Electrical characterization and vibrational spectroscopic investigations of order-disorder phase transitions in [N(C3H7)4]2CoCl4 compound

Accepted Manuscript Electrical characterization and vibrational spectroscopic investigations of orderdisorder phase transitions in [N(C3H7)4]2CoCl4 compound N. Moutia, M. Ben Gzaiel, A. Oueslati, K. Khirouni PII:

S0022-2860(17)30009-1

DOI:

10.1016/j.molstruc.2017.01.009

Reference:

MOLSTR 23314

To appear in:

Journal of Molecular Structure

Received Date: 30 November 2016 Revised Date:

31 December 2016

Accepted Date: 2 January 2017

Please cite this article as: N. Moutia, M. Ben Gzaiel, A. Oueslati, K. Khirouni, Electrical characterization and vibrational spectroscopic investigations of order-disorder phase transitions in [N(C3H7)4]2CoCl4 compound, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.01.009. 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 Graphical Abstract

358K 363K 368K

373K 378K

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333K 338K 343K 348K

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ACCEPTED MANUSCRIPT

Electrical characterization and vibrational spectroscopic investigations of order-disorder phase transitions in [N(C3H7)4]2CoCl4 compound N. Moutia a, M. Ben Gzaiel b, A. Oueslati , b K. Khirouni a a

Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à l’Environnement, Faculté des

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Sciences de Gabès cité Erriadh, 6079 Gabès, Tunisia b

University of Sfax, Condensed Matter Laboratory, Faculty of Sciences BP. 1171, 3000 Sfax, Tunisia *

: Corresponding author e-mail address: [email protected]

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Abstract

The present paper accounts for the vibrational spectroscopy and electrical characterization of

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a bis-tetrapropylammonium tetrachlorocobaltate grown at room temperature by slow evaporation of aqueous solution. The Raman spectra were studied in the range of 50-3500 cm1

as a function of temperature of 318 K to 421 K. The most important changes are observed

for the band at 1032 cm-1 associated to δ(C− C− C) + t(CH 2 ) + ω(CH 2 ) . A detail analysis of

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the frequency and half-width is quantitatively described in term of an order-disorder model allowed to obtain information relative to the thermal coefficient and activation energy. The decrease of the activation energy with increasing temperature has been interpreted in term of a

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change in the re-orientation motion of the cationic parts [N(C3H7)4]+. Besides, the impedance measurements indicate that the electrical properties are strongly temperature dependent.

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Nyquist plots (-Z’’versus Z’) show that the conductivity behavior is accurately represented by an equivalent circuit models which consists of a series combination of grains interior and grains boundary. The conductivity follows the Arrhenius relation with different activation energies and conduction mechanisms: three temperature regions with activation energies EaI = 0.78 eV and EaII = 0.81 eV and EaIII = 0.93 eV. Furthermore, the modulus plots can be characterized by full width at half height or in term of a non-experiential decay function φ(t) = exp(

−1 β ) . τ

ACCEPTED MANUSCRIPT Keywords: Bis-tetrapropylammoniumtetrachlorocobaltate, Phase transition, Raman spectroscopy; Electrical characterization I. Introduction

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In recent years, much attention has been paid to a novel class of materials, namely organic– inorganic hybrid materials. This arrangement gives the opportunity of gathering the properties of organic and inorganic compounds at the molecular level. The class of these hybrid compounds is very wide and features a large set of various structures, properties, and

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applications [1]. In particular, the organic-inorganic compounds of the general formula

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A2BX4, where A is the tetrapropylammonium (TPA) group, B a divalent metal ion and X a halogen, has received much attention since they are the most interesting hybrid family [1] due to the various physical properties they provide such as nonlinear optical magnetic [2, 3] and ferroelectric [4]. Most of these materials exhibit multiple phase transitions attributed to the reorientational dynamics of the substituted ammonium group. Such a mechanism of structural

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phase transitions was classified as “order-disorder” [5-9]. In an attempt to study this class of compounds, a hybrid compound with the formula [N(C3H7)4]2CoCl4 has been synthesized.

