Raman study of order-disorder phase transition in [(C3H7)4N]3Bi3Cl12 compound

Journal of Molecular Structure 1106 (2016) 19e29

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Raman study of order-disorder phase transition in [(C3H7)N]3Bi3Cl12 compound W. Trigui a, *, A. Oueslati a, F. Hlel a, A. Bulou b a b

University of Sfax, Faculty of Sciences, Laboratory of Condensed Matter, BP1171, 3018 Sfax, Tunisia Institute for Molecules and Materials Le Mans, University of Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 May 2015 Received in revised form 11 September 2015 Accepted 23 October 2015 Available online 27 October 2015

The [(C3H7)N]3Bi3Cl12 compound that crystallizes at room temperature in a centrosymmetric triclinic system (P1 space group) is studied on heating up to its melting temperature. Differential scanning calorimetry investigations disclosed an irreversible phase transition at T ¼ 414 K. Raman scattering studies (140e3500 cm1) were investigated in the temperature range 398e438 K. A detailed analysis of the frequency, half-width and reduced intensity of the bands associated to y(BieCl) (at 254 cm1), y(Bi eCl) (at 287 cm1) and ys(NeCeC)þds(CeNeC) (at 1034 cm1) is quantitatively described in term of an order-disorder phase transitions. The temperature dependence of the correlation lengths of these three bands is consistent with a disordered state at high temperature. This behavior is presumably governed by the reorientational motion of the alkylammonium chains of the cations between two sites coupled with changes of interactions with the anionic groups. © 2015 Elsevier B.V. All rights reserved.

Keywords: Tri-tetrapropylammonium dodeca chlorobismuthate (III) DSC Raman spectroscopy Order-disorder phase transition

1. Introduction The prospect of creating new functional materials with tunable properties provides a great motivation on research of organicinorganic hybrids compound [1,2]. A typical research has been focused on crystals of alkylammonium halogenoantimonates(III) and bismuthates(III) that are very interesting hybrid family due to the various physical properties they provide such as nonlinear optical [3] magnetic [4] and ferroelectric [5]. Most of the phase transitions found in the crystals of this family are classified as “order-disorder” attributed to the reorientation motion of alkyl chains of the cations [6,7]. Usually the alkylammonium groups in such class of compound are placed in large cavities between inorganic moieties through hydrogen bonds and/or electrostatic interactions. Furthermore, halo-bismuthate (III) family in particular the chlorobismuthate, present the tendency to form isolated anionic units or infinite chaînes or two-dimensional structure formed by BiX3 6 octahedra depends on several factors but most important seem to be the nature of the halogen atom (Cl, Br, I), the size and shape of the alkylammonium cation [8e11]. Thus, these compounds can serve

* Corresponding author. E-mail address: [email protected] (W. Trigui). http://dx.doi.org/10.1016/j.molstruc.2015.10.071 0022-2860/© 2015 Elsevier B.V. All rights reserved.

as convenient models for understanding relations between dynamical motion of the organic and inorganic groups with the phase transitions phenomenon [12e14]. The present paper is devoted to the detailed investigation of the dynamical properties of the tetrapropylammonium cations as well as the halogeno-bismuthate anions in order to establish their role in the order-disorder phase transition. So we report herein the thermal evolution of vibrational spectra of tri-tetrapropylammonium dodeca chlorobismuthate (III) crystals investigated by Raman spectroscopy. 2. Experimental The organic-inorganic [(C3H7)4N]3Bi3Cl12 compound crystals were grown by slow evaporation, at room temperature. Details of the growth procedure and single crystal X-ray diffraction study were described elsewhere [15]. 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 aluminum capsule under thermal recording conditions, whose heating and cooling speed was 5  C/min and in the temperature range 293e453 K. Thermogravimetric analysis was performed with a ATG Q500 SETARAM from 298 to 772 K. The sample was heated in a crucible with a heating rate of 5  C/min.

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

For Raman measurement, a good transparent single crystal was selected and the Raman spectra were recorded using a T-64000 (HoribaeJobineYvon) spectrometer in the frequency range 140e3500 cm1. The wave-length radiation for excitation was 647.1 nm using an Ar/Kr laser. The laser power in the samples was kept less than 100 mW in order to avoid sample heating. The position, intensity and the half-width of the Raman bands were refined, at different temperatures, using a combination of Gaussian and Lorentzian functions in the Labspec software.

