We report measurements of X-ray powder diffraction, vibrational study, the differential scanning calorimetry (DSC), and the electric properties of a made-up [C6H9N2]2CuCl4 sample. The alternative current (ac) conductivity of the compound [C6H9N2]2CuCl4 has been measured in the temperature range 356–398 K and the frequency range 209 Hz–5 MHz. The Cole-Cole (the imaginer part () versus real part () of impedance complex) plots are well fitted to an equivalent circuit model which consists of a parallel combination of a bulk resistance () and constant phase elements (CPE). The single semicircle indicates only one primary mechanism for the electrical conduction within [C6H9N2]2CuCl4. The variation of the value of these elements with temperatures confirmed the result detected by DSC and dielectric measurements. Thus the conduction in the material is probably due to a hopping or a small polaron tunneling process.

1. Introduction

Organic-inorganic hybrid compounds can be designed to utilize synergistic interactions between the dissimilar components, which can yield new properties and/or an enhanced performance. Indeed, synergy will be critical in achieving targeted physical properties (e.g., electronic, optical and transport properties). Thus, organic-inorganic hybrid materials combine the advantageous properties characteristic of inorganic solids (e.g., high carrier mobilities, thermal stability) with those of organic molecules (e.g., ease of processing, high fluorescence efficiency, and large polarizability) [111]. A novel group of crystals, containing heteroaromatic cations like, pyridinium, substituted pyridinium, and imidazolium ones, have been recently synthesized and characterized [1113]. Since aromatic heterocyclic cations are bestowed a significant electric dipole moment; thus some halogenoantimonates(III) and halogenobismuthates(III) containing these cations form strongly polar structures. Inorganic component can introduce some special structural units, such as distorted tetrahedron and octahedron. In an attempt to study the electric behavior in this class of compounds we have successfully synthesized a compound of formula bis(2-amino-4-methylpyridinium) tetrachloridocuprate(II) ([C6H9N2]2[CuCl4]). At room temperature, the synthesized compound crystallizes in the monoclinic system (C2/c space group) with and the following unit cell dimensions: (3) Å, (3) Å, (4) Å, and (17)° [11]. The crystal structure contains chains of cations alternating with stacks of tetrahedra anions of tetrachloridocuprate (Figure 1(a)). Both N–HCl and π-π stacking interactions cause the formation of a three-dimensional supramolecular architecture.(i)The bonding between inorganic and organic layer is established by four different hydrogen bonds ([N1–H1A(Cl1,Cl2i), N2–H2ACl1, and N2–H2BCl2ii). The NCl distances vary between 3.294 (6) and 3.359 (6) Å. These interactions and the symmetrically related ones connect the anion to four surrounding cations (Figure 1(b)).(ii)The cations interact via offset face-to-face, π-π stacking interactions leading to chains along the crystallographic axis (Figure 1(c)) [11].

In the present paper we report the synthesis, X-ray powder diffraction patterns, DSC, infrared, Raman, and impedance spectroscopy characterizations of the [C6H9N2]2CuCl4 compound.

2. Experimental

[C6H9N2]2CuCl4 sample was prepared by mixing CuCl2 (purity 98%; FLUKA), dissolved in hydrochloric acid solution (1 M), and the organic compound bis(2-amino-4-methylpyridinium) (purity 99%; FLUKA), in molar ratio 1 : 2. After six days, crystalline samples were obtained by slow evaporation at room temperature.

Schematically the reaction proposed in order to justify the obtaining [C6H9N2]2CuCl4 sample is shown in the following equation: Powder X-ray diffraction was carried out initially to confirm the presence of the [C6H9N2]2CuCl4 compound. A Siemens D500 was employed, using radiation. A scanning rate of 0.028° s−1 was employed.

Differential scanning calorimetry analysis was performed using a DSC NETZSCH 204 between 220 and 475 K at the heating rate of 5 K/min.

The infrared spectrum was recorded in the 400–4000 cm−1 range with a JASCO FT-IR 420 1000 spectrometer using samples pressed in spectroscopically pure KBr pellets.

