[(CH3)3N(CH2)2Br]2[CoBr4] halogenometallate complex: crystal structure, high-temperature reversible phase transition, electrical and optical properties

Imen Dakhlaoui, Karim Karoui, Fadhel Hajlaoui, Nathalie Audebrand, Thierry Roisnel, Fathi Jomni

► To cite this version:

Imen Dakhlaoui, Karim Karoui, Fadhel Hajlaoui, Nathalie Audebrand, Thierry Roisnel, et al.. [(CH3)3N(CH2)2Br]2[CoBr4] halogenometallate complex: crystal structure, high-temperature reversible phase transition, electrical and optical properties. Journal of Molecular Structure, 2021, 1231, pp.129684. 10.1016/j.molstruc.2020.129684. hal-03190402

HAL Id: hal-03190402

https://hal.science/hal-03190402v1

Submitted on 24 Apr 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
[(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] halogenometallate complex: crystal structure, high-temperature reversible phase transition, electrical and optical properties

Imen Dakhlaoui a, Karim Karoui b*, Fadhel Hajlaoui c, Nathalie Audebrand d, Thierry Roisnel d,
Fathi Jomni a

a-Université de Tunis El Manar, Laboratoire LMOP, LR99ES17, El Manar, 2092 Tunis, Tunisia

b Laboratoire des caractérisations spectroscopiques et optique des matériaux, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisia

c Laboratoire Physico-chimie de l'Etat Solide, Département de Chimie, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisia

d Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000 Rennes, France

*Corresponding Authors' E-mail: karouikarim36@yahoo.com

**Abstract**

A new Co(II)-based organic-inorganic hybrid compound, [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄], was synthesized by slow evaporation at room temperature. The compound was characterized by single crystal X-ray diffraction, thermal analysis, optic and dielectric measurements. [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] belongs to the monoclinic P2₁/c space group at 150K. Molecular structure of the compound consists of discrete [CoBr₄]²⁻ anions and (2-bromoethyl) trimethylammonium cations linked together via non-covalent interactions including C-H···Br hydrogen bonding and weak halogen···halogen interactions which lead to a three-dimensional supramolecular network. The thermal analysis reveals a reversible phase transition at 366/337 K upon heating/cooling respectively, which is giving rise to a thermal hysteresis of ~26 K. In addition, the UV/Vis optical spectrum indicates that the material has an indirect band gap of 3.7 eV which can be used in the application of the semiconductor materials. The electrical properties as a function of frequency and temperatures were studied and confirmed the phase transition showed in the thermal analysis. The equivalent circuit was determined to separate grain and grain boundaries response and determine the grain resistivity. The modulus studies confirm the presence of grain and grain boundaries. The AC conductivity was analyzed by Jonscher power law and showed that the
conduction mechanisms are well described by the small polaron tunnel (SPT) and overlapping large polaron tunneling (OLPT) models in the phases I and II, respectively. The variation of $\sigma_g$, $f_p$ and $\sigma_{dc}$ from the simulation of Nyquist diagram, Modulus and AC conductivity proves the transition in the DSC and allows to determine the activation energies for the two phases. These results will provide positive clues to explore the semiconductor and electrical properties of hybrid halogenometallate complexes.

**Keywords: Hybrid halogenometallates, Crystal structure, Phase transition, Optic, Electric properties.**

# I. Introduction

Organic-inorganic hybrid phase transition materials, with a significant number of promising applications such as energy storage, data communication, and thermal capacity, have attracted special attention, owing to their great academic value and rich electric, magnetic and optical properties [1-6]. Although many researchers have been focused on the exploration of phase transition properties, it is still challenging to predict the formation of such materials combining excellent properties, such as the thermal capacity, photoelectric switches, magnetic, ferroelastic and nonlinear optical (NLO) properties [7-13]. Preparing new materials is crucial not only for the theoretical illustration of structure-property relationship but also for the exploration and exploitation of novel properties. One of the most viable approaches is to construct solid-to-solid phase transition materials induced by temperature, of which the structures change from the noncentrosymmetric to centrosymmetric phases. Fortunately, organic-inorganic hybrids have the advantage of adjustability to design and synthesize new phase transition materials with anticipated physical properties such as sensors and NLO responses [14-18]. Due to the advantages of convenient processing, light weight, diverse structures, discrete metal-halides anions and organic cations, the hybrid molecular systems are relatively more interesting [19-22]. The flexibility of organic structural moieties would enable remarkable structural changes, while inorganic frameworks account for superior electronic and optical properties.

