Ionics (2014) 20:231–241 DOI 10.1007/s11581-013-0972-4

ORIGINAL PAPER

Synthesis, crystal structure, dielectric properties, and AC conductivity of tri-tetrapropylammonium dodeca chlorobismuthate(III) W. Trigui & A. Oueslati & I. Chaabane & F. Hlel

Received: 29 April 2013 / Revised: 24 July 2013 / Accepted: 25 July 2013 / Published online: 25 August 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract The new organic–inorganic compound, tritetrapropylammonium dodeca chlorobismuthate(III), has been synthesized and characterized by single-crystal X-ray diffraction at room temperature. It is crystallized in the triclinic system (P1 space group). The atomic arrangement can be described by an alternation of two types of organic–inorganic layers stacked in [010] direction. The nature of the inorganic polyhedra distortion which can be attributed to the stereo activity of the Bi(III) lone electron pair has been studied. Regarding the differential scanning calorimetry, it disclosed one structural phase transition at T=423 (±5)K of the order– disorder type. Furthermore, the dielectric properties of the compound were studied using complex impedance spectroscopy in the frequency range 209 Hz–5 MHz and temperature range 368–458 K. The frequency-dependent AC conductivity is well described by Jonscher’s universal power law. The nature of DC conductivity variation suggests Arrhenius type of electrical conductivity. Keywords Organic–inorganic hybrid material halogenobismuthates(III) . X-ray diffraction . Differential scanning calorimetry (DSC) . Dielectric study . AC conductivity

Introduction Recently, attention has been focused on the synthesis of organic–inorganic hybrid materials, thanks to their interesting architectures and various physical properties such as electronic, optical, thermal, and catalytic ones [1–3]. These proprieties W. Trigui (*) : A. Oueslati : I. Chaabane : F. Hlel Solid State Laboratory, Sfax Faculty of Science, B.P. 802, 3018 Sfax, Tunisia e-mail: [email protected]

are closely related to the structural changes of these compounds under the effect of various factors such as temperature and chemical composition. The halogenoantimonates(III) and halogenobismuthates(III) evoke much interest because their single crystals present polar, nonlinear, or ferroic properties [4, 5]. The change in the cations dynamic state is responsible for the mechanisms of the numerous structural phase transitions found in these materials, which are classified as order–disorder [6–9]. The bismuth compounds represent a potential class of materials with unusual structural archetypes. This is due to the fact that the Bi(III) ion exhibits a great variety of coordination numbers and geometries, depending on crystal packing and hydrogen bonding, as well as halide dimensions [10–12]. Awide variety of stoichiometries belong to this class of compounds, leading to an extensive family of bismuth(III) halogen anions ([BiX4]−, [BiX5]2−, [BiX6]3−, [Bi2X9]3−, [Bi2X10]4−, [Bi2X11]5−, [Bi3X12]3−, [Bi4X18]6−, [Bi6X22]4−, and [Bi8X30]4−) [13–20], which makes the crystal chemistry of chlorobismuthate(III) extremely diverse and complex. In fact, besides the rich structural diversity displayed by these systems, some interest has been directed towards halogenobismuthate(III) compounds in combination with organic cations, due to their very interesting physical properties related to 6 s2 active lone pairs [21, 22]. In this aim, the interest in our laboratory is focused on the investigation of the chemical and physical properties of hybrid materials with organic cation tetraalkylammonium of the general formula [(CnH2n+1)4]+. Such properties are located in cations with n=1 [(CH3)4N]+ and n=2 [(C2H5)4N]]+ that were well stabilized [23–26]. In this work, the synthesis of tri-tetrapropylammonium dodeca chlorobismuthate(III) crystals is presented; a detailed single-crystal structure refinement, the differential scanning calorimetry, and dielectric study are reported and discussed.

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Experimental procedure

Table 1 Crystal data and structure refinement for [(C3H7)4N]]3Bi3Cl12 crystal

Material preparation

Formula Molecular weight (g mol−1) Color/shape Space group

[(C3H7)4N]]3Bi3Cl12 1611.4 Colorless/prismatic P1

Crystal system Temperature (K)

Triclinic 293 (2)

Unit cell dimensions a (Å) b (Å)

15.6830(7) 20.1394(10)

c (Å) α (°) β (°) γ (°) Volume (Å3) Z Absorption coefficient (mm−1) ρcalc. (g/cm3) Radiation type, λ (Å) Monochromateur Crystal size (mm3) ɵ range (°) Range of h. k. l Independent reflections Observed reflections (I>2σ(I)) Rint Refinement on Refined parameters

20.8063(9) 79.809(3) 87.818(3) 70.327(3) 6088.74(5) 2 9.183 1.738 MoKα, 0.71073 Graphite 0.35×0.30×0.22 1.34≤θ≤27.51 0→20, –23→26, –26→26 14153 12866 0.0465 F2 918

Goodness of fit R/wR Δρ (max)/Δρ (min) (e Å−3)