The Raman spectrum of [C6H9N2]2CuCl4 sample is recorded on a Kaiser Optical System spectrometer model-Hololab 5000R in the region 80–2000 cm−1 at room temperature.

The pellets of [C6H9N2]2CuCl4 sample were prepared (diameter 8 mm; thickness 0.6 mm) by compressing the powder in a die using a hydraulic press at a pressure of 3000 kg/cm2. The samples of the appropriate shape were sandwiched between two silver electrodes of the configuration Ag/electrolyte/Ag. 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 356–398 K.

3. Results and Discussion

3.1. X-Ray Powder Diffraction Patterns

The room temperature X-Ray diffraction pattern of [C6H9N2]2CuCl4 is shown in Figure 2. Lattice parameters were calculated with reference to the single-crystal X-ray study of bis(2-amino-4-methylpyridinium) tetrachloridocuprate(II) and using a computer program package Celref 2 based on the least square refinement method. The refined, in the monoclinic system (C2/c space group), of observed interplanar spacing -values from the diffractograms show that refined lattice parameters are (2) Å, (2) Å, (4) Å, and (3)°. From these, it is found that [C6H9N2]2CuCl4 is in a good agreement with the literature values [11].

3.2. Infrared and Raman Vibrational Study

Figures 3(a) and 3(b) show infrared (IR) and Raman spectra, respectively, of the reported compound at room temperature. A detailed assignment of all the bands is difficult, but we can attribute some of them by comparison with similar compounds [1215]. The assignments are listed in Table 1.

The principal bands are assigned to the internal modes of organic cation. The C=C bands exhibit torsion vibration at 425 cm−1 in IR and 430 cm−1 in Raman the stretching vibration appears at 1624 and 1658 cm−1 in IR. The bands observed at 762, 940, 961, 989 and 840, 860, 962, and 984 cm−1 in Raman and IR, respectively, are attributed to CH wagging modes. The CH stretching vibrations are observed at 2362, 2956, and 3010 cm−1 in IR. The bands observed at 3094 and 3166 cm−1 in IR are associated to the asymmetric NH stretching out of plane those observed at 3366 and 3732 cm−1 in IR are ascribed to asymmetric NH stretching in plane.

A free ion under symmetry has four fundamental vibrations the symmetric stretching mode , the bending mode observed at 77 cm−1. The average frequencies observed at 267, 248 and 136, 118 cm−1 are ascribed to the asymmetric stretching mode and asymmetric bending mode , respectively. All the modes are Raman active, whereas only and are active in the IR [16]. Meanwhile, the vibrations of the in this structure are shown in Raman spectra. The mode appears as one strong band at 81 cm−1. The mode appears as one strong band at 279 cm−1 and one shoulder at 223 cm−1. The mode is observed as one strong band at 180 and one shoulder at 135 cm−1. The higher frequency value obtained for the , and modes than those in a free ion confirms the distortion of CuCl4 tetrahedra as is evident from different Cu–Cl bond lengths determined by the X-ray diffraction study [11]. The presence of hydrogen bonds and Jahn-Teller effect may be the reason for the observed distortion in the CuCl4 tetrahedra in this structure [11].

The infrared and Raman studies confirm the presence of the organic group C6H9N2 and the tetrahedral anion .

3.3. D.S.C Analysis

Results of differential scanning calorimetry measurements, recorded in the temperature range 220–475 K, are presented in the thermogram shown in Figure 4. It shows the presence of an endothermic peak located at 436.71 K corresponding to the fusion of the compound [C6H9N2]2CuCl4.

3.4. Electrical Properties

Complex impedance spectroscopies have been used to study electrical properties of the [C6H9N2]2CuCl4 compound. It is useful for analyzing the electrical processes occurring in a compound on the application of a small ac signal as input perturbation. In our case the resultant response (when plotted in a complex plane shown in Figures 5(a) and 5(b)) appears in the form of depressed semicircles. The observed depressed semicircles can usually be reproduced with an equivalent circuit formed by parallel combination of resistance polarization () and constant phase elements (CPEs) [2]. 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.