Recently, Gao et al. employed an organic cation capable of forming halogen···halogen interactions into an $A_2M^{II}X_4$ type structure to obtain a ferroelastic transition with a critical temperature of 387 K [23]. So far, many efforts have been devoted to improve hybrid halogenometallates ferroelastic compounds [24-25]. Other hybrid compounds, for examples $[Br(CH_2)_2NH_3]_2PbI_4$ [26] $[Br(CH_2)_2N(CH_3)_3]_2M^{II}Br_4$, (M = Cd or Cu) [27-28], $[(CH_3)_3NCH_2Cl]_2CoCl_4$ [29], $[C_6H_{15}ClNO]_2CoX_4$ (X = Cl or Br) [30] $[F(CH_2)_2NH_3]_2CdCl_4$ [31] have also been found to be
order-disorder phase transition materials with high Curie temperatures, ferroelectric, dielectric behaviours and optical properties.

The aim of this paper is to pursue our aim of exploration of organic-inorganic halogenometallates materials with captivating properties by the synthesis of a new compound [(2-bromoethyl)trimethylammonium]CoBr4. The title compound adopts a 3D-type structure, displays a phase transition at T = 366 K which is confirmed by DSC measurements. Besides, it exhibits a semiconducting behaviour with a band gap (Eg) of ~ 3.7 eV. This study offers an alternative pathway for designing new multifunctional A2MX4 hybrid materials with expected semiconducting behaviour.

## II. Experimental section

### II.1. Materials

Cobalt(II) bromide (CoBr2), hydrobromic acid HBr (48%), (2-bromoethyl)trimethylammonium bromide Br(CH2)2N(CH3)3Br were purchased from Sigma-Aldrich and used without further purification.

### II.2. Synthesis of [(CH3)3N(CH2)2Br]2[CoBr4]

CoBr2 (1 mmol) and (2-bromoethyl) trimethylammonium bromide Br(CH2)2N(CH3)3Br (2 mmol) were mixed in hydrobromic acid aqueous solution (20 mL). After being stirred for 1 h at room temperature, the reaction solution was evaporated slowly at room temperature for several days. The blue prismatic single crystals of [(CH3)3N(CH2)2Br]2[CoBr4] were collected, washed with a small amount of distilled water and dried in open air. Yield: 85.2% based on Co.

### II.3. Crystal structure

Single-crystal X-ray diffraction data of [(CH3)3N(CH2)2Br]2[CoBr4] were collected at 150 K on a D8 VENTURE Bruker AXS diffractometer equipped with a (CMOS) PHOTON 100 detector and processed with the APEX 3 program suite [32]. The used X-ray wavelength was the Mo-Kα radiation (λ = 0.71073 Å). Frames integration and data reduction were carried out with the program SAINT [33]. The program SADABS [34] was employed for multiscan-type absorption corrections. Structures were determined by dual space methods using the SHELXT program, [35] and refined with full-matrix least-square methods based on F2 (SHELXL-2014) [36] through the WinGX program suite [37]. All non hydrogen atoms were refined with anisotropic atomic displacement parameters. Hydrogen atoms were positioned geometrically and allowed to ride on their parent atoms with constrained thermal parameters. The graphics program Diamond [38] was used to draw the structures. Crystallographic data, details on data collections and refinement parameters of the crystal structures are listed in Table 1. The relevant selected bond lengths and angles are summarized Tables S1-S2.
<table><caption>**Table 1.** Crystallographic data and structure refinement parameters for [(CH<sub>3</sub>)<sub>3</sub>N(CH<sub>2</sub>)<sub>2</sub>Br]<sub>2</sub>CoBr<sub>4</sub>.</caption><tr><td>Empirical formula</td><td>C<sub>10</sub>H<sub>26</sub>N<sub>2</sub>Br<sub>6</sub>Co</td></tr><tr><td>Formula weight (g.mol<sup>-1</sup>)</td><td>712.72</td></tr><tr><td>Temperature (K)</td><td>150</td></tr><tr><td>Crystal system</td><td>Monoclinic</td></tr><tr><td>Space group</td><td>P2<sub>1</sub>/c</td></tr><tr><td>a (Å)</td><td>19.0838 (15)</td></tr><tr><td>b (Å)</td><td>19.0525 (19)</td></tr><tr><td>c (Å)</td><td>24.541 (2)</td></tr><tr><td>α (°)</td><td>90</td></tr><tr><td>β (°)</td><td>94.090 (3)</td></tr><tr><td>γ (°)</td><td>90</td></tr><tr><td>V (Å<sup>3</sup>)</td><td>8900.2 (13)</td></tr><tr><td>Z</td><td>16</td></tr><tr><td>λ (MoKα) (Å)</td><td>0.71073</td></tr><tr><td>ρ<sub>cal</sub> (g.cm<sup>-1</sup>)</td><td>2.128</td></tr><tr><td>Absorption correction</td><td>Multi-scan</td></tr><tr><td>μ (mm<sup>-1</sup>)</td><td>11.54</td></tr><tr><td>Crystal size (mm<sup>3</sup>)</td><td>0.48 × 0.25 × 0.23</td></tr><tr><td>Crystal color/shape</td><td>blue/prism</td></tr><tr><td>hkl range</td><td>-24 ≤ h ≤ 21 ; -22 ≤ k ≤ 24 ; -31 ≤ l ≤ 31</td></tr><tr><td>θ range for data collection (deg)</td><td>2.14 - 27.48</td></tr><tr><td>Refinement method</td><td>Full-matrix least-squares on F<sup>2</sup></td></tr><tr><td>No. of collected reflections</td><td>73913</td></tr><tr><td>No. of independent reflections</td><td>20290</td></tr><tr><td>Observed reflections / restrains / parameters / refined parameters</td><td>11826 / 0 / 451</td></tr><tr><td>R<sub>int</sub></td><td>0.148</td></tr><tr><td>F(000)</td><td>5392</td></tr><tr><td>Goodness of fit</td><td>1.07</td></tr><tr><td>Transmission factors</td><td>Tmin = 0.2491, Tmax = 0.7456</td></tr><tr><td>R indices</td><td>R1 = 0.087, wR2 = 0.201</td></tr><tr><td>Largest difference map hole (e Å<sup>-3</sup>)</td><td>Δρ<sub>max</sub> = 2.27, Δρ<sub>min</sub> = -2.10</td></tr><tr><td>CCDC number</td><td>2011128</td></tr></table>