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This compound is crystallized in the centrosymmetric monoclinic system with P21/C space group; the unit cell parameters are a=15.241(4) Å, b= 15.155(4) Å, c=15.804(4) Å and

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β=113.931(11)°. The atomic arrangement can be described by an alternation of organic and organic-inorganic layers parallel to (001) plan. The organic part is made up by (C3H7)4N+(1) and the organic-inorganic part is made up by (C3H7)4N+(2) cation and CoCl4 anion (Fig.1) [10]. The aim of this paper is to study the Raman scattering and electric propriety in order to investigate the phase transition for this compound.

ACCEPTED MANUSCRIPT II. Experimental II.1. Synthesis The single crystal of [N(C3H7)4)]2CoCl4 have been grown using CoCl2 (purity 98%; FLUKA) and [N(C3H7)4]Cl (purity 97%; FLUKA) which were dissolved in a HCl (1M)

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aqueous solution in a molar ratio of 1:1. At room temperature and after a few days, the crystals were obtained by slow evaporation. The single crystal was selected by using the microscope. After that it has been washed by absolute ethanol and dried in vacuum

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desiccators two days before the measurements II.2. Raman measurements

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Raman scattering spectra was recorded using a Horiba-Jobin-Yvon T64000 Raman spectrometer in the frequency range 50-3500 cm-1 ), using the 514.5 nm radiation of an Ar/Kr laser as excitation. All measurements were done under microscope (X50 objective with long working distance) in backscattering geometry on transparent single crystal using the parallel

stage up to 421 K.

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polarization. The studies as a function of temperature were performed in a Linkam heating The wavenumbers and widths of the Raman lines were determined by

fitting using the LabSpec5 software with a combined Lorentzian–Gaussian band shape.

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II.3. Electrical measurements

The electrical measurements were performed using a two-electrode configuration on

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polycrystalline sample. In fact, this compound was pressed into pellet of 8mm diameter and 1.1mm thickness using 3t/cm2 uniaxial pressure. These measurements were registered in a frequency ranging from 200 Hz to 5MHz with the TEGAM 3550 ALF automatic bridge monitored by a microcomputer between 318 and428 K.

ACCEPTED MANUSCRIPT III. Results and discussion III. 1. Raman spectrum at room temperature The assignment of the internal modes of the most important bands, observed in the Raman spectrum (Fig. 2), is based on the comparison with the similar compounds [11-13]. The

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observed wavenumber and tentative assignments are listed in Table 1.

We note, the bands corresponding to the internal vibrational modes of the anions; ν1 and ν 2 (CoCl4) appear in the spectral range below 310 cm-1. The bands at 101 and 123 cm-1 were

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assigned to the ν2(CoCl4) mode, while the bands observed at 267 and 308 cm-1 were assigned

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to the ν1(CoCl4) mode. The bands from the range 303 to 1034 cm-1 are mainly emanate from the pseudo NC4 tetrahedron part of the TPA entity and the C-N-C and C-C-C bending vibrations. The frequency observed at 1103 cm-1 is related to the rocking ρ(CH3) vibration mode. The lines at higher wavenumber are due to the symmetric and asymmetric stretching vibrations. More particularly, the bands observed near 2840 and 2935 cm-1 were ascribed to

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the νs (CH 3 ) and ν as (CH 2 ) stretching vibrations, respectively. III. 1. Temperature evolution of the Raman spectra

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The Raman spectra of the title compound have been collected in the temperature range 308 to 400 K and in the frequency range 50- 3500 cm-1 (Fig. 3). However, several bands show a

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significant shift in their band position and half-width in the vicinity of the order-disorder phase transition detected by Differential Scanning Calorimetry. This behavior can be related to changes of interactions between organic and inorganic parts increasing with temperature which can be attributed to the increase of the dynamical motion of alkyl chains and/or the degree of distortion of the anions [14]. The position and half-width of the Raman lines were refined using a combination of Gaussian and Lorentzian function. Fig. 4 shows an example of the deconvolution of the spectrum recorded at 333K in the 50-400, 600-1600 and 2700-3100 cm-1 spectral range.

ACCEPTED MANUSCRIPT The temperature behavior of some Raman shifts and half-widths obtained between 90 and 380cm-1 where the anionic part is found are shown in Fig. 5 (a and b).It is obviously seen that these bands exhibit small changes in their band positions and half-widths around the T1 and T2 phase transitions.

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The position and width at half maximum for selected lines obtained between 600 at 1600 cm-1 are depicted in Fig. 6 (a and b). In this region, the most interesting change in the vicinity of the phase transition are observed for the symmetric bending mode of CH3 near 1450 cm-1.

