Journal of Chemistry

Volume 2013 (2013), Article ID 435462, 10 pages

http://dx.doi.org/10.1155/2013/435462

Research Article

Structural Characterization and Infrared and Electrical Properties of the New Inorganic-Organic Hybrid Compound

A. Oueslati,¹ I. Chaabane,¹ K. Adil,² and F. Hlel¹

¹Laboratoire de l'État Solide, Faculté des Sciences de Sfax, Université de Sfax, 3000 Sfax, Tunisia

²Laboratoire des Oxydes et Fluorures, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 05, France

Received 9 June 2012; Accepted 11 August 2012

Academic Editor: Adriana Szeghalmi

Copyright © 2013 A. Oueslati et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

New inorganic-organic hybrid [(C₃H₇)₄N]₂Hg₂Cl₆ compound was obtained and characterised by single-crystal X-ray diffraction, infrared, and impedance spectroscopy. The latter crystallizes in the monoclinic system (space group C 2/c, ) with the following unit cell dimensions: (1) Å, (6) Å, (2) Å, and (2). Besides, its structure was solved using 84860 independent reflections leading to . Electrical properties of the material were studied using impedance spectroscopic technique at different temperatures in the frequency range of 209 Hz to 5 MHz. Detailed analysis of the impedance spectrum suggested that the electrical properties of the material are strongly temperature-dependent. The Nyquist plots clearly showed the presence of bulk and grain boundary effect in the compound.

1. Introduction

The contemporary deep interest for inorganic-organic hybrid materials is due to various reasons. An obvious advantage of hybrids is that they favorably combine the properties of the organic materials to that of the inorganic one and thus to exhibit specific properties, such as electronic, optical, thermal, and catalytic [1–4]. Among halometallates (II), chloromercurates (II) have been of special interest for their structural flexibility. In fact the spherical d¹⁰ configuration of Hg²⁺ ion is associated with a flexible coordination environment so that the geometries of these complexes can vary from tetrahedral to octahedral and severe distortions in the ideal polyhedron occur easily, depending on crystal packing and hydrogen bonding, as well as halide dimensions [5–11]. In the last years, the structural and electrical properties of [(C₃H₇)₄N]₂Cu₂Cl₆ and [(C₃H₇)₄N]₂Cd₂Cl₆ compounds have been reported [12, 13]. In spite of attractive properties of mercury(II) compounds in terms of their potential applications in thermometers, manometers, energy efficient fluorescent light bulbs, and mercury batteries (although somehow limited due to mercury’s toxicity), we have successfully synthesized a new compound of formula [(C₃H₇)₄N]₂Hg₂Cl₆.

In the present paper we report on the synthesis, the structural, and spectroscopic characterizations of the [(C₃H₇)₄N]₂Hg₂Cl₆ compound.

2. Experimental

2.1. Preparation of [N(C₃H₇)₄]₂Hg₂Cl₆

The synthesis of [N(C₃H₇)₄]₂Hg₂Cl₆ was performed from precursors [N(C₃H₇)₄]Cl (purity 97%; FLUKA) and HgCl₂ (purity 99.5%; FLUKA). The reactions sequence for the synthesis is shown in the following equation:

The solution was obtained by mixing 0.441 g of [N(C₃H₇)₄]Cl and 0.218 g of HgCl₂ in 10 ml of HCl (1 M) aqueous solution. After few days, crystalline samples were obtained by slow evaporation at room temperature.

2.2. Crystallographic Studies

Single crystals of [(C₃H₇)₄N]₂Hg₂Cl₆ are obtained by slow evaporation, at room temperature, of an hydrochloric acid solution containing stoichiometric amounts of [N(C₃H₇)₄]Cl (purity 97%; FLUKA) acidified with HCl (1 M) and HgCl₂ (purity 99.5%; FLUKA).

A single crystal with 0.2 × 0.19 × 0.17 mm³ dimensions was selected by optical examination. The crystal data were collected on a Siemens APEX II four-circles diffractometer using a monochromatic Mo radiation (λ = 0.71073 Å). The absorption corrections were based on multiple and symmetry-equivalent reflections in the data set using the SADABS program [14]. The structure was solved by direct methods and refined in the anisotropic approximation using SHELXS-97 [15] and SHELXL-97 [16]. The main crystal data, the parameters used for intensity data collection, and the reliability factor are gathered in Table 1. The obtained solution permits us to localize the positions of mercury and chlorine atoms. Positions of N and all carbon atoms were located after subsequent Fourier series analysis. The nitrogen H-atoms were successively located in a difference Fourier maps. The other hydrogen atoms were placed in calculated positions. Main geometrical features, bond distances, and angles are reported in Table 2.