1.156 0.0465/0.158 1.986/–2.204

The single crystal of [(C3H7)4N]]3Bi3Cl12 compound was prepared in two steps. Firstly, Bi2O3 (0.332 g, 1 mmol) dissolved in concentrated HCl. Secondly, the obtained solution was added in molar ratio of 1:2 to (C3H7)4NCl (0.233 g, 2 mmol), respectively. The reactions sequence for the synthesis is shown in the following two equations: Bi2 O3 þ 6 HCl → 2 BiCl3 þ 3 H2 O     3 ðC3 H7 Þ4 NCl þ 3 BiCl3 → ðC3 H7 Þ4 N 3 Bi3 Cl12 After a few days, colorless prismatic crystals were obtained by slow evaporation at room temperature. Material characterization The single-crystal X-ray data collection was performed on prismatic single crystal with dimensions (0.35×0.30×0.22 mm3) chosen by optical microscope. Data were obtained using a Bruker APEXII diffractometer with monochromated graphite MoKα radiation (λ=0.71073 Å). The crystal structure was solved and refined in triclinic system with P1 space group using the SHELX-97computer program included in the WINGX soft ware package [27]. As for the bismuth and the chlorine atom positions, they were located with the Patterson method using the SHELXS-97 [28] program, while the remaining atoms were found from successive difference Fourier maps with the SHELXL-97 [29] program. The structural figures were carried out with Diamond 2.1 supplied by crystal impact [30]. Most agitated atoms, chlorine and carbon, were duplicated. The anisotropic thermal factors for the non-hydrogen and duplicated atoms were determined. The crystal data, collected reflections and parameters of the final refinement, are reported in Table 1. The differential scanning calorimetry (DSC) measurement was performed on a NETZSCH DSC 204 calorimeter, by putting the powder sample (about 15.60(1)mg) in an aluminum capsule under thermal recording conditions, whose heating speed was 5°/min and in the temperature range of 180 to 500 K. The finely grained samples were pressed into pellets of 8mm diameter and 1.2-mm thickness using a hydraulic press and then placed between two parallel platinum electrodes. The dielectric properties of this compound were measured in the frequency range 209 Hz–5 MHz with a TEGAM3550 ALF (0.10 % error) automatic bridge monitored by a microcomputer. Measurements were made over the temperature range 368–458 K. Temperature was measured using a thermocouple with 2° precision.

Results and discussion Structure description The interatomic distances and angles of inorganic and organic groups are shown, respectively, in Tables 2 and 3. The [(C3H7)4N]]3Bi3Cl12 compound is crystallized in the triclinic system (P1 space group). The asymmetric unit contains six crystallographically independent [(C3H7)4N]]+ cations, one and two half of discrete trinuclear anions [Bi3Cl12]3−. The most striking originality in this structure is the presence of the discrete [Bi3Cl12]3− trinuclear anion. A recent literature investigation shows that this anionic geometry has been reported only in α-(BETS)6Bi3Cl12 compound [31]. The packing of [(C3H7)4N]]3Bi3Cl12 compound viewed along the [001] direction (Fig. 1a) can be described by an

a

Cl62 96.10(3) 2.51(9)

Cl52 84.00(2) 2.70(4)

Cl51 2.69(5)

Cl53 85.15(1) 85.17(1) 2.73(4)

Cl6 93.61(3) 12.60(3) 2.53(10)

Cl72 98.29(2) 81.71(2) 2.69(4)

Cl71a 180.00(2) 2.71(5)

Cl71 2.71(5)

Cl61 2.53(5)

Cl12 94.06(1) 85.94(1) 2.71(4)

Cl11a 180.00(1) 2.71(4)

Cl11 2.71(4)

Symmetry codes: ‘−x, −y, −z’

Bi3Cl12(III) Bi6 Cl62 Cl62 Cl6 Cl63 Cl54 Cl56 Bi5 Cl51 Cl52 Cl53 Cl54 Cl55 Cl56

Bi3Cl12(I) Bi1 Cl11 Cl11a Cl12 Cl12a Cl13 Cl13a Bi3Cl12(II) Bi7 Cl71 Cl71a Cl72 Cl72a Cl73 Cl73a

Cl54 96.18(1) 96.91(1) 177.63(1) 2.71(4)

Cl63 93.90(2) 98.70(5) 86.60(5) 2.47(6)

Cl72a 81.71(2) 98.29(2) 180.00(1) 2.69(4)

Cl12a 85.94(1) 94.06(1) 180.00(1) 2.71(4)

Cl55 98.45(2) 176.43(1) 92.44(1) 85.43(1) 2.69(4)

Cl54 91.24(1) 162.91(5) 174.31(4) 96.2(2) 3.02(4)

Cl73 84.37(1) 95.63(2) 94.44(2) 85.56(2) 2.72(5)

C1l3 92.50(2) 87.50(1) 82.06(1) 97.94(2) 2.72(4)