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This band splits into two components below the T1 temperature transition. The change of these bands can be related to a gradual change in the motion state of the (CH3) cation,

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indicating that this motion is directly involved in the disordering process [15-16]. The band assigned to the rocking vibration of CH2 groups appears at 1315 cm-1, shifts to low frequency by 4 cm-1 after the first transition (T1) and by 2 cm-1 around T2. While the half-width of this band shows few variations during the phase transition. The band at 1156 cm-1 attributed to vanishes

above

T1.

The

other

band

at

1032

cm-1

associated

to

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t(CH3)

δ(C − C − C) + t(CH2 ) + ω(CH2 ) exhibits a visible change in their positions and half-widths in the vicinity of the phase transitions around 332 and 376 K.

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The temperature dependence of the Raman spectra in the region of the stretching symmetric

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and asymmetric vibrations between 2800 and 3100 cm-1 is represented in Fig. 7 (a and b). The biggest change in the bands positions are observed for the mode lying at 2950 cm-1 and 2960 cm-1 assigned to the asymmetric stretching νas (CH2) mode. These two bands tend to merge in a single band above 333K. The band at 2984 cm-1 related to the asymmetric stretching νas (CH3) mode shifts to higher wavenumber approaching the phase transition 333K. A significant jump in the half-width of this band observed at T1 and T2. The important changes in the Raman spectrum are observed at 332 and 376 K for some internal mode of cations. The changes around the temperature transition suggests that the

ACCEPTED MANUSCRIPT dynamics of the tetra-propylammonium cations evidence the contribution of the mechanism of phase transition in [N(C3H7)4)]2CoCl4 [17]. In order to verify whether the phase transition are correlated with changes in the dynamical

δ (C-C-C) + t (CH2) + ω (CH2) has been undertaken. III. 2. Temperature dependence of wavenumber

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state of the anions and cations, a quantitative study of the band at 1032 cm-1 associated at

According [18, 19], the temperature dependence of the Raman wavenumber of a phonon

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connected to an order-disorder mechanism can be described by:

ν 2 = ν 02 [1 + γ (T − TC ) ]

(1)

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where γ is the thermal coefficient and ν 0 is the ‘’hard-core wavenumber’’ at temperature transition TC. The value of γ are small (it's usually the case), the wavenumber variation can write as:

(2)

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 γ  ν = ν 1 + (T − TC )  2  0 

The thermal coefficient depends to the variation of the wavenumber position and the volume of the crystal according to the expression: ∆ν i V × ν i ∆V

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γi = −

(3)

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where ∆ν i and ∆V are the variation of the wavenumber position and the volume respectively, V symbolizes the original volume and ν i the band position of the i mode at room temperature.

According to the approximation of Gruneisen, the relative change of any vibration is directly proportional to the relative change in the volume [20].

ACCEPTED MANUSCRIPT Fig. 8, show the dependency of the Raman wavenumber versus temperature for the analyzed band at 1032 cm-1 fitted using Eq. (2). We obtain the expansion coefficient γ = 4 10-5 K-1 for TTC2. Hence, the decreasing of the thermal coefficient related to the changes of these bands

increase of the molecular motion of the cations and the anions.

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positions indicates an increase of the volume of the crystal [21]. This behavior is related to an

III. 3. Temperature dependence of the full width of half maximum

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In order to analyze the full width at half maximum (FWHM), we follow the analysis by

with an order-disorder mechanism [21].

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Carabatos-Nédelec and Becker, which is based on the theory used from damping associated

The temperature dependence of the line width associated with the order-disorder mechanism is given by the generalized Langevin equation as a function of the correlation time τc by [22]: τc 1 + ω2 τc2

(4)

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Γ(ω) = (a + bT) + c

in which first linear part represents the influence of the vibrational relaxation or the anharmonicity and the second term represents the thermal orientational mechanism of diffuse

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nature. ω is the frequency of a particular phonon mode.