## II.4. Thermal measurements

DSC measurements were performed with a NETZCSCH DSC 200 F3 instrument (Pt crucibles, Al<sub>2</sub>O<sub>3</sub> as a reference) under nitrogen flux at a heating/cooling rate of 5 K min<sup>-1</sup> over the temperature range of 240-435 K.

## II.5. Optical and electrical measurements
The UV-visible spectra of [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] compounds were obtained using a conventional UV visible absorption spectrometer "Shimadzu UV-3101PC" in the 200-800 nm wavelength range. The electrical measurements of the real and imaginary components of the impedance parameters (Z' and Z'') were performed in the 10⁻¹ – 10⁶ Hz frequency range with a Solartron1260A impedance analyzer coupled with a dielectric interface in the 323-453 K temperature range.

### III. Results and discussion

#### III.1. Crystal structure

To further understand the details of the structure of [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄], single-crystal X-ray diffraction data were collected at 150 K. The results revealed that the compound crystallizes in a monoclinic P2₁/c space group with cell parameters: *a* = 19.0838(15) Å, *b* = 19.0525(19) Å, *c* = 24.541(2) Å, β = 94.090(3)°, and V = 8900.2(13)Å³ (Table 1). The asymmetric unit consists of four [CoBr₄]²⁻ anions and eight symmetrically independent [Br(CH₂)₂(CH₃)₃N]⁺ cations (Figure 1).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** The asymmetric unit of [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄]. Hydrogen bonds are shown with red dotted lines.</figcaption></figure>
The central metal cations are coordinated to four Br ions in the shape of distorted tetrahedron geometry. The Co-Br bond distances vary from 2.392 (2) Å to 2.431 (2) Å. The Br-Co-Br bond angles range between 106.31 (1) and 112.27 (9) °, which are slightly deviated from the ideal values of 109.5 ° (Table S1). The geometrical features of CoBr₄ entities agree well with those reported for other cobalt (II) salts containing isolated tetrahedra [39].

The [Br(CH₂)₂(CH₃)₃N]⁺ cations neutralize the negative charge of the anionic part. The C–C and N–C bond lengths vary from 1.440 (2) to 1.544 (2) Å. The selected bond lengths and bond angles within the cationic part indicate that the N-C, C-C distances and the N-C-C, C-N-C, Br-C-C angles are comparable with those observed in other similar hybrid metal-halides with the same organic cations [40].

The crystal structure of [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] contains infinite chains of [CoBr₄]²⁻ anions tetrahedra along *a*-axis separated by (2-bromoethyl)trimethylammonium cations leading to layers that are parallel to (*ac*) plane (Figure 2).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** The packing structure of [(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] viewed along *b*-axis. Hydrogen bonds are shown with red dotted lines.</figcaption></figure>
The terminal Br atoms of the $[Br(CH_2)_2(CH_3)_3N]^+$ cations participate in building intermolecular hydrogen bonds (C29···Br19 and C18···Br21), in which the distances C···Br fall in the range of 3.731 (16) - 3.846 (1) Å, and the C-H···Br bond angles range from 134 to 146°. Meanwhile, the four Br atoms of $[CoBr_4]^{2-}$ anions serve as acceptors and form numerous C-H···Br hydrogen bonds with the organic cations creating a 3D supramolecular assembly architecture (Figure 2). The hydrogen bonding data are summarized in Table S2. It is worth mentioning that no typical halogen bonds contact below 4 Å was found for terminal Br atoms in the compound. The short intermolecular Br···Br contact of 4.140(3) Å between Br2 atom and Br23 atom, leads to the formation of weak halogen bonding interactions [41].