Cl56 166.13(1) 95.60(3) 99.40(3) 91.70(2) 75.54(1) 2.97(4) Cl56 177.37(1) 93.51(1) 93.82(1) 84.93(1) 84.00(1) 2.72(4)

Cl73a 95.63(2) 84.37(1) 85.56(2) 94.44(2) 180.0(3) 2.72(5)

Cl13a 87.50(1) 92.50(1) 97.94(1) 82.06(2) 180.00(3) 2.72(4)

Bi3 Cl31 Cl32 Cl33 Cl51 Cl52 Cl53

Bi4 Cl71a Cl72 Cl73 Cl41 Cl42 Cl4 Cl43

Bi2 Cl11 Cl12 Cl13 Cl21 Cl22 Cl23

Table 2 Principal interatomic distances (angstrom) and angles (degree) of inorganic groups Bi3Cl123−

Cl31 2.52(5)

Cl71 3.03(5)

Cl11 2.98(4)

Cl32 95.24(2) 2.52(6)

Cl72 72.20(1) 2.96(5)

Cl12 77.04(3) 2.96(4)

Cl33 93.20(2) 94.11(3) 2.50(6)

Cl73 74.36(1) 76.08(1) 2.99(5)

Cl13 76.98(1) 72.81(1) 3.05(4)

Cl51 95.75(2) 163.11(2) 98.10(2) 3.09(6)

Cl41 166.70(1) 97.62(1) 95.17(2) 2.52(5)

Cl21 168.03(3) 91.42(2) 96.74(2) 2.49(4)

Cl52 98.18(2) 93.62(2) 165.61(2) 72.15(2) 3.04(1)

Cl42 98.40(3) 85.50(3) 161.50(3) 89.00(4) 2.51(8)

Cl22 96.84(1) 171.69(1) 100.50(1) 94.31(2) 2.56(4)

Cl53 169.40(2) 93.59(2) 92.00(2) 74.36(1) 75.38(1) 2.97(1)

Cl4 91.60(4) 93.91(4) 164.6(3) 97.70(4) 12.91(3) 2.55(11)

Cl23 91.06(2) 93.68(1) 163.54(4) 92.88(2) 92.03(1) 2.49(4) Cl43 98.85(1) 170.57(1) 98.92(2) 90.76(1) 99.00(4) 89.30(5) 2.55(4)

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Table 3 Principal interatomic distances (angstrom) and angles (degree) of organic groups [(C3H7)4N]+ [(C3H7)4N]+(1) Distances (Å) N1-C11 N1-C12 N1-C13 N1-C14

1.58(2) 1.53(2) 1.56(2) 1.49(2)

C11-C15 C12-C16 C13-C17 C14-C18 C15-C19 C16-C110 C17-C111 C18-C112

1.52(2) 1.57(2) 1.49(2) 1.55(2) 1.51(2) 1.49(3) 1.54(2) 1.55(3)

Angles (°) C13-N1-C11 C13-N1-C12 C11-N1-C12 C13-N1-C14

103.8(1) 111.8(1) 113.2(1) 111.3(1)

C11-N1-C14 C12-N1-C14 C15-C11-N1 C16-C12-N1 N1-C13-C17 C18-C14-N1 C11-C15-C19 C12-C16-C110 C111-C17-C13 C112-C18-C14

111.5(1) 105.5(1) 113.4(1) 114.5(1) 114.6(1) 114.5(1) 105.0(1) 107.8(2) 108.4(1) 104.8(2)

[(C3H7)4N]+(3) Distances (Å) N3-C31 N3-C32 N3-C33 N3-C34 C31-C35 C32-C36 C33-C37 C34-C38 C35-C39 C36-C310 C37-C311 C38-C312 C32-C361 C35-C391 C38-C3

C33-N3-C32 C33N3-C31 C32-N3-C31 C33-N3-C34 C32-N3-C34 C31-N3-C34 N3-C31-C35 C36-C32-N3 N3-C33-C37 C38-C34-N3 C31-C35-C39 C32-C36-C310 C34-C38-C312 C33-C37-C311 C36-C32-C361 N3-C32-C361 C31-C35-C391 C32-C361-C310

113.3(1) 112.3(1) 104.5(1) 103.9(1) 111.6(1) 111.5(1) 113.2(1) 118.8(2) 115.3(1) 117.5(2) 101.0(2) 111.0(2) 123.0(4) 105.3(1) 24.2(1) 109.9(2) 113.0(1) 100.0(1)

C3-C38-C34

106.0(3)

[(C3H7)4N]+(5) Distances (Å) N5-C51 N5-C52 N5-C53 N5-C54 C51-C55

1.52(2) 1.55(2) 1.54(2) 1.52(2) 1.56(3)

Angles (°) C54-N5-C51 C54-N5-C52 C51-N5-C52 C54-N5-C53 C51-N5-C53

1.56(2) 1.53(2) 1.51(2) 1.45(3)

