indicates the capacitive nature of the element. The obtained parameters for the circuit elements are summarized in Table 1. The angular frequency dependence of the experimental values of Z′ and -Z” for some temperatures with simulated ones using the extracted parameters of the equivalent circuit model are shown in Figs 2 and 3, respectively. The excellent agreement between experimental data and simulated lines indicates that the equivalent circuit describes well the electrical properties of this compound. The amplitude of Z’ decreases with the rise in temperature at low frequency due to the increase in ac conductivity and all curves merge at high-frequency region for all temperatures. This behavior suggests a possible release of space charge and a consequent lowering of the barrier properties in the materials [17]. With the increase of frequency, the 3
ACCEPTED MANUSCRIPT magnitude of -Z” increases at the beginning, reaches a peak (-Z”max) and then decreases with the rise in frequency at all measured temperatures. The broadening of peak and its shift towards higher frequency side with the increase of temperature demonstrate the presence of temperature dependent electrical relaxation phenomenon, and the relaxation time decreases with increasing temperature [18]. At higher frequency side all the curves are merged for all
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temperatures which might be caused by the reduction in space charge polarization at this frequency region. 3.3 Conductivity study 3.3.1 DC conductivity
σ =
×
(3)
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Dc conductivity σdc can be calculated using the values of the extracted circuit parameters as:
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where e, S and R represent, the thickness, the area of the pellet and the resistance, respectively. The temperature dependence of the conductivity Ln(σdc.T) versus (1000/T) in the studied temperature range is given in Fig. 4. The linearity of the obtained experimental points shows firstly that this compound does not have any phase transition in the studied temperature range which confirms the results of DSC measurements carried out by G. Bhagavannarayana et al [15], and secondly the σdc.T exhibits an Arrhenius type behavior
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described by the following expression:
σ T = Bexp −
!"
#
(4)
where B is a pre-exponential factor, kB the Boltzmann’s constant and Ea is the thermal
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activation energy of ion migration. The activation energy value obtained from a linear fit of data points is E = 1.7 eV.
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3.3.2 AC conductivity
The frequency variation of AC conductivity at different temperatures for the title compound is shown in Fig. 5. It is seen that the conductivity pattern can be categorically divided into two parts. At lower frequencies, the ac conductivity remains almost constant which corresponds to D.C. conductivity while it shows dispersion with increasing angular frequency, which is a characteristic of ωs. Angular frequency-dependent conductivity is well described by the Jonscher universal power law [19]:
σ ω = σ + Aω
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(5)
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temperature range and are almost in the vicinity of (0.62 ± 0.07). For each temperature, the crossover frequency from dc to the dispersive region of the ac conductivity is characterized by a change in slope at a certain value of frequency which is known as hopping frequency ωh and it can be calculated directly from ac conductivity data using the following formula [20]: ,
#
(6)
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σ*+ /
ω) =
The hopping frequency is temperature dependent and it obeys the Arrhenius equation
#
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ω) = ω. exp −
!"
(7)
Where ω. is the pre-exponential of hopping frequency and E the activation energy for the hopping frequency [20]. The variation of Ln(ωh) with temperature is shown in Fig. 6. The activation energy calculated from linear fit of experimental data is 1.68 eV. It is worthwhile to note that the calculated activation energy determined from D.C. conductivity and that
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estimated from hopping frequency are very close, suggesting that the mobility of the charge carriers is due to a hopping mechanism. In order to determine the ions responsible for the conduction in this compound we refer to the material structure. To have a look into the structure arrangement, we have presented in Fig. 7 the atomic arrangement projection along
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the b axis of the title compound. From the single crystal XRD data, both amino groups of the leucinium residues connect to carboxyl O atoms of symmetry-related leucine residues, thus
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forming infinite chains along the a axis [14]. From the structure disposition and since the leucinium cation is 1.72 times lighter than the picrate anion, we can deduce that the conductivity in this sample is may be assured by the contribution of the movements of cationic parts along the a-axis. Ac conductivity can be expressed by a function with a non-dimensional frequency through Ghosh model [21]: σ + ω σ*+
= F # ω ω
0
(8)
In this scaling model, the ac conductivity axis is scaled by σdc and the frequency axis by ωh, σdc is a parameter obtained from the fit of the conductivity as a function of angular frequency. 5
ACCEPTED MANUSCRIPT Scaling conductivity plots of the LLLLP compound at different temperatures are shown in Fig. 8 such that all normalized curves overlap each other forming a single curve which implies that the relaxation dynamics of charge carriers is independent of temperature. 3.4 Modulus analysis
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Electric modulus formalism is used in order to study the electrical relaxation process and it is suitable to extract the electrode polarization. The complex electric modulus M is defined by the reciprocal of the complex permittivity ε* (M* = 1/ε*) and can be calculated from the
M ∗ = M 3 + jM′′
M 3 = ωC. Z′′
(10) (11)
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M 33 = ωC. Z′
(9)
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following equations:
