Elemental analysis
Figure. 1 shows the EDX spectra of doped [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O compound. EDX spectra revealed the presence of all elements of [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O compound other than hydrogen: copper, chromium, nitrogen, potassium, chloride and oxygen., which confirms that there is no loss of any integrated element during substitution, within the experimental errors. An analysis of all elements was carried out to ascertain the constituents and purity of the synthesized [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O compound. Calculated elemental analysis of K = 5.76, N = 7.77, Cu = 15.83, Ni = 5.97 and Cl = 49.77. However, the experimental values are K = 5.71, N =7.30, Cu = 16.26, Ni = 5.09 and Cl =50.13. The experimental and calculated values matched ±0.05%. The purity of the compound was >99% (Table. 6).
Crystal structure
The [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O compound crystallizes in the tetragonal system with the centrosymmetric space group P4(2)/mnm and the following lattice parameters: a = b = 7.4557(6); c = 7.9595(6) Å and β = 90°. The structure of this material consists of an alternation of wavy lines of cationic groups [(NH4)0,79/K0,21]+ and anionic groups [Cu0,71Ni0,29Cl4.(H2O)2]2-crystallographically independent (Figure. 2). The anionic part is formed by the octahedron [Cu0,71Ni0,29Cl4.(H2O)2]2-, whose copper/Nickel atom has an oxidation degree +II. Each substituted atom is coordinated by four atoms of chlorine and two oxygen atoms of water molecules (Figure. 3). The anion [Cu0,71Ni0,29Cl4.(H2O)2]2- constitutes an outer orbital complex in molecular orbital theory, it is a complex with weak field chloride ligands and strong spin complex giving rise to octahedral bursting of 3d orbitals.
The environment of copper/nickel in this compound is formed by four Cu/Ni-Cl bonds with distances of 2.2374 (7) Å and the angles Cl1-Cu/Ni-Cl2 in the order of 180°. Two Cu/Ni-O bonds with a length of 1.959 (2)Å, the angles O1-Cu/Ni-O2 with a value of 180° and O-Cu/Ni-Cl is about 90° (Table. 3 and 4). The value of the distortion indice of different bonds in the octahedron [Cu0,71Ni0,29Cl4.(H2O)2]2- is calculated by:
The calculated bond distortion indices is low (ID (Cu/Ni-Cl) = 0.023), the [Cu0,71Ni0,29Cl4(H2O)2]2- octahedron are slightly distorted at local D2h symmetry instead of the ideal octahedral symmetry Oh. It can be seen that the copper/Nichel atoms are placed approximately at positions x = 0 and z = 0 forming with the chlorines octahedron isolated from each other with a minimum distance Cu/Ni-Cu/Ni = 7.564Å, such a result leads us to think about the existence of interesting magnetic properties in this compound. Besides, this deformation is due to the constraints imposed by the hydrogen bonds in the structure. Indeed, the presence of two water ligands exerts an axial compression favoring release of the JT distortion at moderate pressures below the metallization pressure.
The cationic sub-network is a sequence of cation planes [(NH4)0.79 /K0.21]+ parallel to the c axis. This arrangement shows that these entities do not have the same orientation in the structure. The entities located at z = 0 are oriented in an opposite way to those located at z = 1(Figure.2).
The cohesion and the stability of the atomic arrangement is ensured, on the one hand by ionic bonds between the chloride ions and the potassium ions, on the other hand by hydrogen bonds N-H ... Cl between the ammonium NH4+ groups and the chlorine atoms and by O-H… Cl bonds hydrogen between chlorine atoms and water molecules (Table. 5).
Spectroscopic studies at room temperature
Figure. 4 shows the IR spectrum of [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O. The wide band at 3143 cm-1 is attributed to the elongation vibration of water molecule. The characteristic bands of the asymmetric and symmetric bending vibration of NH4+ appeared at 3040 and 2824 cm-1. The deformation mode of O-H is observed by a sharp peak around 1609 cm-1 and by a ray at 1629 cm-1 on the Raman spectrum. The fine and most intense peak observed at 1405 cm-1 is attributed to the sway mode of N-H. Finally, the corresponding line appear around and 541 cm-1 [11, 12].