C21-C25 C22-C26 C23-C27 C24-C28 C25-C29 C26-C210 C27-C211 C28-C212 C27-C2 C23-C271 C271-C2

Angles (°) C24-N2-C22 C24-N2-C23 C22-N2-C23 C24-N2-C21

111.1(1) 103.5(2) 115.0(1) 112.1(2)

1.48(3) 1.54(3) 1.49(2) 1.46(2) 1.60(3) 1.62(3) 1.50(2) 1.53(2) 1.25(7) 1.70(5) 1.67(8)

C22-N2-C21 C23-N2-C21 N2-C21-C25 C26-C22-N2 C27-C23-N2 C28-C24-N2 C29-C25-C21 C22-C26-C210 C211-C27-C23 C24-C28-C212 N2 C23 C271 N2 C241 C28 C241 C24 N2 C241 C24 C28 C271 C27 C23 C2 C27 C23

103.7(1) 111.8(1) 115.4(2) 114.4(1) 126.1(3) 120.2(3) 105.5(2) 101.9(2) 111.0(3) 110.1(3) 102.0(2) 90.0(5) 106.2(7) 114.1(7) 95.0(5) 148.0(5)

N4-C41 N4-C42 N4-C43 N4-C44 C41-C45 C42-C46 C43-C47 C44-C48 C45-C49 C46-C410 C47-C411 C48-C412

1.53(2) 1.50(2) 1.53(2) 1.54(2) 1.46(2) 1.48(3) 1.54(3) 1.55(2) 1.53(2) 1.56(3) 1.53(2) 1.49(3)

C44-N4-C42 C44-N4-C41 C42-N4-C41 C44-N4-C43 C42-N4-C43 C41-N4-C43 N4-C41-C45 C46-C42-N4 N4-C43-C47 N4-C44-C48 C41-C45-C49 C410-C46-C42 C411-C47-C43 C44-C48-C412

105.1(1) 109.3(1) 115.6(1) 110.6(1) 103.4(2) 112.5(1) 114.5(2) 112.0(2) 119.9(2) 116.3(2) 103.1(2) 109.2(2) 108.1(2) 116.0(2)

[(C3H7)4N]+(6) Distances (Å) N6-C61 N6-C62 N6-C63 N6-C64 C61-C65

1.57(2) 1.56(3) 1.56(2) 1.51(2) 1.52(4)

Angles (°) C64-N6-C62 C64-N6-C63 C62-N6-C63 C64-N6-C61 C62-N6-C61

104.0(2) 113.0(2) 101.0(2) 110.9(2) 113.9(2)

[(C3H7)4N]+(4) Distances (Å)

Angles (°) 1.52(2) 1.57(2) 1.53(2) 1.54(2) 1.50(2) 1.49(2) 1.59(2) 1.62(3) 1.66(4) 1.50(2) 1.48(3) 1.28(7) 1.60(3) 1.54(4) 1.54(7)

[(C3H7)4N]+(2) Distances (Å) N2-C21 N2-C22 N2-C23 N2-C24

113.3(1) 103.0(1) 113.3(1) 109.4(1) 107.0(1)

Angles (°)

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Table 3 (continued) C52-C56 C53-C57 C54-C58 C55-C59 C56-C510 C57-C511 C58-C512 C55-C591 C58-C5

1.55(2) 1.55(2) 1.54(3) 1.52(2) 1.52(2) 1.52(2) 1.55(4) 1.45(8) 1.65(5)

C52-N5-C53 C55-C51-N5 C56-C52-N5 C57-C53-N5 N5-C54-C58 C51-C55-C59 C52-C56-C510 C53-C57-C511 C54-C58-C512 C591-C55-C51

111.0(1) 115.7(1) 112.8(1) 113.7(2) 112.1(1) 99.2(3) 109.4(1) 111.0(6) 108.1(2) 124.2(6)

C54-C58-C5

106.0(2)

alternation of two types of organic–inorganic layers stacked along [010] direction, observed at b=0 (layer 1) and b=1/2 (layer 2). The first layer is composed of only one type of [Bi3Cl12]3−, noted [Bi3Cl12]3−(III), and four types of TPA noted [(C3H7)4N](3)]+, [(C3H7)4N](4)]+, [(C3H7)4N](5)]+, and [(C3H7)4N](6)]+. The tripolyhedra [Bi3Cl12]3− are arranged in discrete infinite chains parallel to the [100] direction (Fig. 1b). As regards the organic groups, they are located between the anionic groups belonging to adjacent layers. Each cation directs one alkyl ramification between the neighboring trinuclear polyhedra. In the second layer (Fig. 1c), the inorganic groups can be described by an alternation of two types of discrete infinite chains running along a axis. The first chain located at c=0 is composed of the [Bi3Cl12]3−(II) anions and the second chain observed at c=1/2 is formed by [Bi3Cl12]3−(I) anions. As for the cationic groups, the [(C3H7)4N](1)]+ and [(C3H7)4N](2)]+ form a corrugated chain sandwiched between two different inorganic chains parallel to [100] direction. In fact, the organic cations and the inorganic groups are held together by weak electrostatic interactions between the external chlorine atoms of anions and the nitrogen atoms of tetrapropylammonium cations. The length of the alkyl chains widened the distances between the opposite load and decreased the electrostatic interactions, which explains the low tridimensional cohesion of the compound (melting point, Tm =467 K). Geometry and coordination of the dodeca chloro-tribismuthate anion 3−