where M’, M”, Z’ and Z” are the real and imaginary parts of the complex modulus M* and electric impedance Z*, respectively. C0=ε0S/e is the vacuum capacitance of the measuring cell. In order to determine some characteristic parameters of the charge carriers such as their activation energy and relaxation frequency, numerical simulations of the modulus spectra are interesting. We have fitted the imaginary part of the electric modulus for different temperature
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with an approximate frequency representation of the Kohlrausch–Williams–Watts (KWW) function, proposed by Bergman [22]:
9": ;
β β ω ω <β= #@β : ; #= # A >?β ω ω: ;
(12)
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M"ω =
where M”max and ωmax is the peak maximum and peak angular frequency of imaginary part of
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the modulus, respectively. β is the well-known Kohlrausch parameter. This parameter characterizes the degree of non-Debye behavior which decreases with the increase in the relaxation time distribution (0 ≤ β ≤ 1). The variation of the experimental and simulated data of the imaginary part of the modulus complex as a function of frequency at several temperatures is shown in Fig. 9. The M” spectra for the studied compound clearly show one relaxation peak observed for each temperature representing the bulk grain behavior. Moreover, the peak corresponding to M”max shifts to higher frequencies with elevation of temperature, which indicates a thermally activated dielectric relaxation process in which the hopping mechanism of charge carriers dominates intrinsically [23]. The relaxation frequency fp corresponds to M”max expressed as:
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ωDEF GH
(13)
The variation of the frequency fp with temperature follows the Arrhenius relation and the calculated activation energy from a linear fit to the data points is 1.69 eV (Fig. 6). This result is in good agreement with the activation energies determined from σ . T and ω) . This close resemblance indicates that the mobility of the charge carriers in the title compound can be
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described through a simple hopping mechanism, dominated by the motion of leucinium cations along the “a” direction. Fig. 10 shows the normalized plots of M”/M”max at different temperature for LLLLP compound. All the peaks merge into one master curve and almost perfectly overlap at different temperature. This result is in good agreement with the β value
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which remains constant at different temperature (Fig. 10 inset). It suggests that the distribution of the relaxation time is independent of temperature [24]. At high frequency, the
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variation of M”/M”max vs log(f/fmax) represents the range of frequencies in which the charge carriers are confined to their potential wells, and therefore they can make only localized motions inside the well. While, at a frequency range below M”max peak, the charge carriers are mobile over long distances. The peak indicates the transition from the long-range to the shortrange mobility with increasing frequency [25]. Fig. 11 shows the imaginary part of the electrical modulus M" as a function of the real part M′ at different temperatures. In the studied
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frequency range, one relaxation peak is observed which is attributed to the grain effect. The well superposition of the curves at several temperatures shows that the Kohlrausch parameter β is temperature independent which is in concordance with the previous result and indicating that the temperature does not affect the relaxation dynamics. The combined plot of M” and (-
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Z”) versus frequency can distinguish whether the short range or long range movement of charge carries is dominant in a relaxation process [24]. Thus, Fig. 12 presents the plots of
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M”/M”max and –Z” at T= 415K. This figure unambiguously illustrates that the peak maximum of the two curves do not occur in the same frequency indicating that the relaxation process is dominated by the short range movement of charge carriers and the departure from the ideal Debye type behavior [26]. Conclusions: In summary, the electric and dielectric properties of L-leucine L-leucinium picrate compound were investigated as a function of frequency (209Hz - 1MHz) and temperature (393 - 433K). The complex impedance plots have revealed the presence of only one semicircular arc ascribed to grain effect which is well fitted to an equivalent circuit model. The ac conductivity spectra are found to obey the Jonscher’s universal power law at different temperatures. 7
ACCEPTED MANUSCRIPT Moreover, the superposition of scaling conductivity spectra into one master curve using the Ghosh model at several temperatures implies that the relaxation dynamics of charge carriers is independent of temperature. The dielectric data have been analyzed in modulus formalism indicating the presence of a single relaxation peak. The variation of the bulk relaxation frequency fp with temperature is governed by a typical Arrhenius behavior showing one
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straight line region with calculated activation energy E = 1.69 eV. Since the activation energy determined from the electric modulus study was in concordance with those obtained from conductivity, we can conclude that the transport is due to a hopping mechanism and probably
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dominated by the motion of cationic parts “L-Leucinium” along the “a” direction.
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ACCEPTED MANUSCRIPT References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99–117. [2] B.J. Leever, C.A. Bailey, T.J. Marks, M.C. Hersam, M. Durstock, Adv. Energy Mater. 2 (2012) 120–128.
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[3] M. Obulichetty, D. Saravanabharathi, Spectrochim. Acta A 118 (2014) 861–866. [4] J. Jayabharathi, V. Thanikachalam, M. Padmavathy, N. Srinivasan, Spectrochim. Acta A 81 (2011) 380–389.
[5] S. Anandhi, T.S. Shyju, R. Gopalakrishnan, J. Cryst. Growth 312 (2010) 3292–3299.