Table 1: Summary of crystal data, X-ray diffraction intensity measurements, and refinement parameters for [N(C₃H₇)₄]₂Hg₂Cl₆.

Formula: [(C₃H₇)₄N]₂Hg₂Cl6
= 986.6 

Color/shape
Colorless/prismatic

Crystal dimensions
0.22 0.18 0.11 mm

Crystal system
Monoclinic

Space group
C 2/ (N° 15)

 Å, Å,
= 7409.0(7) Å³

 Å, β =91.686
= 8, = 8.726 mm⁻¹

Temperature (K)
173

Radiation, λ (Å), monochromator
Mo, 0.71069, graphite

Scan angle (°)
θ-θ

θ Range (°)
1.4–30

Range of , ,
−26→27, −16→18, −40→40

Reflections collected/unique
42266/10658 ( = 0.047)

Observed reflections [Fo > 2σ(Fo)]
8486

Absorption correction
Multiscan (SADABS)

,
0.542, 0.823

Structure resolution
Paterson methods: SHELXS-86

Structure refinement with
SHELXL-97

Refinement
full matrix

Refined parameters
348

Goodness of fit
1.177

Final and Rw
0.038, 0.087

Final and Rw (for all data)
0.057, 0.097

Largest feature diff. map
1.565, −1.354 e⁻/Å³

= 1/[ σ ²(Fo)² + (0.035 )² + ]
= [Fo² + 2Fc²]/3

Table 2: Selected bond distances (Å) and bond angles (°) of [N(C₃H₇)₄]₂Hg₂Cl₆ (e.s.d. are given in parentheses).

Distances (Å)
Angles (°)

Hg Cl₄

    Hg1−ClE1

ClE1−Hg1−ClE2

    Hg1−ClE2

ClE1−Hg1−ClL1

    Hg1−ClL1

ClE1−Hg1−ClL2

    Hg1−ClL2

ClE2−Hg1−ClL1

ClE2−Hg1−ClL2

ClL1−Hg1−ClL2

Hg Cl₄

    Hg2−ClE3

ClE3 −Hg2−ClE4

    Hg2−ClE4

ClE3 −Hg2−ClL1

    Hg2−ClL1

ClE3 −Hg2−ClL2

    Hg2−ClL2

ClE4 −Hg2−ClL1

ClE4 −Hg2−ClL2

ClL1−Hg2−ClL2

(C₃H₇)₄N⁺

    N1− C21

C21−N1−C41

    N1−C41

C21−N1−C11

    N1−C11

C41−N1−C11

    N1−C31

C21−N1−C31

    C11−C12

C41−N1−C31

    C12−C13

C11−N1−C31

    C21−C22

C12−C11−N1

    C22−C23

C11−C12−C13

    C31−C32

C22−C21−N1

    C32−C33

C23 −C22−C21

    C41−C42

C32−C31−N1

    C42−C43

C33 −C32−C31

C42−C41−N1

C41−C42−C43

(C₃H₇)₄N⁺

    N2−C81

C81−N2−C51

    N2−C51

C81−N2−C61

    N2−C61

C51−N2−C61

    N2−C71

C81−N2−C71

    C51−C52

C51−N2−C71

    C52−C53

C61−N2−C71

    C61−C62

N2−C51−C52

    C62−C63

C53 −C52−C51

    C71−C72

C62−C61−N2

    C72−C73

C61−C62−C63

    C81−C82

C72−C71−N2

    C82−C83

C71−C72−C73

N2−C81−C82

C81−C82−C83

2.3. Infrared Spectroscopies

The infrared spectrum was recorded in the 400–4,000 cm⁻¹ range with a Perkin-Elmer FT-IR 1000 spectrometer using samples pressed in spectroscopically pure KBr pellets. Spectral resolution is better than 4 cm⁻¹.

2.4. Impedance Spectroscopy Analysis

The finely grain samples were pressed into pellets of 8 mm diameter and 1.1 mm thickness using a hydraulic press. The pellet discs were coated with Ag paste to ensure good electrical contact. The ac impedance data, , and phase angle were obtained in the frequency range 209 Hz–5 MHz using TEGAM 3550 impedance analyzer over the temperature range 293–375 K.