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The correlation time τc is the mean reorientational time of the atoms to jumps from one potential well to another, it given by: τc = τ0 exp(

Ea ) k BT

(5)

where τ0 a magnitude of the relaxation time at infinite temperature, Ea is the activation energy for the mode connected to the order-disorder transition and kB is the Boltzman constant. In the case ω2 τc2 >>1, the Eq. (4) is reduced to [23, 24]:

ACCEPTED MANUSCRIPT FWHM(T) = (a + bT) + c exp( −

Ea ) k BT

(6)

where a, b, c and Ea are the fitting parameters. The experimental values of the full-width at half maximum for the band at 1032 cm-1 at

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various temperatures were fitted on the basis of Eq. (6) and the results are represented in Fig.

9. Eq. (6) allow us to estimate the values of a, b, c as well as the activation energy Ea. It shows that the activation energy play a major role in the behavior of the phase transition. The

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estimated activation energy values are: Ea= 55.26 kJ mol-1 for TT2. In fact, the decrease of activation energy values below

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the order-disorder phase transition is probably due to the decrease of the population involved in this vibration which can be due to the change of the conformation of the

[N(C 3 H 7 ) 4 ]+ cation. III. 4. Impedance analysis

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The complex impedance spectra (−Z’’ versus Z’) of the [N(C3H7)4)]2CoCl4 compound at different temperatures is shown in Fig. 10 (a and b). It can be also seen from this figure that the complex impedance data are represented by depressed semicircle (i.e., centres of

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semicircle lie below the abscissa axis), which indicates a Cole– Cole empirical behavior [25]. It is observed that with the increase in temperature the radius semicircles decreases and they

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bent towards real (Z’) axis, indicating an activated thermal conduction mechanism. In order to study the electric properties of this compound, we proposed an equivalent electrical circuit adapted to the spectra of complex impedances. The impedance data are fitted using Zview software and the best fit is obtained when employing an equivalent circuit (insert Fig. 10) consisting essentially by two element which consisting of parallel combination of bulk resistance R1 (polarization resistance), fractal capacity CPE1 (capacity of the fractal interface)

ACCEPTED MANUSCRIPT and capacitance C in series with parallel combination of bulk boundary

resistance R2 and

fractal capacity CPE2. The CPE impedance (ZCPE) is given by the flowing relation [26]: ZCPE = 1/ Q (jω)α

(7)

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where Q indicates the value of the capacitance of the CPE element and α (0<α<1) is the exponent which determines a constant phase angle equal to (α /2).

Fig. 11 shows the variation of imaginary part of impedance (Z") with frequency for some

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representative temperatures. Each spectrum is characterized by the appearance of a peak which shifts to higher frequencies with increasing temperature. Such behavior indicates the

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presence of relaxation process in the system [27]. The height of the relaxation peaks decreases gradually with increasing temperature. This observation indicates the drop in the resistive properties [28].

The frequency dependence of real part of impedance (Z ') of some representative temperatures

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is illustrated in Fig. 12. We note, at lower frequencies Z ' has higher value but the value of Z ' merges in the high frequency region irrespective of temperature. This result indicates the presence of space charge polarization [29] and may be a responsible factor for the

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enhancement of conductivity of the material with temperature at high frequencies. We also

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observed where Z ' start to decrease, the relaxation frequency shifts to higher frequency side. It is worth mentioning that a decrease in Z ' when temperature increases indicating an increase of ac-conductivity [30].

The electrical conductivity σ b was obtained from (Rb) by means of the relation:

σb =

e R bS

where (e/S) represents the sample geometrical ratio.

(8)

ACCEPTED MANUSCRIPT The temperature dependence of the conductivity is presented in the form of Ln(σ p T) versus 1000/T plot (Fig. 13). All phase transitions appearing in DSC diagrams and Raman spectroscopy are affirmed by the change of the curve slope at T1 and T2. The values of the

Ea(II) = 0.81 eV in region II and Ea(III) = 0.97 eV in region III.

III. 5. Modulus analysis

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activation energy determined from linear fit to the data points are Ea(I) = 0.76 eV in region I,

The electrical modulus analysis is very useful in the analysis of the electrical properties. This

at different temperatures and frequencies.