The trioctahedra ([Bi3Cl12] (I) and (II)) have Ci point group symmetry; the inversion center coincides with the bismuth atom of the central octahedra (Fig. 2a). There is a shift of the lone electron pair in the direction of the bridging chlorine

C62-C66 C63-C67 C64-C68 C65-C69 C66-C610 C67-C611 C68-C612 C61-C651 C65-C691 C651-C691

1.36(3) 1.49(2) 1.50(2) 1.51(4) 1.43(4) 1.49(6) 1.52(5) 2.02(1) 1.25(1) 1.51(2)

C63-N6-C61 C65-C61-N6 C66-C62-N6 N6-C63-C67 C68-C64-N6 C61-C65-C69 C62-C66-C610 C63-C67-C611 C64-C68-C612 N6-C61-C651

113.3(2) 125.1(3) 112.1(2) 127.0(2) 108.1(2) 105.3(3) 110.0(3) 111.0(2) 106.1(3) 102.1(2)

C691-C65-C61 C691-C651-C61

153.0(5) 97.2(4)

atoms. This leads to the increase in Bi–Cl bond length (Bi2– Cl13=3.05(4) and Bi4–Cl71=3.03(5)Å). While the distortion indices for central octahedra are ID(I)=0.001 and ID(II)=0.004, and those for terminal octahedra are ID(I)=0.088 and ID(II)=0.083. The angle distortion indices indicate that the central octahedra (ID, 0.049–0.078) are less distorted than terminal ones (ID, 0.068–0.088). The configuration of the Bi(1) and Bi(7) atoms is less distorted, probably because its octahedron involves only bridging Bi–Cl bonds (Bi–Cl≈2.72 Å). The atoms Bi(2) and Bi(4) are bonded to three terminal chlorine atoms (Bi–Cl≈2.53 Å) and to three bridging chlorine atoms (Bi–Cl≈3.00 Å). Whereas the tripolyhedra ([Bi3Cl12]3−(III)) have C1 point group symmetry and is formed by two octahedra sharing oneface chlorine atoms and linked by axial chlorides to a square pyramid, the Bi5-Bi6-Bi3 angle is equal to 175° (Fig. 2b). This incomplete geometry of the trioctahedra is probably correlated to deformations resulting from the stereochemical activity of Bi(III) lone electron pair. The length distortion indices ID of anionic entity (III) are 0.004 for central octahedra and 0.094– 0.081 for terminal octahedra. The angle distortion indices are 0.058 for central octahedra and 0.087 for terminal octahedra. So, the terminal octahedra are more distorted than the central ones. The configuration of the Bi(5) atom is less distorted because it involves only bridging Bi–Cl bonds (Bi– Cl≈2.71 Å). The atoms Bi(3) and Bi(6) are bonded to three terminal chlorine atoms (Bi–Cl≈2.52 Å) and to three bridging chlorine atoms (Bi–Cl≈2.93 Å). It can be said that in [(C3H7)4N]]3Bi3Cl12 compound, the anionic groups (I) and (II) are less distorted than the anionic group (III). Geometry and coordination of the tetrapropylammonium cation The organic groups have C1 point group symmetry. The geometry of [(C3H7)4N]]+ cations is isolated tetrahedra. Four

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a

Layer 2

Layer 1

b

In the first layer, the C–N bond lengths vary from 1.50(2) to 1.57(2)Å for N(4) and N(6) and from 1.52(2) to 1.57(2)Å for N(3) and N(5). The C–C bond lengths are in the range of 1.36(4)–1.56(2)Å for N(4) and N(6) and in the range of 1.28(1) to 1.66(4)Å for N(3) and N(5). The C–N–C angles are in the range of 101.0(2)–115.6(1)° for N(4) and N(6) and in the range of 103.9(1)–113.3(1)° for N(3) and N(5) (Table 3). The configurations of the [(C3H7)4N](4)]+ and [(C3H7)4N](6)]+ are more distorted than the two author cations. This is related to the disposition of these cations between the tripolyhedra of the anionic chains [2]. In the second layer, the C–N bond lengths vary from 1.45(3) to 1.58(2)Å for N(1) and from 1.53(2) to 1.57(3)Å for N(2). The C–C bond lengths vary from 1.25(7) to 1.70(5)Å for N(1) and from 1.44(3) to 1.81(1)Å for N(2). The C–N–C angles are in the range of 103.8(1)–113.2(1)° for N(1) and in the range of 103.0(1)–113.3(1)° for N(2) (Table 3). The configuration of the [(C3H7)4N](2)]+ cation is more distorted than the [(C3H7)4N](1)]+ cation, probably because it is located in a position in which the negative charges, [Bi3Cl12]3− trioctahedra, are very close to the cation. Calorimetric study