3. Results and Discussions

3.1. Crystal Structure Description

The representations of asymmetric unit showing the ellipsoid thermal mean square displacements are represented in Figure 1. The structural arrangement of [N(C₃H₇)₄]₂Hg₂Cl₆ is layered organization of organic-inorganic sheets staked along direction, Figure 2. Each layer is translated via its neighbor by . The organic-inorganic layer can be described as an alternation of inorganic chain and organic double chain parallel to the [] direction. The organic double chain is made up of two crystallographic different [N(C₃H₇)₄]⁺ cation, Figure 3. Unlike the reported structure, atomic arrangement of similar compound, [N(C₃H₇)₄]₂Cu₂Cl₆ [17] and [N(C₃H₇)₄]₂Cd₂Cl₆ [13], can be described as a sequence of alternating organic-inorganic layers. In the organic layer, N(C₃H₇)₄ groups orient their aliphatic chains parallel to organic sheets. For [N(C₃H₇)₄]₂Hg₂Cl₆ structure, the alkylammonium cation shows a tetrahedral environment. However, each (C₃H₇)₄ fragment develops along the N–C direction of the NC₄ tetrahedra. Consequently, each two fragments made a “V” shape and were observed in the same plane. Plans contain all (C₃H₇)₄ are perpendicular. Two (C₃H₇)₄ chains are observed in the plane of the organic-inorganic layer. The other fragments, localized in the plan perpendicular to layer, are oriented in both sides of the inorganic chain. Both cations have C1 point group symmetry. C–C–C, C–N–C and N–C–C angles and C–C and C–N distances, Table 2, are commonly observed [17].

Figure 1: Representation of the ellipsoid of thermal mean square displacements of nonhydrogen atoms at 50% of the asymmetric unit.

Figure 2: view showing the organic-inorganic layers staked along direction; Hg₂Cl₆ schematized by grey tetrahedra, grate empty, and small and full circles represent nitrogen, hydrogen, and carbon atoms, respectively.

Figure 3: View of an organic-inorganic layer of [N(C₃H₇)₄]₂Hg₂Cl₆.

The inorganic chain is made up of one Hg₂Cl₆ dimers. The anion is built up by two independent HgCl₄ tetrahedra and shares one bridging chlorine atoms (1 and 2) as shown in Figure 1. The Hg₂Cl₆ dimers can be described as two strongly distorted HgCl₄ tetrahedra. The Hg₂Cl₆ tetrahedron presents a C1 punctual symmetry. The M₂X₆ groups are commonly observed in this family. Two types of ligands are usually mentioned, XE (external) and (linkage). Moreover, bibliographic investigations show that M–X distances and X–M–X angles verify (M–XL > M–XE) (I) and (XL–M–XE and XE–M–XE are generally bigger than XL–M–XL) (II) [18, 19]. The geometry of Hg₂Cl₆ dimers verifies the reported relations, Table 2. The anion, –Hg––Hg, is pseudosquare. These results are in agreement with the above comparison (I and II).

3.2. Infrared Spectra

The infrared spectrum of the [N(C₃H₇)₄]₂Hg₂Cl₆ compound at room temperature is shown in Figure 4. A detailed assignment of most important bands is realized by comparison with similar compounds [20–27]. The wavenumbers and proposed band assignments are listed in Table 3.

Table 3: Observed IR frequencies (in cm⁻¹) for [N(C₃H₇)₄]₂Hg₂Cl₆ at room temperature.

IR wavenumbers

(cm⁻¹)
Assignment

516

609

755
(NC₄)

851
(CH₂)

874
(CH₃) + (CH₂)

920
(CH₂)

968
(NC₄)

986
(C–C)

1038
(C–C–N)

1107

1138
(C–C–C–N)

1173

1274
τ(CH₂)

1328
ω(CH₂)

1357
(CH₃)

1392

1458
(CH₃)

1471

2413

2734

2878
(CH₂)

2940
(CH₂)

2972
(CH₃)

3701

Figure 4: Infrared spectrum of the [N(C₃H₇)₄]₂Hg₂Cl₆ compound at room temperature.