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technique also provides an insight about charge transport processes occurring in the materials

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Fig. 14 represents the variation of the imaginary parts of the electrical modulus as a function of angular frequency at various temperatures. This figure shows a slightly asymmetric peak at each temperature. This peak shifted towards higher relaxation frequencies as temperature increases. This nature of dielectric relaxation suggests that the hopping mechanism of charge

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carriers dominates intrinsically in thermally activated process [31]. The dielectric relaxation process, in general, can be characterized by a no-exponential decay function. The stretched exponential function is defined by the numerical Laplace transform of

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 t  φ(t) = exp  −( )β   τ 

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the Kohlraush-Williams-Watts (KWW) function [32]: 0 < β <1

(9)

where the exponent β characterizes the degree of non-Debye behavior and τ is the conductivity relaxation time. φ(t) is related to the modulus in the angular frequency domain by the expression [33, 34]:

 ∞ dϕ(t)  M = M ∞ 1 − ∫ exp − iωt (− )dt  dt  0  *

(10)

ACCEPTED MANUSCRIPT Recently, Bregman has proposed an approximate KWW function which allows a more direct and easier analysis of the imaginary part of the modulus [35]:

M max

(11)

 β  ωm ω  )  β( ) + ( ) β    (1 − β) + ( 1+ β  ω ωm   

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M" =

1 where M max is the peak maximum of the modulus and ωm = ( ) is the peak frequency of the τ imaginary part of the modulus. The value of β is positioned in the (0-1) range, which reflects

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the important of coupling between mobiles ions in the conduction process.

In the present case, the temperature dependence of β , obtained by fitting curve to Eq. (11) are

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depicted in Fig. 15. From this figure we can seen that the value of β are ranging between 0.47 and 0.65. The change in the slope is detected around 333 and 376 K, this result is in agreement with the temperature of the phase transition determined from DSC measurement, relaxation process and the conductivity. We can conclude that the interactions between the

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charge carriers vary with the temperature.

M " typically exhibits a well-defined maximum to which a characteristic relaxation rate can be

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associated and displays asymmetric frequency dependence. The relaxation frequency dependence of temperature (Fig.16) is expressed by the Arrhenius relation  − Em    K BT 

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ωp = ω0 exp 

(12)

where, ω0 is the angular frequency at infinite temperature and Em is the activation energy for conductivity relaxation.

A change of the curve slope is noted around 332 and 376 K which confirmed the occurrence of the phase transition detected by DSC measurement and Raman spectroscopy. The calculated activation energy from linear fit of the data point are Em1= 0.78 eV, Em2=0.81 eV and Em3=0.93 eV. It’s worth noticing that the energy Em is in good agreement with electrical

ACCEPTED MANUSCRIPT measurements, which means that the relaxation process and the electrical conductivity are ascribed to the same effect.

IV. Conclusion In this work, the Raman spectra at several temperatures were studied. It’s clearly shown that

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the Raman spectra presented drastic changes around 332 and 373 K. The reported transitions around these temperatures are of order-disorder type. We showed that the careful analysis of the wavenumber and the bands width of the order-disorder or structural phase transition are

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consistent with a dynamics reorientation of the tetrapropylammonium cation. The complex impedance analysis reveals the bulk contribution to electrical properties. A detailed analysis

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of the arcs reveals the presence of a grains interior and grains boundary as an equivalent electrical circuit in different regions. Besides, the variations of the elements values, which correspond to electric measurements with temperature, confirmed the occurrence of the change detected by the thermal analysis and Raman spectroscopy. Furthermore, the dielectric

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data have been analyzed in modulus formalism using KWW stretched exponential function. The near values of activation energies determined from the impedance and the electric

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[26]

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Table. 1

Intensity Raman (cm-1)

Assignements ν2(CoCl4) ν2(CoCl4) ν1(CoCl4) ν1(CoCl4)

330 377 782 843 1032

δ(NC4)+δ(C-C-C) ν2(NC4) ν1(NC4) δ(C-N-C)+ δ(C-C-C) δ(C-C-C)+t(CH2)+ω( CH2)

1103 1160 1316 1453

ρ(CH3) t(CH2) ω(CH2) δs(CH3)

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101 123 267 308

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2840 νs(CH3) 2879 νs(CH2) 2935 νas(CH2) 2988 νas(CH3) νs: symmetric stretching; νas: asymmetric stretching; δas: asymmetric bending;

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ω: wagging; t: twisting; δ: bending; ρr: rocking.