Figure. 5 shows the Raman spectrum of [(NH4)0,79K0,21]2Cu0,71Ni0,29Cl4.2H2O recorded between 50 and 4000 cm-1 at room temperature, it is characterized by the presence of vibration peaks of the anionic entity. An attempt to assign the vibrational frequencies of the [Cu0.71Ni0.29Cl4. (H2O)2]2- group is made based on homologous compounds in the literature [12-14]. The peaks appearing at 116 and 225 cm-1 correspond to the elongation vibration νCu-Cl(1) and νCu-Cl(2). The 247 cm-1 band can be attributed to the δCl(2)-Cu-OH2 modes. The νRCuCl4 rotary mode appeared in the low frequency of the FT-Raman spectrum at 83 cm-1. All of the more or less intense peaks observed around 3100 cm-1 confirm the presence of H2O molecules.
Thermal analysis
The evolution of the mass (TG signal) when the sample is submitted to a constant temperature rise is shown in Figure. 6. The sample is stable up to 90 ˚C. The decomposition is triggered by an exothermic process. This process starts by the evaporation of the water molecules shown at DSC spectra at 160 ˚C and it is not thermally activated. The process shifts to higher temperature when the heating is increased. Three decomposition stages can be clearly distinguished. The first of which is exothermic and appears as a shoulder at 220 ˚C. As for the second stage, it is located around 280 ˚C. Concerning the third stage, involves the larger mass loss. It is located approximately between 300 ˚C and 400 ˚C and is also an exothermic process (Figure. 7) [15].
Electric and Dielectric properties
Figure. 8 reveals the Nyquist representations (Z’’ vs Z’) used to determine the contributions in the conduction mechanism such as that of grains and grain boundaries. All impedance spectra display some dispersion instead of a semicircle centered on the real axis, indicating relaxation of a non-Debye type [16-19]. The arc of semi-circles decreases with temperature rise, which confirms that the conduction mechanism is thermally activated for all measurement temperatures, and indicates the semi-conducting behavior of this compound [20]. Zview software was used to fit these curves. The indicated electrical equivalent circuit in the inset of Figure. 8 can be used to correlate the electrical properties of the investigated compound with its microstructure. It corresponds to the parallel combination of grain resistance Rg and a term of complex elements: constant phase elements CPE.
Figure. 9 shows the variation of the (Z′) (real part of the impedance) versus frequency for different temperatures. At lower frequencies, it’s clear that Z′ amplitude is significant and it decreases as the measurement temperature rises. This behavior indicates that the AC-conductivity increases with the frequency rise. This observation can be attributed to the release of charges as the barrier layer decreases [21, 22]. However, at high frequencies, the curves of Z′ merge which suggests the existence of the space charge region (SCR) [23, 24].
Figure. 10 illustrates the variation of Z′′ (imaginary part of the impedance) versus frequency at different temperatures. With the increase of frequency, imaginary part of impedance (Z′′) increases initially, reaches its maximum value at a specific frequency (fr) called the relaxation frequency and then decreases with frequency at all measured temperatures. The broadening of peak and its shift towards higher frequency side with temperature indicate the presence of temperature dependent electrical relaxation phenomenon, as the temperature increases, the maximum impedance and the width of the peaks decrease. These peaks shift towards higher frequencies with temperature increase.
Figures. 11 and 12 show the thermal evolutions of the real part of the permittivity e' and the imaginary part e"at selected frequencies, our measurements being conducted in the 290–470 K temperature range. An overview of the results obviously shows the existence of a dielectric anomaly around T = 433 K. This behavior confirms the anomalies detected by thermal analysis. The temperature anomalies does not change with increasing frequency. In fact, this result suggests that this sample does not present any type of dielectric relaxation in the investigated frequency range [25]. It is clear from these figures that at low temperature, the variations of ɛ′ and ɛ″ with temperature remain constant. This is due to the fact that at relatively low temperature, the charge carriers cannot orient themselves with respect to the direction of the applied field. Therefore, they possess a weak contribution to the dielectric constant and to the polarization [26]. Above, T=410 K, a rapid jump of ε' and ε" is clear. This increase is mainly due to the contribution of the migration polarization of NH4+ and K+ ions. So that, the charge carriers orient themselves conforming the direction of the electric field [27, 28]. Above the anomalies temperature, T > 433 K, the reorientational dynamics of alkyl cations is activated. The cation gets enough excitation thermal energy to be able to respond to the change in the external electric field more easily. This in turn enhances the contribution to the polarization leading to an increase of dielectric behavior.