The principal bands are assigned to the internal modes of the propyl group: strong absorption band appears near 2972 cm⁻¹ which was assigned to the asymmetric methyl stretching mode (CH₃). Two bands are observed at 2940 and 2878 cm⁻¹, which were assigned to the (CH₂) and (CH₂) stretching modes, respectively. A strong absorption band appears near 1458 cm⁻¹ which was assigned to the asymmetric methyl deformation mode (CH₃). Another band, assigned to the symmetric methyl (CH₃) deformation modes, was observed near 1357 cm⁻¹. The splitting (C–C–C–N) bending mode at 1138 cm⁻¹ may correspond to different conformers of the organic chains. The bands observed around 968 cm⁻¹ were assigned to (NC₄) stretching modes. A weak band which appeared at 874 cm⁻¹ is related to the (CH₃) (CH₂) rocking vibration mode, while the deformation mode (NC₄) appears below 755 cm⁻¹.

3.3. Electrical Properties

3.3.1. Impedance Studies

Impedance spectroscopy is a useful method to resolve the contributions of various processes such as bulk, grain boundary, and electrode effect in the specified frequency domain. In addition, the resistance and the capacitance associated with the solids could be estimated using impedance spectroscopy.

Figure 5 shows the plot of () versus taken over the frequency range (209 Hz to 5 MHz) at different temperatures (293 K ≤ T ≤ 375 K). The inset in Figure 5 shows the plot of () versus at 338 K.

Figure 5: Impedance spectra of [(C₃H₇)₄N]₂Hg₂Cl6 sample at different temperature. The inset shows the plot of () versus at 338 K.

Two arcs are obtained which may be ascribed to the circuit shown in Figure 6 where and CPEgb, are respectively, the resistance and the constant phase elements arising out from the grain boundaries response of the sample showed by the second arc observed at low frequency, while and CPEb are, respectively, the resistance and the constant phase elements due to the bulk response of the compound showed by the first arc observed at high frequency [28–30].

Figure 6: Equivalent circuit model of the [(C₃H₇)₄N]₂Hg₂Cl₆ compound.

The impedance of CPE is . Usually is considered to be a dispersive capacitance. is the measure of the capacitive nature of the element: if the element is an ideal capacitor, if it behaves as a frequency independent ohmic resistor, whereas if it behaves as an inductance. (Note that there are known cases when cannot be treated this way; e.g., the Warburg impedance, which is connected with diffusion, is a CPE element with .)

The real and the imaginary components of the whole impedance of this circuit were calculated according to the following expressions:

The curves of and versus frequency at several temperatures are fitted by (2) and (3), respectively. Figures 7 and 8 represent and versus frequency at several temperatures together with fits to the equivalent circuit represented in Figure 6. All fitted curves at each temperature show the good conformity of calculated lines with the experimental data indicating that the suggested equivalent circuit describes the crystal-electrolyte interface reasonably well.

(a)

(b)

Figure 7: Frequency dependence of real part of impedance of [(C₃H₇)₄N]₂Hg₂Cl₆ at different temperatures.

(a)

(b)

Figure 8: Frequency dependence of imaginary part of impedance of [(C₃H₇)₄N]₂Hg₂Cl₆ at different temperatures.

The temperature dependences of the fitted parameters , , , , , and are given in Table 4. As temperature decreases, the grain boundaries resistance increases with the arc increasing. The bulk resistance increases as temperature decreases as is expected for an activated conduction mechanism that is reflected as an increase in the radius of the arc as temperature is lowered.

Table 4: Fitting parameter obtained from impedance spectroscopy at different temperature ranges.

(K)
(M)
(10⁻⁹ F)

(M)
(10⁻¹² F)

323
5.12
1.57
0.745
2.64
12.2
0.77

326
5.10
1.42
0.759
2.24
21.8
0.78

328
4.82
1.47
0.761
1.87
28.0
0.78

331
4.26
1.53
0.764
1.47
34.2
0.82

333
3.56
2.07
0.751
0.853
34.7
0.83

338
3.09
2.77
0.739
0.689
38.5
0.87

343
2.40
3.74
0.725
0.531
61.9
0.89

348
1.98
4.98
0.705
0.407
67.8
0.91

353
1.44
8.86
0.665
0.255
88.7
0.92

358
1.22
11.8
0.647
0.221
89.2
0.95

363
0.859
18.8
0.626
0.237
1.05
0.97

368

0.588
22.7
0.628
0.254
1.13
0.97

The variation of fitted parameters versus temperature shows that the constant phase elements (CPE) represents a leaking (non ideal) capacitor as it contains both imaginary and real parts and constitutes energy dissipation because of the presence of the impedance real part.