The crystal structure of $[(CH_3)_3N(CH_2)_2Br]_2[CoBr_4]$ is similar to those of the Cu(II) and Cd(II) templated by (2-bromoethyl)trimethylammonium itself [40]. The only differences are observed in the direction of the charged-layers alternation, in the hydrogen bonds and the halogen interactions established between organic cations and metal tetrahedra. Consequently, these differences are likely due to the size and the shape of the metal ions involved in these structures.

### III.2. Thermal analysis

Differential Scanning Calorimetry (DSC) experiments were recorded on heating and cooling in the 240–435 K temperature range with a heating rate of 5 °C/min in order to identify the phase transitions. (Fig. 3) exhibits the thermogram obtained by DSC.

<figure><img src="image_4.png" /></figure>
**Figure 3.** Differential scanning calorimetry of [(CH₃)₃N(CH₂)Br]CoBr₄ compound on heating and cooling.

We notice only one endothermic peak at 366K. This transition is reversible and an exothermic peak is observed at 337K on cooling. The entropy value calculated for this transition is ΔS=28.46 J mol⁻¹ K⁻¹ suggesting that this transition can be classified as an order-disorder type [42]. The sharp peak-like anomalies and large thermal hysteresis (~26K) are indicative of a first-order phase transition [43-45].

### III.3. Optical properties

The absorption spectrum between 200 and 800nm excitation length of [(CH₃)₃N(CH₂)Br]CoBr₄ at room temperature is shown in **Fig.4**.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Variation of the absorbance with wavelength of [(CH₃)₃N(CH₂)Br]CoBr₄</figcaption></figure>

The excitation peaks are observed at 297, 394, 429, 466 and 590 nm. The higher absorption observed at lower energy with an intense peak at 297nm in the ultraviolet spectral region suggests a
direct band gap and indicates the transition between the conduction and valence bands [46, 47]. The other peaks are attributed to the excitonic transition. Experimentally, the optical energy gap of the title compound has been determined using the Tauc plot [48]:

$$ (\alpha h\nu)^2 = B (h\nu - E_g) \quad (1) $$

where B is a constant, $E_g$ is the optical band gap and $\alpha$ is the optical absorption coefficient deduced from the absorbance using the relation:

$$ \alpha = 2.303 A / d \quad (2) $$

width A is the absorption and d the thickness of the samples.

In fact, the optical energy gap is estimated from the intersection of the extrapolated linear part of the $(\alpha h\nu)^2$ vs $h\nu$ curve with the energy axis ($h\nu$) (Fig.5).

<figure><img src="image_6.png" /><figcaption>Figure 5. $(\alpha h\nu)^2$ vs. $h\nu$ plots of [(CH$_3$)$_3$N(CH$_2$)$_2$Br]CoBr$_4$.</figcaption></figure>

The experimental $E_g$ value was found to be 3.7 eV. This value shows that the material can be classified as semiconductor [49]. The band gap value of [(CH$_3$)$_3$N(CH$_2$)$_2$Br]$_2$[CoBr$_4$] was found to be comparable to those of (demethyltropinone)$_4$Pb$_3$Br$_{10}$ ($E_g$=3.23 eV) [50], and [(CH$_3$)$_3$N(CH$_2$)$_2$Br]$_2$[CdBr$_4$] ($E_g$=4.96eV) [51] but obviously lower than those of (C$_3$H$_{12}$N$_2$O)CdCl$_4$ ($E_g$=4.40 eV) [52], [C$_7$H$_{16}$N$_2$][CuCl$_4$] ($E_g$=4.61eV) [53], [C$_7$H$_{16}$N$_2$][ZnCl$_4$] ($E_g$=4.69eV) [54].
### III.4. Electrical properties

#### III.4.1. Impedance measurement

The complex impedance is a useful technique to analyze electrical properties of materials and separate the contributions from various processes such as grain, grain boundary, and the electrode/material interface effect in the specified frequency domain. Complex impedance spectra of the [(CH₃)₃N(CH₂)Br]CoBr₄ compound recorded at different temperatures (353–463 K) are shown in Fig. 6 a and b.