1.5 1.0 0.5 W/g

20

0.0 (I)

-0.5

3. Results and discussion

-1.0

3.1. Crystal structure The [(C3H7)N]3Bi3Cl12 compound was crystallized in a centrosymmetric triclinic system (P1 space group) with the lattice parameters: a ¼ 15.683 (7) Å, b ¼ 20.139 (10) Å, c ¼ 20.806 (9) Å, a ¼ 79.809 (3) , b ¼ 87.818 (3) and g ¼ 70.327 (3) [15]. The asymmetric unit contains six crystallographically independent [(C3H7)4N]þ cations, one and two halves distinct trioctahedral anions [Bi3Cl12]3. The arrangement of this compound, viewed along [001] direction (Fig. 1), can be described by an alternation of two types of organic-inorganic layers stacked along [010] direction, observed at b ¼ 0 (layer 1) and b ¼ 1/2 (layer 2).

(II)

endo

-1.5 414 K -2.0 280

300

320

340

360 380 400 Temperature (K)

420

440

460

Fig. 2. DSC diagram in the temperature range 193e453 K on heating and cooling showing the irreversible phase transition at 414 K.

110 100

3.2. Thermal analysis (DSC, TGA) 90 80 Weight (%)

The thermal analysis DSC and TGA were carried out to characterize the thermal stability of the [(C3H7)4N]3Bi3Cl12 compound, Figs. 2 and 3, respectively. The DSC thermogram shows the existence of an irreversible phase transition centered at 414 K. The solidesolid phase transition from phase I to phase II present the values of enthalpy DH ¼ 13.78 kJ mol1 and entropy DS ¼ 32.5 J mol1 K1 [15]. In view of the entropy change at the phase transition, assuming the relation DS ¼ R lnU it is concluded that the number of equivalent positions U ¼ 50, which implies that the [Bi3Cl12]3 anions and/or the [(C3H7)4N]þ cations acquire a quite large part of their motional freedom at the order-disorder phase transition. The TGA curve, reported in Fig. 3, shows that this compound start to decompose at 600 K. Besides no weight losses was observed between 298 K and 550 K. The degradation of this compound gives rise to a black carbon residue.

70 60 50 40 30 20 300

400

500 600 Temperture (K)

700

800

Fig. 3. TGA curve of the compound.

4

5x10

4

Intensity (a.u)

4x10

4

3x10

4

2x10

4

1x10

0 0

500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm ) Fig. 1. Projection of the atomic arrangement of [(C3H7)4N]3Bi3Cl12 compound according to [001] direction.

Fig. 4. Raman spectra of [(C3H7)4N]3Bi3Cl12 compound at room temperature.

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

21

Table 1 Assignments of Raman wavenumbers of [(C3H7)4N]3Bi3Cl12 crystal.

3.3. Raman scattering at room temperature The Raman spectrum recorded at room temperature is shown in Fig. 4. A detailed assignment of the most important bands is dare by comparison with similar compound [16e23]. The wavenumbers and proposed bands assignment are listed in Table 1. 3.3.1. Vibrational modes of [(C3H7)4N]þ The Raman spectrum shows the bending vibration of d(NC4) and d(CeCeC) in 333e510 cm1 spectral range. The stretching vibration y1, y3 and y4 of (NC4) are found at 967, 754 and 782 cm1, respectively. The band observed at 804 cm1 arises from the symmetric stretching vibration ys(CeCeC), while the symmetric bending vibrations ds(CeCeC) are located at 845, 881, 903 and 1056 cm1. The bands observed at 845 and 1057 cm1 are attributed to the symmetric stretching vibration ys(NeC). The bands related to the symmetric stretching ys(CeNeC) are detected at 845 cm1. The

symmetric bending vibrations ds(CeNeC) is observed in three bands 882, 904 and 1034 cm1. The vibrational band located at 940 cm1 is attributed to the symmetric bending ys(CeC), while the antisymmetric stretching band yas(CeC) is observed at 1005 cm1. The bands arising from the vibration of the cation (skeleton) occurs at 1157 cm1. The rocking mode rr(CH2) is observed in three bands at 754 and 870 cm1, while the torsion mode of (CH2) is detected at 1157 and 1270 cm1. On the other hand, the wagging mode u(CH2) is located in two bands at 1316 and 1377 cm1. The symmetric and antisymmetric bending of methyl groups (CH3) are observed at 1350 and 1452 cm1, respectively. The bands observed between 2877 and 2985 cm1 are due to symmetric and antisymmetric stretching vibration of CH2 and CH3 groups. 3.3.2. Vibrational modes of Bi3 Cl3 12 The stretching vibrations of the (BieCl) are cited in the 150e300 cm1 frequency range.