c

Fig. 1 a Projection of the atomic arrangement of [(C3H7)4N]]3Bi3Cl12 compound according to [001] direction. b Projection in the plane (a, c) of the layer 1. c Projection shows the layer 2

cations, [(C3H7)4N](1)]+, [(C3H7)4N](2)]+, [(C3H7)4N](3)]+, and [(C3H7)4N](5)]+, present a cross configuration (Fig. 2d), while the two others, [(C3H7)4N](4)]+ and [(C3H7)4N](6)]+ present a broken cross configuration (Fig. 2c).

The results of the calorimetric study of [(C3H7)4N]]3Bi3Cl12 were performed by heating a sample (15.6 mg mass) with a scanning rate (5°/min). An overview of the results (Fig. 3) obviously shows the existence of two distinct endothermic peaks detected at 424 and 467 K. Regarding the first peak, it corresponds to the phase transition from phase I to phase II, and the values of enthalpy and entropy were ΔH=13.78 kJ mol−1 and ΔS=32.5 J mol−1 K−1, respectively. As for the second one, it presents the melting point of the compound, and the values of enthalpy and entropy were ΔH=49.86 kJ mol−1 and ΔS=0.11 kJ mol−1 K−1, respectively. This experimentally observed entropies can be interpreted in terms of Boltzmann’s principle ΔS=R Ln Ω=R Ln N1\N2 where N1 and N2 are the number of distinguishable orientations allowed in the high and low-temperature phases. The ΔS=R lnΩ relation of transition brings about a number of equivalent positions Ω=50, which implies that the [N(C3H7)4]+ and/or the [Bi3Cl12]3− anions acquire quite a large part of their motional freedom at phase transition. The entropy value of the transition is larger than R Ln2 and the phase transition at T=424 K can be classified as an order–disorder. Dielectric study The study of the dielectric properties is an important source of valuable information about conduction processes since the origin of the dielectric losses, the electrical and dipolar relaxation time, and its activation energy can be determined [32, 33].

Ionics (2014) 20:231–241 Fig. 2 (a) and (b) Configurations of the anionic groups [Bi3Cl12]3−. (c) and (d) Configurations of the cationic groups [(C3H7)4N]+

237

a

b

[Bi3Cl12]3-(I and II)

c

[Bi3Cl12]3-(III)

d

[(C3H7)4N]+ (4 and 6)

The dielectric relaxation is described by a non-Debye model which gives the frequency-dependent complex permittivity in the form [34]:

ε*ðωÞ ¼

¼ ε∞

εs −ε∞ σ0 þ  1−α þ iε 0ω 1 þ ωiω1

[(C3H7)4N]+(1, 2, 3 and 5)

  1−α  ð1−αÞπ  ðεs −ε∞ Þ 1 þ ωω1 cos 2   ε0 ðωÞ ¼ ε∞ þ  1−α  2ð1−αÞ ð 1−α Þπ cos 1 þ 2 ωω1 þ ωω1 2

ð2Þ ð1Þ



 1−α



ð1 − αÞπ σ0 2 ε ðωÞ ¼  1−α  ð1 − αÞπ   2ð1−αÞ þ ε ω 0 ω ω þ ω1 cos 1 þ 2 ω1 2 00

ð εs − ε∞ Þ

ω ω1

sin

ð3Þ where σ0 represents the specific conductivity, εs is the static permittivity, ε0 is the permittivity of the free space, and ε∞ is the high frequency value of ε*. The real part ε′ of dielectric constant describes the stored energy while the second term is the imaginary part ε″ of dielectric constant, which describes the dissipated energy. The real and imaginary part of the ε* have been determined from the following relations:

While the first part in Eq. (3) is related to the thermal polarization, the second is related to the electrical conductivity. Frequency dependence The angular frequency dependence plots of the real ε′ part of complex dielectric permittivity ε* at several temperatures