The impedance of the equivalent circuit shown in Figure 6 can be expressed as follows: where The curves of and versus frequency at several temperatures are fitted by (2) and (3), respectively. In Figures 7(a) and 7(b) are represented and versus frequency at 381 K, respectively, together with fits to the equivalent circuit presented 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.

The resistance , the capacitance , and have been simulated using a mean square method which consists to minimize the difference between the experimental and calculate data. The values of the equivalent circuit elements have been listed in Table 2.

The temperature dependences of the fitted parameters   () show that the constant phase elements (CPEs) represent a leaking (nonideal) capacitor as it contains both imaginary and real parts and constitute 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 literature that the dispersion in the boundaries and in the grain bulk interior has a capacitance value in the range of nF and pF, respectively, [1720]. In our case the capacitance values () vary between 50.584 pF and 234.86 pF in the temperature range 356 K–398 K. This implies that the single semicircular response is from grain interiors, which is expected from the sample where no grain boundaries are involved.

Knowing the bulk resistance, obtained from equivalent circuit parameter values, and the dimensions of the sample, the conductivity () has been calculated at each temperature by means of the relation: where represents the sample geometrical ratio. The temperature dependence of the conductivity is represented in the form of versus 1000/T plot in Figure 8. An Arrhenius type behaviour, , is shown in the temperature range  K, where is the dc conductivity at temperature , the preexponential factor, the Boltzmann’s constant, and is the thermal activation energy for the ion migration. The activation energies obtained from the slopes of [C6H9N2]2CuCl4 sample are Ea = 1.347 eV. The variation of electrical conductivity versus the angular frequency of [C6H9N2]2CuCl4 sample at various temperatures is plotted in Figure 9. The electric response of the low conductivity materials is usually characterized by the well-known universal dynamic response [21]. Consider where is the dc conductivity in the particular range of temperature, is a temperature dependent parameter, and is the temperature-dependent exponent in the range of [2123]. The exponent represents the degree of interaction between mobile ions with the lattices around them and the prefactor exponent determines the strength of polarizability.

In order to explain the behaviour of with both frequency and temperature, different theoretical models have been proposed to correlate the conduction mechanism of AC conductivity with behaviour. According to quantum mechanical tunneling model [22, 24, 25], the exponent is temperature independent. The large overlapping polaron model [25] predicted that decreases with increasing temperature up to a certain temperature degree, after which, it begins to increase with further rise in temperature. The small polaron tunneling model and the classical hopping model over a barrier separating two sites [26] predicted that decreases with increasing the temperature.

The temperature dependence on the fitted frequency exponent is shown in Figure 10. The values decreases with increasing the temperature. Comparing our results of shown in Figure 10 with the abovementioned models, it can be concluded that the classical hopping or small polaron tunneling model is the most probable conduction mechanism for the [C6H9N2]2CuCl4 crystal.

4. Conclusions

The diffraction of X-rays powder shows that the compound [C6H9N2]2CuCl4 crystallizes in the monoclinic system (space group C2/c, ) with the following unit cell dimensions: (2) Å, (2) Å, (4) Å, and (3)°, similar of the X-ray diffraction data collection described by Al-Far and Ali.

The differential calorimetric analysis shows the presence of only one endothermic peak located at 436.71 K which corresponds to the fusion of material. The analysis by infrared and Raman spectroscopy made it possible to check the presence of the functional groupings, of the ionic groupings of material.

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 [C6H9N2]2CuCl4. The variations of the values of elements of this equivalent circuit with temperature confirmed the result detected by DSC and dielectric measurements. The temperature dependence of conductivity was analyzed using the Arrhenius approach.

The AC conductivity is analyzed by Jonscher's law, and suggests that the classical hopping or small polaron tunneling model is the most probable conduction mechanism for the [C6H9N2]2CuCl4 crystal.