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Complex impedance for different temperatures, **a** (353K-393K) and **b** (403K-453K), with electrical equivalent circuit of [$(CH_3)_3N(CH_2Br)CoBr_4$.</figcaption></figure>

The dielectric response illustrated in a complex impedance plot enables to separate two
contributions which appear in the form of semicircles arcs. These plots are characterized by the
presence of two semi-circles associated with grain and grain boundary. In general, the peak of
frequency for the contribution of grain boundaries is much lower than that for grains due to their
large resistance and capacitance. The center of each observed semi-circle is shifted down to the real
axis indicating a non-Debye type relaxation process and confirms that the complex impedance
curves of these compounds obeyes the Cole–Cole formalism. Furthermore it also manifests that
there is a distribution of relaxation time instead of a single relaxation time in the material [55, 56].

The complex impedance spectra are modeled by the same equivalent circuit which is formed by two
cells connected in series before the transition temperature T= 366K (Fig. 6a), the first one being
formed by a parallel combination of resistance, capacity and fractal capacity constant phase element
(CPE) and the second one being formed by resistance parallel to a fractal capacity. After this
transition, an other cell (Fig. 6b) appears which is formed by resistance parallel to a fractal
capacity. The resistance (Rg) values obtained from the data fit of the impedance spectra were used
to trace the variation of the electrical conductivity for the grain as where defined by the following relation:

$$ \sigma_g = \frac{e}{sR_g} \quad (3) $$

where e is the thickness, s is the cross-sectional area. In order to confirm the existence of the phase transition the plot of the variation of Ln (σg) as a function of 1000/T has been drawn (Fig. 7).

<figure><img src="image_9.png" /><figcaption>Figure 7. Variation of ln (σg) versus 1000/T for [(CH3)3N(CH2)Br]CoBr4</figcaption></figure>

The change of the slope is observed at T=363 K corresponding to the transition detected by the calorimetric study. From the linear fitting, the values of the activation energy are as follow: EaII = 0.43eV and EaI=0.12 eV.

### III.4.2. Complex modulus analysis

The complex modulus enables to study space-charge relaxation phenomena and to distinguish the microscopic process responsible for the localized dielectric relaxations and long-range conduction. The variation of the imaginary part of the electrical modulus with frequency is illustrated in Fig. 8 a and b.
<figure><img src="image_10.png" /><figcaption>**Figure 8 (a and b).** Frequency dependence of the imaginary part of the electric modulus at several temperatures of [(CH<sub>3</sub>)<sub>3</sub>N(CH<sub>2</sub>)Br]CoBr<sub>4</sub>.</figcaption></figure>
At low frequencies, the values of M" are close to angular zero, which confirms that the effect of the electrode can be neglected [57]. We can observe two peaks, the first one is at in low frequency due to the grain boundaries effect then the second peak in the high frequency is well attributed to the grain effect. When the temperature increases, these peaks shift to higher frequencies evoking an ionic conducting character of the sample.

The adjustment function of Kohlrausch-Williams-Watts (KWW) allowing a direct analysis in the frequency domain was modified by Bergman [58]. The imaginary part of the M"(ω) has been approximated as [59]:

$$ M'' = M''_{1max} / ((1 - \beta_1) + (\beta_1 / 1 + \beta_1))[(\omega_{1max} / \omega + (\omega / \omega_{1max})\beta_1)] + [M''_{2max} / ((1 - \beta_2) + (\beta_2 / 1 + \beta_2)) + (\beta_2 / 1 + \beta_2))[(\omega_{2max} / \omega) + (\omega / \omega_{2max})\beta_2]] \quad (4) $$

where ωmax is the frequency corresponding to the maximum modulus M", β reflects the importance of coupling between mobile ions in the conduction process and is positioned in the (0–1) range. The imaginary M" part of modulus, modeled with the Bergman function shows a well agreement between the theoretical and the experimental data Fig.8a and b. The maximum in the well-defined peak of the imaginary parts of M curves corresponds to the bulk relaxation frequency fp expressed as:

$$ f_p = Ae^{-E_a/K_b} \quad (5) $$

where A is the pre-exponential factor, E_a is the activation energy and K_b is the Boltzmann's constant. A change of the curve slope is noted near to the transition at 366K, which confirms the DSC study Fig.9.
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Temperature dependence of the relaxation frequency of [(CH₃)₃N(CH₂)Br]CoBr₄</figcaption></figure>

The calculated activation energy from linear fit to the data points are E_a(II) = 1.2 eV and E_a(I) = 0.25eV.