238

Ionics (2014) 20:231–241

between 373 and 453 K is represented in Fig. 4. The increase in the dielectric of the sample is due to the electric field which is accompanied with the applied frequencies. Such a field will cause some ordering inside the sample as well as the formation of an electrical moment in the entire volume of the dielectric and in each separate polarizing molecule. The molecular dipoles in polar material cannot orient themselves at low temperature. When the temperature rises, the dipole orientation becomes easy, and this increases the dielectric constant ε′. In slowly varying fields at low frequency, the dipoles align themselves along the field direction and fully contribute to the total polarization. As the frequency increases, the variation in the field becomes too rapid for the molecular dipoles to follow, so their contribution to the polarization becomes lower with a measurable lag because of internal fractional forces [35]. Therefore, the space charge polarization is expected to give a drastic increase in ε′ around 423 K and is responsible for the behavior of the dielectric constant at different frequencies. As the ε′ decreases with the increase in frequency and approaches a limiting constant value, ε′∞(ω) at high frequencies, which can be interpreted as a result of rapid polarization processes with no ionic motion contribution because the frequency is too high and the ions can only oscillate without reaching the sample–electrode interface [36]. In the intermediate frequency range, Fig. 4 shows a plateau. The rise of ε′ is due to the sample–electrode interface polarization. The presence of a little spike in the complex plane impedance representation also confirms that the electrode polarization is responsible for the increase of ε′ at low frequency and high temperature [35, 37]. The intermediate frequency plateau, which was found to shift to a higher frequency with the increase in temperature, corresponds to the limiting low frequency dielectric constant ε′s [38]. Moreover, the variation of capacitance (C=ε′ε0 A/t) with frequency at different temperatures is shown in the inset of Fig. 4. The capacitance decreases with the increase in the

0

W/g

-5

(I)

Liq

(II)

40 1.5x10

-6

1.2x10

-6

C (F)

30 ε'

9.0x10

-7

6.0x10

-7

3.0x10

-7

10

4

ω (rad.s-1)

10

5

10

6

373 K 383 K 393 K 403 K 413 K 423 K 433 K 443 K 453 K

20

10

10

4

10

5

10

6

-1

ω (rad.s )

Fig. 4 Frequency dependence of real part of complex permittivity at several temperatures

applied frequency. This effect may be accredited to the screening of the electric field across the sample by charge redistribution [39, 40]. Figure 5 shows the frequency dependence of the imaginary part of dielectric constant ε″ from fitting Eq. (3). The experimental data of fitting parameters are presented in Table 4. It is important to mention that there are no substantial relaxation peaks in the frequency range employed in this study. Furthermore, the dielectric loss rises sharply at low frequency indicating that the electrode polarization and space charge effects have occurred confirming non-Debye dependence [41–43]. The temperature dependence of the conductivity (Log(σ0T) versus 1,000/T) in the studied temperature range is given in Fig. 6 and proves that σ increases with the increase in temperature according to Eq. (4): σT=A exp (−Ea/KBT). As to the phase transition, it is confirmed by the change of the slope at 423 (±5)K. Indeed, following the Arrhenius law, the obtained activation energy is equal to Ea2 =0.63 eV in phase I and Ea1 =1.01 eV in phase II. This discontinuity at 423 (±5)K is in agreement with the DSC measurements.

endo

Temperature dependence

424 K

-10

-15

-20 467 K

-25 200

250

300 350 Temperature (K)

400

450

500

Fig. 3 Differential scanning calorimetry curve of [(C3H7)4N]]3Bi3Cl12 compound

Figures 7 and 8 show the temperature dependence of the real part ε′ of permittivity, and the dielectric loss ε″ of [(C3H7)4N]]3Bi3Cl12 compound measured between 500 Hz and 1 MHz. The variation of ε′ and ε″ shows a weak dispersion in lowtemperature region. This behavior can be explained by the charge carriers which, on most cases, cannot orient themselves with respect to the direction of applied field; therefore, they possess a weak contribution to the polarization and the dielectric behavior [37].

Ionics (2014) 20:231–241

239

800

400

(II) Ea2 = 0.63 eV

-5

-1

ε''

-1

600

-4

Log(σ0T)[Ω cm K]

373 K 383 K 393 K 403 K 413 K 423 K 433 K 443 K fit

-6 (I) Ea1 = 1.01 eV

-7

200 -8

0

-9 2.2

10

4 -1

ω (rad.s )

10

5

10

2.3

2.4

6

Fig. 5 Frequency dependence of imaginary part of complex permittivity at several temperatures

2.5

2.6

2.7

-1

1000/T (K )

Fig. 6 Variation of Log(Tσ0) with the inverse of absolute temperature (1,000/T)

As the temperature rises, the variation of ε′ and ε″ increases and shows a strong dispersion. The imperfections/disorders are created in the lattice and the mobility of the majority charge carriers increases. This may be possibly due to the ion jump, the orientation, and space charge effect resulting from the increased concentrations of the charge carriers.

appears for each temperature corresponds to σDC. Second, the high frequency part of the conductivity is governed by Aωn. The frequency at which the dispersion takes place is known as hopping frequency [44]. Generally, the conductivity behavior in all temperature ranges is well described by Jonscher’s universal power law [45] feature governed by the relation:

AC conductivity

σAC ðωÞ ¼ σDC þ Aωn

Figure 9 shows the frequency dependence of the AC conductivity at different temperatures. AC conductivity spectra can be split into two parts. First, at low frequency, a plateau which Table 4 Experimental data of fitting parameters of the imaginary permittivity 1/ω (s rad−1)

α

Σ (Ω cm−1)