ACCEPTED MANUSCRIPT

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Fig. 1

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L1

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Intensity (a.u)

6000

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8000

EP

Fig 2

L2

4000

2000

0 500

1000

1500

2000 -1

Wavenumber (cm )

2500

3000

3500

ACCEPTED MANUSCRIPT Fig 3

308K 318K 323K 328K

333K 338K 343K 348K

358K 363K 368K

373K 378K

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30000

0 50

100

150

M AN U

Intensity (a.u)

60000

200

250

300

350

400

450

-1

Wavenumber (cm )

358K 363K 368K

373K 378K

EP

30000

333K 338K 343K 348K

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Intensity (a.u)

60000

TE D

308K 318K 323K 328K

0

750

900

1050

1200 -1

Wavenumber (cm )

1350

1500

500

ACCEPTED MANUSCRIPT

308K 318K 323K 328K

Intensity (a.u)

90000

333K 338K 343K 348K

358K 363K 368K

373K 378K

RI PT

60000

0 2800

2900

3000

3100

M AN U

2700

SC

30000

-1

Wavenumber (cm )

Fig. 4

50 000

30 000 25 000

0

EP

100

150

264.4

287.3

5 000

AC C

10 000

125.2

118.1

91.4

15 000

69.8

20 000

200

250 Wavenumber (cm-1)

300

369.7

Intensity (a.u)

35 000

329.5

40 000

305.4

TE D

45 000

350

1450.4

ACCEPTED MANUSCRIPT 16 000

1032.5

Intensity (a.u)

12 000

1461.3

14 000

10 000

0 700

800

2 600

2 650

2 700

900

1 000

1 100 Wavenumber (cm-1)

1 200

1477.3

1 400

1510.9

1353.1

1 300

1 500

1 600

3 050

3 100

SC

600

1378.7

RI PT 1315.0 1331.6

1297.2

1154.3 1174.5

2 000

969.7

915.6 933.5

872.3

751.3

4 000

843.4

778.1

6 000

1133.7

1100.1

8 000

30 000 25 000 20 000

10 000 5 000

2 750

AC C

EP

0

2774.0

2740.6

TE D

15 000

2 800

2967.5 2987.7

Intensity (a.u)

35 000

2843.6

2877.1

40 000

2952.2

45 000

2908.0

M AN U

2934.6

50 000

2 850 Wavenumber (cm-1)

2 900

2 950

3 000

ACCEPTED MANUSCRIPT Fig. 5 (a) 374 372 370

ν2(NC4) δ(NC4)+δ(C-C-C)

126

ν1(CoCl4)

ν1(CoCl4)

124 98 ν2(CoCl4) 96 94 92 315 330

345

SC

266 264 262 260

RI PT

328

360

375

M AN U

-1

Wavenumber (cm )

332

390

405

T(K)

(b)

δ (NC4) + δ (C-C-C)

TE D

20

ν2(NC4)

ν1(CoCl4)

18

ν1(CoCl4)

EP

-1

∆ν (cm )

26 24 22 20 36 30 24

30 24 18

ν2(CoCl4)

AC C

21 18 15

315

330

345

360 T(K)

375

390

405

ACCEPTED MANUSCRIPT Fig. 6 (a) 1476 1470 1464

δs(CH3)

RI PT

ω(CH2)

1314 1312 1156.2

t(CH2)

1155.6 1034 1033 1032 1031

δ(C-C-C)+t(CH2)+ω( CH2)

315

330

345

360

28 24 20 16 17.8 17.6 17.4

δs(CH3)

ω(CH2)

405

(b)

t(CH2)

δ(C-C-C)+t(CH2)+ω( CH2)

EP

24 22 20

390

TE D

-1

∆ν (cm )

45 30 15

375

M AN U

T(K)

SC

-1

Wavenumber (cm )

1316

AC C

315

330

345

360 T(K)

375

390

405

ACCEPTED MANUSCRIPT Fig. 7

(a)

νas(CH3)

2988 2982

2880 2878 2876

RI PT

νas(CH2)

2970 2960 2950

νs(CH2)

2844

SC

-1

Wavenumber (cm )

2976

νS(CH3)

315

330

M AN U

2840 345

360

375

390

405

T (K)

TE D

40 30 20 10 21 20 19 18 40 36 32 28

νas(CH3)

(b)

νas(CH2)

νs(CH2)

EP

-1

∆ν (cm )

32 30 28 26

AC C

νS(CH3)

315

330

345

360 T(K)

375

390

405

ACCEPTED MANUSCRIPT Fig. 8

1034

γ= 1 10

-5

γ= 2 10 1033

Tr2

RI PT

-1 Wavenumber(cm )