### III.4.3. AC electrical conductivity

The frequency variation of AC conductivity for the [(CH₃)₃N(CH₂)Br]CoBr₄ compound is shown in **Fig.10**.
<figure><img src="image_12.png" /><figcaption>Figure 10. Frequency dependence of the AC conductivity at various temperatures for [(CH₃)₃N(CH₂)Br]CoBr₄.</figcaption></figure>

This variation obeys to the Jonscher power law given by the following equation [60]:

$$ \sigma_{ac} = \sigma_{dc} + A\omega^s \quad (6) $$

where $\sigma_{dc}$ is the conductivity at low frequencies, A is a parameter dependent on temperature and s is the power law exponent with values in the range $0 < s < 1$.

The AC conductivity increases with the increase in frequency. The spectra of the conductivity can be described by two regions (i) at low frequency where the conductivity has a plateau ($\sigma_{dc}$) defined by the first term of the Jonscher equation, then (ii) at high frequency, where the conductivity has an asymptotic form (or dispersive regime) that can be defined by the second term of equation (6) where the charge species that have been accumulated at the grain boundaries have sufficient energy to jump over the barrier on increasing temperature. The fitted values of the (dc) conductivity are versus 1000/T show in Fig 11.
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Variation for universal exponents s versus 1000/T of [(CH₃)₃N(CH₂)Br]CoBr₄.</figcaption></figure>

One changes of the slope is found at T=363K, which confirms the obtained result by the variation of σg and fp and the calorimetric study. The calculated activation energies in the two phases from a linear fit to the data points are Ea(II)=0.29 eV and Ea(I)=0.11 eV. The activation energies values calculated from the conductivity and modulus spectra in the two phases I and II are different, suggesting that the transport is not through ion hopping mechanism in the investigated sample [61]. In order to identify the conduction mechanism for the hybrid compound [(CH₃)₃N(CH₂)Br]CoBr₄, we carried out a study of the variation of the exponent (s) as a function of the temperature Fig.12.
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Variation for universal exponents s as a function of temperature of [(CH₃)₃N(CH₂)Br]CoBr₄.</figcaption></figure>

The variation of (s) reflects the existence of two conduction models for two phases. In phase I, the values of (s) increase with the increasing temperatures. Therefore, the model of small polaron tunnel (SPT), is in good agreement with this variation. In phase II the values of (s) decreases with increasing temperature up to a minimum and then increases. This behavior indicates that the overlapping large polaron tunneling (OLPT) model is suitable for the conduction process [62].

## Conclusion

[(CH₃)₃N(CH₂)₂Br]₂[CoBr₄] was synthesized and physically characterized by multiple techniques. Compound belongs to the monoclinic system space group P2₁/c. The crystal packing is guided by C-H···Br hydrogen bonds and by weak means of halogen···halogen interactions. These interactions contribute to the overall stabilization of the structures which exhibits a 3D network. The optical properties show that the sample has a semiconducting character where the gap energy is 3.7 eV. The thermal study of the hybrid halogenometallate complex shows that this compound exhibits a reversible phase transition at 366K. The dielectric and the electrical properties were studied as
function of frequencies and temperatures. The Nyquist diagram show the presence of the grain and grain boundaries and their simulation allows determining the resistivity of the materials. The equivalent circuit was determined. The variation of σg, fP and σdc confirms the phase transition detected by the thermal analysis. The activation energies for the two phases was determined and discussed. The conduction mechanism in the two phases was studied based on the Elliot theory.

REFERENCES

[1] F. Hao, C. C. Stoumpos, R. P. H. Chang, M. G. Kanatzidis, J. Am. Chem. Soc., 2014, **136**, 8094–8099.

[2] D. A. Egger, A. M. Rappe, L. Kronik, Acc. Chem. Res., 2016, **49**, 573–581.

[3] A. Mardana, S. Ducharme, S. Adenwalla, Nano Lett., 2011, **11**, 3862-3867.

[4] H. Wu, N. Reeves-McLaren, J. Pokorny, J. Yarwood, A. R. West, Cryst. Growth Des., 2010, **10**, 3141-3148.

[5] P. P. Shi, Y. Y. Tang, P. F. Li, W. Q. Liao, Z. X. Wang, Q. Ye, R. G. Xiong, Chem. Soc. Rev., 2016, **45**, 3811-3827.

[6] S. Burger, M. G. Ehrenreich, G. Kieslich, J. Mater. Chem. A, 2018, **6**, 21785–21793.

[7] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391-392.

[8] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759-765.

[9] E.K.H. Salje, Annu. Rev. Mater. Res. 42 (2012) 265-283.

[10] W.Y. Zhang, Q. Ye, D.W. Fu, R.G. Xiong, Adv. Funct. Mater. 27 (2017), 1603945.

[11] M.D. Hollingsworth, M.L. Peterson, J.R. Rush, M.E. Brown, M.J. Abel, A.A. Black, M. Dudley, B. Raghothamachar, U. Werner-Zwanziger, E.J. Still, Cryst. Growth Des. 5 (2005) 2100-2116.