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[6] T. Dhanabal, G. Amirthaganesan, M. Dhandapani, Samar K. Das, J. Mol. Struct. 1035 (2013) 483–492.
[7] S.A. Martin Britto Dhas, G. Bhagavannarayana, S. Natarajan, J. Cryst. Growth 310 (2008)
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3535– 3539.
[8] A. Antony Joseph, I. John David Ebenezar, C. Ramachandra Raja, Optik 123 (2012) 1436–1439.
[9] V.V. Ghazaryan, M. Fleck, A.M. Petrosyan, J. Mol. Struct. 1015 (2012) 51–55. [10] K. Anitha, R.K. Rajaram, Acta Crystallogr. E61 (2005) o589–o591.
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[11] K. Anitha, S. Athimoolam, R.K. Rajaram, Acta Crystallogr. E62 (2006) o8–o10. [12] K. Anitha, S. Annavenus, B. Sridhar, R.K. Rajaram, Acta Crystallogr. E60 (2004) o1722–o1724.
[13] S. Guidara, H. Feki, Y Abid, Spectrochim. Acta A 115 (2013) 437–444.
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[14] K. Anitha, S. Athimoolam, R. K. Rajaram, Acta Crystallogr. E61 (2005) o1604–o1606. [15] G. Bhagavannarayana, B. Riscob, Mohd. Shakir, Mater. Chem. Phys. 126 (2011) 20–23. [16] J. R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987 (Chapter 4).
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[17] A. Kumar, B.P. Singh, R.N.P. Choudhary, Awalendra K. Thakur, Mater. Chem. Phys. 99 (2006) 150–159.
[18] M. Ram, Solid State Sci. 12 (2010) 350–354. [19] A.K. Jonscher, Nature 267 (1977) 673–679. [20] D.P. Almond , A.R. West, Solid State Ionics 9 & 10 (1983) 277–282. [21] A. Ghosh, A. Pan, Phys. Rev. Lett. 84 (2000) 2188–2190. [22] R. Bergman, J. Appl. Phys. 88 (2000) 1356–1365. [23] B. Behera, P. Nayak, R.N.P. Choudhary, Mater. Res. Bull. 43 (2008) 401–410. [24] L. Ying, Y. Haibo, L. Miao, Z. Ge, Mater. Res. Bull. 51 (2014) 44–48.
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ACCEPTED MANUSCRIPT [25] P.S. Das, P.K. Chakraborty, B. Behera, R.N.P. Choudhary, Physica B 395 (2007) 98– 103.
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[26] M. Pant, D. K. Kanchan, N. Gondaliya, Mater. Chem. Phys. 115 (2009) 98–104.
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Table 1: The equivalent circuit parameters for the LLLLP
C1(pF)
Q(nF)
α
C2(pF)
151.00 85.00 50.40 39.60 26.40 19.50 13.00 10.70 6.23 4.32 2.73 2.14
2.51 2.51 2.52 2.52 2.53 2.54 2.57 2.57 2.61 2.60 2.68 2.68
0.20 0.27 0.44 0.43 0.56 0.72 1.07 1.19 1.87 2.56 4.21 4.51
0.40 0.40 0.39 0.40 0.40 0.40 0.38 0.39 0.37 0.37 0.34 0.34
1.97 2.80 2.51 3.02 3.26 3.22 3.22 3.34 3.39 3.46 3.57 3.65
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R (MΩ)
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T(K) 393 398 403 406 408 411 413 415 418 423 428 433
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Fig. 1: The Nyquist plots for LLLLP at different temperatures.
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Fig. 2: The angular frequency dependence of Z’ at some measurement temperatures of LLLLP.
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Fig. 3: The angular frequency dependence of -Z” at some measurement temperatures of LLLLP.
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Fig. 4: Temperature dependence of Ln (σdc.T) versus reciprocal temperature of LLLLP.
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Fig. 5: Angular frequency dependence of the AC conductivity at different temperatures of
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LLLLP.
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Fig.6: Temperature dependence of the relaxation and hopping frequency of LLLLP.
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Fig.7: Projection along the b axis of the atomic arrangement of LLLLP [14].
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Fig. 8: Plot of (σac/σdc) versus (ω/ωh) at different temperatures of LLLLP.
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Fig. 9: Angular frequency dependence of the imaginary part of electric modulus at several temperatures of LLLLP.
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Fig. 10: Modulus scaling behavior of M”/M”max versus Log(f/fmax) of LLLLP compound.
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(inset) Temperature dependence of the Kohlrausch parameter β.
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Fig.11: Variation of M″ as a function of the real part M′ of the electrical modulus of LLLLP.
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Fig. 12: The M”/M”max and (-Z”) spectra at T= 415 K of LLLLP.
ACCEPTED MANUSCRIPT Highlights ► The electrical properties were studied using the impedance measurements. ► The ac conductivity of L-Leucine L-leucinium picrate follows the Jonscher’s law.
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► The mobility of the charge carriers is due to a hopping mechanism.