-5

γ= 4 10

-5

1032

1031 315

345

360

375

390

M AN U

330

SC

Tr1

T(K)

Fig. 9

-1

Ea= 6.56 KJ mol

-1

Ea= 24.16 KJ mol

23 E =55.26 KJ mol-1 a

EP

-1

∆ν (cm )

24

Tr2

1032

TE D

25

22

Tr1

AC C

21

20

315

330

345

360 T(K)

375

390

405

405

ACCEPTED MANUSCRIPT Fig. 10

8

8

1.6x10

8

1.4x10

8

-Z″(Ω)

1.2x10

8

1.0x10

RI PT

348K 353K 358K 363K 368K 373K 378K Fit

1.8x10

7

8.0x10

7

SC

6.0x10

7

4.0x10

(a)

7

0.0 0.0

M AN U

2.0x10

7

2.0x10

7

4.0x10

AC C

EP

TE D

Z′(Ω)

7

6.0x10

7

8.0x10

8

1.0x10

ACCEPTED MANUSCRIPT 7

1.8x10

7

1.6x10

(b)

7

1.4x10

7

1.2x10

7

383K 388K 393K 398K 403K 408K 413K 416K 421K fit

6

8.0x10

6

6.0x10

6

4.0x10

6

2.0x10

0.0 0.0

8.0x10

6

1.6x10

7

7

7

2.4x10

3.2x10

7

4.0x10

M AN U

SC

Z′ (Ω)

RI PT

-Z″ (Ω)

1.0x10

7

1.8x10

7

1.6x10

(b)

7

1.4x10

TE D

7

1.2x10

7

-Z″ (Ω)

1.0x10

6

8.0x10

6

6.0x10

383K 388K 393K 398K 403K 408K 413K 416K 421K fit

EP

6

4.0x10

6

2.0x10

AC C

0.0

0.0

6

8.0x10

7

1.6x10

Z′ (Ω)

7

2.4x10

7

3.2x10

7

4.0x10

ACCEPTED MANUSCRIPT Fig. 11 7

1.8x10

383 K 388 K 393 K 398 K 403 K 408 K 421 K FIT

7

RI PT

1.2x10 -Z″ (Ω)

6

6.0x10

2

10

3

SC

0.0 4

10

5

10

-1

ω (rad s )

7

4x10

7

TE D

3x10

7

Z′ (Ω)

10

M AN U

Fig. 12

6

10

2x10

7

EP

1x10

383 K 388 K 393 K 398 K 403 K 408 K 421 K Fit

AC C

0

2

10

3

10

4

5

10

10 -1

ω (rad s )

6

10

ACCEPTED MANUSCRIPT Fig. 13 7

4x10

383 K 388 K 393 K 398 K 403 K 408 K 421 K Fit

7

RI PT

3x10

7

Z′ (Ω)

2x10

7

SC

1x10

0

2

3

4

10

5

10

10

M AN U

10

-1

ω (rad s )

Fig. 14

353 K 358 K 363 K 368 K 373 K 378 K 383 K 388 K 393 K 398 K 403 K 408 K 421 K FIT

TE D

0.06 0.05 0.04

M″

EP

0.03 0.02

AC C

0.01 0.00

2

10

3

10

4

5

10

10 -1

ω (rads

)

6

10

10

6

ACCEPTED MANUSCRIPT Fig. 15

0.66 (III)

0.63 0.60 (I)

RI PT

0.57

β 0.54 (II)

0.51

0.45 300

320

340

360

380

400

420

440

M AN U

T (K)

SC

0.48

Fig. 16

14 (III)

TE D

13

Ea=0.93 eV

11

EP

(II) Ea=0.81 eV

10 9

AC C

-1

ln (ωp) (rad. s )

12

(I) Ea=0.78 eV

8 7

2.3

2.4

2.5

2.6

2.7

2.8 -1

1000/T (K )

2.9

3.0

3.1

3.2

ACCEPTED MANUSCRIPT Highlights •

This compound is crystallized in the monoclinic system (P21/n space group)

•

The phase transition has been analyzed by Raman spectroscopy. The half-width is quantitatively described in term of an order-disorder mode

•

Electric measurement confirms the conclusion drawn from the DSC and Raman.

AC C

EP

TE D

M AN U

SC

RI PT

•