[12] H. Yu, C.-C. Chung, N. Shewmon, S. Ho, J.H. Carpenter, R. Larrabee, T. Sun, J.L. Jones, H. Ade, B.T. O'Connor, F. So, Adv. Funct. Mater. 27 (2017), 1700461.

[13] T. Khan, M.A. Asghar, Z. Sun, A. Zeb, C. Ji, J. Luo, J. Mater. Chem. C 5 (2017) 2865-2870.

[14] K. Karoui, A. Ben Rhaiem, K. Guidara, Ionics 17 (2011) 517-525.

[15] (a) Q. Gu, M. Yuan, S. Ma, G. Sun, J. Lumin. 192 (2017) 1211-1219; (b) W. Xu, C. He, C. Ji, S. Chen, R. Huang, R. Lin, W. Xue, J. Luo, W. Zhang, X. Chen, Adv. Mater. 28 (2016) 5886-5890.

[16] (a) N. Mahfoudh, K. Karoui, M. Gargouri, A. Ben Rhaiem, Appl. Organomet. Chem. 34 (2020) 5404-5411; (b) F. Hajlaoui, I. Ben Hadj Sadok, H. Alamri Aeshah, N. Audebrand, T. Roisnel, N. Zouari, J. Mol. Struct. 1182 (2019) 47-53; (c) I. Ben Hadj Sadok, F. Hajlaoui, H. Ben
Ayed, N. Ennaceur, M. Nasri, N. Audebrand, T. Bataille, N. Zouari, J. Mol. Struct. 1167 (2018) 316-326.

[17] F. Hajlaoui, N. Audebrand, T. Roisnel, N. Zouari, Appl. Organomet. Chem. 34 (2) (2019) 5293.

[18] B. Saparov, D.B. Mitzi, Chem. Rev. 116 (2016) 4558-4596.

[19] V.P. Ananikov, Nanomaterials 9 (2019) 1197-1202.

[20] W.J. Peveler, J.C. Bear, P. Southern, I.P. Parkin, Chem. Commun. 50 (2014) 14418-14420.

[21] (a) S. Pandey, S.B. Mishra, J. Sol. Gel Sci. Technol. 59 (2011) 73-94; (b) M. Faustini, L. Nicole, E. Ruiz, Hitzky, C. Sanchez, Adv. Funct. Mater. 28 (2018) 1704158.

[22] (a) W. Sun, J. Shi, C. Chen, N. Li, RSC Adv. 8 (2018) 10040e10056; (b) C. Sanchez, B. Juli\_an, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559-3592.

[23] J.-X. Gao, X.-N. Hua, P.-F. Li, X.-G. Chen, W.-Q. Liao, J. Phys. Chem. C 2018, **122**, 23111-23116.

[24] C.-R. Huang, X. Luo, W.-Q. Liao, Y.-Y. Tang, R.-G. Xiong, Inorg Chem., 2020, **59**, 829-836.

[25] X.-G. Chen, Y.-Z. Zhang, D.-S. Sun, J.-X. Gao, X.-N. Hua, W.-Q. Liao, Dalton Trans., 2019, **48**, 11292-11297.

[26] S. Sourisseau, N. Louvain, W. Bi, N. Mercier, D. Rondeau, J.-Y. Buzaré, C. Legein, Inorg. Chem., 2007, **15**, 6148-6154.

[27] Y. Sui, Y.-S. Zhong, J.-J. Wang, Q. Xia, L.-J. Wang, D.-S. Liu, J. Mater. Chem. C 2019, **7**, 14294-14300.

[28] Y. Sui, D.-S. Liu, W.-T. Chen, L.-J. Wang, Y.-X. Ma, H.-Q. Lai, Y.-W. Zhou, H.-R. Wen, J. Chem. Asian, 2020, **15**, 1-7.

[29] X.-N. Hua, C.-R. Huang, J.-X. Gao, Y. Lu, X.-G. Chen and W.-Q. Liao, Dalton Trans., 2018, **47**, 6218-6224.

[30] Y. Sui, Y.-S. Zhong, W.-T. Chen, G.-X. Zhang, C.-L. Liu, D.-S. Liu, J. Phys. Chem. C 2019, **123**, 14647-14651.

[31] N. Song, S.-P. Chen, X.-W. Fan, Y.-H. Tan, W.-J. Wei, Y.-Z. Tang, ACS Omega 2020, **5**, 6773-6780.

[32] APEX3 program suite V2016.1-0, Bruker AXS Inc., Wisconsin, USA.

[33] G. M. Sheldrick, SAINT Version 8.37A, Bruker AXS Inc., Wisconsin, USA 2013.