The variation of the real and the imaginary part of dielectric permittivity (ε″) and with frequency at different temperature are showed in Figures 13 and 14. It is clear that both ε″ and tan δ decrease with increasing frequency for all the temperatures. The higher values at low frequency suggest the presence of all types of polarization (i.e., interfacial, dipolar, atomic, ionic, and electronic) at room temperature and ε″ shows a dispersive behavior. There are no relaxation peaks in the frequency range employed in this study. Besides, this dielectric relaxation is described by a non-Debye model which gives the frequency-dependent complex permittivity in the form:
The real and imaginary part of the ε* have been determined from the following relations:
While the first part in Eq. (2) is related to the thermal polarization, the second is related to the electrical conductivity.
While it merges at frequencies above 100 kHz, this behavior is due to the effect of the applied electric field which causes some disorder inside the sample. This causes an increase in the density of charge carriers and consequently an improvement in the conductivity of the material.
In order to identify the conduction mechanism in our structure; the AC conductivity of the sample has been widely studied. The figure. 15 shows the variation of the AC conductivity with the frequency at various temperatures showing. This behavior can be analyzed in the higher frequency region by the Jonscher's power law according to the following equation [22]:
σac = dc + σAc n
Each conductivity plot exhibits two distinct regions. The first is an independent low-frequency domain (the presence of plateau), which refers to direct current conductivity (rdc) because of the long range movement of free charge [29]. The second, at higher frequencies, the AC conductivity dependence occurred with changes in slope and increased gradually with an increase in frequency. In general, the effect of temperature on the exponent (n) plays a key role in the estimation of conduction mechanism in disordered materials [30, 31]. Several theoretical models of AC conductivity have been present in the literature to justify the evolution of the exponent n with temperature. These different formalisms are:
The quantum mechanical tunnel conduction model (QMT), where the exponent n is almost constant around the value 0.8 and increases a little with increasing temperature [32].
For the OLPT model, the exponent n decreases at first with the rise in temperature, reaching a minimum value and increases thereafter with temperature increase [33].
Contrarily, in the CBH model, the charge carriers hop over the potential barrier between two charged defect states and predict a decrease in the value of n with increase in temperature [34].
In the NSPT model, the exponent s which is temperature dependent increased with the increase in temperature. In the present work the variation of frequency exponent n as a function of temperature is shown in Figure. 16, which present that n increases with the increase in temperature. Concerning the values of s and its variation versus temperature, it is found that n stay less than a unity. Consequently, the non-overlapping small polaron tunneling (NSPT) appear to be the most interesting model related to the obtained results [35].
The variation of AC conductivity as a function of reciprocal temperature, at different frequencies, is given in figure 17. Three distinct regions are observed, confirming the detected anomalies by thermal analysis technique. In all phases, the activation energies decrease with the increase of frequency. This suggests that the applied field frequency improves the ionic jumps between the localized states which further confirm that the hopping conduction is the dominant mechanism [36]. Moreover, at high frequency region, the conductivity is governed by the mobility of charge carriers over short distances. While, in low frequency region, the needed energy is lower than the required one for their mobility over long distances.
From Figure. 18, the values of M’ approaches to zero at low frequencies indicating that the electrode effects have a tendency to be eliminated in modulus representation [37]. Then M’ attains a maximum below 105 Hz and decreases above 106 Hz for all temperatures. This behavior may be interpreted by the accumulation of charges at the interface between the sample and the electrode, space charge polarization [38, 39].