The capacitance values of the equivalent circuit element are critical to the identification of the grain boundary and grain interior contribution. It has already been established in the literatures that the dispersion in the grain boundaries and in the bulk has a capacitance value in the range of nF and pF, respectively. In our case the capacitance values vary between 1.05 pF and 89.2 pF and vary in the range 1.42 nF–22.7 nF. This implies that the first arc response is due to grain interiors response and the second arc arising out from the grain boundaries response.

The temperature dependence of the conductivity obtained from the fits of the first arc observed at high frequency is shown in Figure 9. It is noted that the variation of the conductivity with temperature obeys an Arrhenius equation as follows:

where is the conductivity at temperature the preexponential factor, the Boltzmann’s constant, and is the thermal activation energy. The value of the activation energy is about 0.46 eV obtained from the least squares straight line fits.

Figure 9: Temperature dependence of versus reciprocal temperature for [N(C₃H₇)₄]₂Cu₂Cl₆.

3.3.2. Modulus Studies

The modulus formalisma is particularly suitable to extract phenomena such as electrode polarization and conductivity relaxation times [31, 32].

Consider the following:

The dependence of electric modulus on frequency can be written also as [33]:

where gives the time evolution of the electric field with in the dielectric.

Figure 10 shows the variation of as a function of frequency at various temperatures. At lower frequencies, tends to be very small, confirming that the electrode effects make a negligible contribution and hence may be ignored when the data are analyzed in modulus formalism. attains a maximum below 10⁷ Hz and decrease above for all temperatures. The decreases of above 10⁷ Hz may be interpreted as the accumulation of charges at the interface between the sample and the electrode, that is, space charge polarization [34, 35].

Figure 10: Variation of real part of electric modulus with frequency for [(C₃H₇)₄N]₂Hg₂Cl₆ at several temperatures.

Figure 11 shows the temperature-frequency dependence of imaginary part of electric modulus (). The asymmetric modulus peaks shifts towards higher frequency side indicating correlation between motions of mobile ions charges [36]. The asymmetry in peak broadening shows the spread of relaxation times with different time constant, and hence relaxation is of non-Debye type. The existence of low frequency peaks suggests that the ions can move over long distances whereas high-frequency peaks suggest confinement of ions in their potential well. The peak in the plot shifts toward higher frequencies with increase in temperature and also the peak height increases. This reveals that when the frequency is high, the temperature for which the measuring frequency is equal to is also high.

Figure 11: Variation of imaginary part of modulus with frequency for [(C₃H₇)₄N]₂Hg₂Cl₆ at different temperatures.

The conductivity relaxation frequency is given by the relation , where is the characteristic phonon frequency, is the activation energy for conductivity relaxation, is the Boltzmann’s constant, and is the temperature. The temperature dependence of the conductivity relaxation frequency is plotted in Figure 12. It is well described by the Arrhenius relation. The activation energy obtained from the modulus spectra is about 0.4 eV.

Figure 12: Dependence of Ln() versus reciprocal temperature for [(C₃H₇)₄N]₂Hg₂Cl₆ sample.

The activation energies issued from the impedance (0.46 eV) and modulus (0.4 eV) spectra are not very different, to suggest that the ion transport in polycrystalline sample is probably due to a hopping mechanism [37].

4. Conclusion

Crystals of a new hybrid material, [(C₃H₇)₄N]₂Hg₂Cl₆, have been prepared by slow evaporation of aqueous solution HCl (1 M), N(C₃H₇)₄, and HgCl₂ at room temperature. The atomic arrangement is layered organization of organic-inorganic sheets staked along direction. Those layers spreading in this network are interconnected by van Der Waals interaction. The analysis of the frequency dispersion of the real imaginary components of the complex impedance allowed us to determine an equivalent electrical circuit for the electrochemical cell with [(C₃H₇)₄N]₂Hg₂Cl₆. The temperature dependence of conductivity was analyzed using the Arrhenius approach. Impedance spectra revealed the presence of grain and grain boundary contribution whereas electric modulus spectra showed only bulk (grain) contribution in the electrical properties.

The analysis of the temperature variation of peak indicates that the observed relaxation process is thermally activated. The near value of activation energies is obtained from the relaxation times (0.4 eV) and conductivity data (0.46 eV) suggesting that the ionic transport in the investigated material can be described by a hopping mechanism.

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