[34] G. M. Sheldrick, SADABS version 2014/5, SADABS Bruker AXS Inc., Madison, Wisconsin, USA.

[35] G. Sheldrick, Acta Crystallogr. A 2015, **71**, 3-8.

[36] G. Sheldrick, Acta Crystallogr. C 2015, **71**, 561-563.

[37] L. Farrugia, J. Appl. Crystallogr. 2012, **45**, 849-854.
[38] K. Brandenburg, M. Berndt, Diamond (version 3), Crystal Impact, Bonn, 2001.

[39] A. Mukherjee, S. Tothadi, G. R. Desiraju, Acc. Chem. Res., 2014, **47**, 2514–2524.

[40] G. R. Desiraju, Acta Cryst. C., 2019, **75**, 1188-1189.

[41] M. B. Andrews, C. L. Cahill, Dalton Trans., 2012, **41**, 3911-3914.

[42] H. Kchaou, A. Ben Rhaiem, K. Karoui, F. jomni, K. Guidara. J. Appl. Phys. A, DOI 10.1007/s00339-015-9574-5.

[43] N. Mahfoudh, K. Karoui, K. Khirouni, A. Ben Rhaiem. Physica B: Condens. Matter., 2019, **554**, 126-136.

[44] P. P. Shi, Q. Ye, Q. Li, H. T. Wang, D. W. Fu, Y. Zhang, R. G. Xiong, Chem. Mater., 2014, **26**, 6042-6049.

[45] W. Zhang, H. Y. Ye, H. L. Cai, J. Z. Ge, R. G. Xiong, S. D. Huang, J. Am. Chem. Soc., 2010, **132**, 7300-7302.

[46] E. R. Dohner, E. T. Hoke, H. I. Karunadasa, J. Am. Chem. Soc., 2014, **136**, 1718-1721.

[47] T. Hu, M. D. Smith, E. R. Dohner, M.-J. Sher, X. Wu, M. T. Trinh, A. Fisher, J. Corbett, X.-Y. Zhu, H. I. Karunadasa, A. M. Lindenberg, J. Phys. Chem. Lett., 2016, **7**, 2258-2263.

[48] J. Tauc, Amorphous and liquid semiconductors. Plenum Press, New York (1974).

[49] S. Ilican, Y. Caglar, M. Cagla. J. Optoelectron. Adv. Mater., 2008, **10**, 2578-2583.

[50] H.-J. Li, Y.-L. Liu, X.-G. Chen, J.-X. Gao, Z.-X. Wang and W.-Q. Liao, Inorg. Chem., 2019, 58, 10357-10363.

[51] Yan Sui, Dong-Sheng Liu, Wen-Tong Chen, Liang-Jun Wang, Yu-Xiao Ma, Hui-Qi Lai, Yu-Wei Zhou, and He-Rui Wen. *Chem Asian J.* 2020, 15, 1-7.

[52] C.-F. Wang, X.-W. Fan, Y.-H. Tan, W.-J. Wei and Y.-Z. Tang, Eur. J. Inorg. Chem., 2019, 2907-2911.

[53] Ines Ben Hadj Sadok, Fadhel Hajlaoui, Karim Karoui, Nathalie Audebrand, Thierry Roisnel, Nabil Zouari. Journal of Molecular Structure 1186 (2019) 118-126.

[54] Ines Ben Hadj Sadok, Fadhel Hajlaoui, Karim Karoui, Nathalie Audebrand, Thierry Roisnel, Nabil Zouari. Journal of Physics and Chemistry of Solids 129 (2019) 71-80.

[55] M. Ram, Journal of Alloys and Compounds, 287 (2011) 244-256.

[56] S. Sen, R. Choudhary, P. Pramanik. Phys. B Condens. Matter., 2007, **387**, 56-62.

[57] B. Louati, K. Guidara, M. Gargouri, J. Alloy. Compd., 2009, **472**, 347-351.

[58] R. Bergman, J. Appl. Phys., 2000, **88**, 1356-1365.

[59] M. D. Migahed, N. A. Bakr, M. I. Abdel-Hamid, O. El-Hanafy, M. El-Nimr, J. Appl. Polym. Sci., 1996, **59**, 655-662.

[60] A. K. Jonscher, J. Phys. D: Appl. Phys., 1999, **32**, 57-70.

[61] K. Karoui, A. Ben Rhaiem, K. Guidara, Physica B, 2012, **407**, 489-493.
[62] S. Elliott, Adv. Phys., 1987, **36**, 135-217.

**Conflicts of interest**

There are no conflicts to declare.

**Appendix**

CCDC 2011128 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

**Acknowledgments**

The authors thank the University of Sfax-Tunisia for financial support and CDIFX (UMR 6226, Univ. Rennes 1) for X-ray diffraction data collections.