16.967 29.370 36.783 84.040

8.701×10−04 1.480×10−03 2.730×10−03 1.952×10−03

0.555 0.540 0.528 0.485

5.651×10−07 6.811×10−07 9.262×10−07 1.180×10−06

91.809 174.566 207.314 20.621 18.998 17.890 16.409 18.881 15.629 23.718 17.870 65.981 34.700

4.741×10−03 77.000×10−03 4.691×10−03 7.010×10−05 5.000×10−05 3.000×10−05 2.000×10−05 2.000×10−05 1.000×10−05 2.000×10−05 1.020×10−05 2.300×10−04 3.001×10−05

0.436 0.418 0.372 0.302 0.266 0.258 0.252 0.343 0.396 0.449 0.451 0.519 0.586

1.341×10−06 1.850×10−06 1.972×10−06 4.123×10−06 6.121×10−06 8.340×10−06 1.001×10−05 1.020×10−05 2.000×10−05 2.000×10−05 2.002×10−05 3.000×10−05 3.000×10−05

T (K)

e s–

373 378 383 388 393 398 403 408 413 418 423 428 433 438 443 448 453

∞

0<

n <1

ð4Þ

where σDC is the direct current conductivity of the sample, A is constant for a particular temperature, and n is the degree of interaction between mobile ions and the environments surrounding them. The transport mechanism can be explained by the thermally activated hopping process between two sites separated by an energy barrier. The above mentioned Eq. (4) has been used to fit the AC conductivity data. In the fitting 40 35 30

596 Hz 1.4 kHz 5.6 kHz 10 kHz 102 kHz 1 MHz

ε'

25 20 15 10 5 380

400

420

440

460

Temperature (K)

Fig. 7 Temperature dependence of the real part ε′ of complex permittivity at several frequencies

240

Ionics (2014) 20:231–241

800

0.60 596 Hz 1.4 kHz 5.6 kHz 10 kHz 102 kHz 1 MHz

600 ε''

0.55 n

400

0.50

200

0.45

0 380

400

420

440

0.40

460

390

400

410

Temperature (K)

Fig. 8 Temperature dependence of the imaginary part ε″ of complex permittivity at different frequencies

procedure, A and n values have been varied simultaneously to get the best fits. It was found that the goodness of fit was satisfactory in all cases. The fitted values of n at various temperatures are shown in Fig. 10. The change of the slope at 423 (±5)K confirms the phase transition observed in the temperature dependence of the electric permittivity ε′ spectra (Fig. 7) ascribed to order disorder phase transition. Figure 11 shows the variation of Log(TσDC) with the inverse of absolute temperature (1,000/T). The bulk conductivity of the material was evaluated from the complex impedance plots of the sample at selected temperatures. This plot is explained by Arrhenius type. The change in the slope of the linear curve is detected around 423 (±5)K. This discontinuity is in agreement with the DSC measurements. The values of the activation energies determined in regions I and II are 0.56 and 1.01 eV, respectively. These Ea values obtained from the DC conductivity are close to the activation energies calculated from the permittivity, suggesting that the mobility of the

440

450

charge carrier is probably due to a hopping mechanism [46]. This behavior is related to the reorientational dynamics of the tetra-ammonium alkyl chains [(C3H7)4N]]+ [47].

Conclusion The present work is devoted to the synthesis and physical– chemical properties of tri-tetrapropylammonium dodeca chlorobismuthate(III), [(C3H7)4N]]3Bi3Cl12. This compound is crystallized at room temperature in the triclinic system (P1 space group). As for the crystal packing, it is governed by electrostatic interactions between the external chlorine atoms of [Bi3Cl12]3− and the nitrogen atom of tetrapropylammonium cations forming a three-dimensional network. DSC studies reveal the existence of one phase transitions at T=423 (±5)K, which can be classified as an order–disorder type. The dielectric properties were studied as a function of

-9.0

-7

-7

-1 -1

(II) Ea2 = 0.56 eV

-9.5

-1 -1

398 K 403 K 408 K 413 K 418 K 423 K 428 K 433 K 438 K 443 K 448 K

σAC(Ω m )

4.0x10

430

Fig. 10 Variation of the exponent n in function of temperature

Log(σDCT)[Ω cm K]

6.0x10

420 Temperature (K)

-10.0 -10.5

(I) Ea1 = 1.01 eV

-11.0 2.0x10

-7

-11.5 -12.0 2.20

0.0 10

4

10

5

10

6

2.25

2.30

2.35

2.40

2.45

2.50

2.55

-1

1000/T (K )

-1

ω (rad.s )

Fig. 9 Frequency dependence of AC conductivity at several temperatures

Fig. 11 Variation of Log(TσDC) with the inverse of absolute temperature (1000/T)

Ionics (2014) 20:231–241

frequency and temperature in the ranges 209 Hz–5 MHz and 368–453 K, respectively. The temperature dependence of DC conductivity was analyzed using the Arrhenius type. The dependence of the imaginary part of the complex permittivity shows the non-Debye-type of behavior.

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