22

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

500 P4 P3

400

Intensity (a.u.)

398 K 400 K 403 K 408 K 413 K 418 K 420 K 422 K 425 K 428 K 433 K 438 K

P22

300

200

100

0 500

1000

1500 2000 2500 -1 Wavenumber (cm )

3000

3500

Fig. 5. Temperature evolution of the Raman spectra in the 140e3500 cm1 frequency range.

3.4. Temperature evolution of the Raman spectra The Raman spectra of [N(C3H7)4]3Bi3Cl12 compound have been collected in the temperature range 398e438 K and in the frequency range 140e3500 cm1 (Fig. 5). The careful analysis of those spectra clearly shows that the shapes of most analyzed bands remain constant in the temperature range 398e418 K. However, several bands show a significant shifts in their band position, half-width and intensity in the vicinity of the order-disorder phase transition detected by Differential Scanning Calorimetry. This behavior can be related to changes of interactions between organic and inorganic parts increasing with temperature which can be attributed to the increase of the dynamical motion of alkyl chains and/or the degree of distortion of the anions [24,25]. Fig. 6(aec) show an example of fits of the Raman spectrum collected at 418 K in the 140e600, 700e1600 and 2700e3100 cm1 spectral range. The temperature dependence of the positions and half-widths of some bands are presented in Figs. 7 and 8(a and b), respectively. The analyzed bands exhibit a visible shift of their positions in the vicinity of the phase transition around 414 K. The Raman bands associate to the stretching vibration of (BieCl) present a deviation from about 4 cm1 of their positions and a shift of their half-width about 5 cm1. The Raman bands connected to the vibration modes of tetrapropylammonium cation [(C3H7)4N]þ appear also sensitive to the phase transition. The bands related to the symmetric stretching vibration of (NeCeC) and symmetric bending vibrations of (CeNeC) located at 1034 cm1, show a variation in their positions to the low frequency by 3 cm1 and in their half-widths by 2 cm1. The antisymmetric stretching vibration of (CeC) observed at 979 cm1 present a variation in the position by 4 cm1 and in the half-widths by 2 cm1. The bands corresponding to the wagging mode u(CH2) and t(CH2) shift to low frequency by 3 cm1, while the half-widths of these bands increase up to 5 cm1 during the phase transition. In order to verify whether the phase transition is connected to a change in the dynamical state of the anions and the cations, we have focused our study on three selected bands P3 (at 254 cm1)

and P4 (at 287 cm1) associated to the stretching vibration y(BieCl) and P22 (at 1034 cm1) associated to the symmetric stretching vibration ys(NeCeC) and symmetric bending vibration ds(CeNeC) (Fig. 5).

3.5. Temperature dependence of wavenumber Andrade and Porto have shown that the temperature dependence of the Raman wavenumber of a phonon connected to an order-disorder mechanism can be described by Refs. [26,27]:

n2 ¼ n20 ½1 þ gðT  Tc Þ

(1)

where g is the thermal coefficient of the substance and n0 is the hard-core wavenumber at Tc. Owing to that the values of g are small, the wavenumber variation (Eq. (1)) can be expressed as [26,27]:

h i g n ¼ n0 1 þ ðT  Tc Þ 2

(2)

Moreover, the thermal coefficient is related to the volume of crystals according to the following expression:

Dn V gi ¼  i  DV ni

(3)

where DV and Dni are the volume and the variation of the wavenumber position respectively, V symbolizes the original volume and ni the band position of the i mode at room temperature. According to Gruneisen the relative variation of any vibrational frequency is directly proportional to the relative variation in the volume [28]. Fig. 9(aec) shows the experimental values of the wavenumber of the P3, P4 and P22 modes at various temperatures fitted using Eq. (2). The fitting parameters for the analyzed bands are summarized in Table 2. The expansion coefficient obtained: g(3) ¼ 1.58 104 K1, g(4) ¼ 1.39 104 K1, g(22) ¼ 6.22 105 K1 for T < TC and

23

307.4

150

200

250

300

509.8

374.0

328.7

212.6

147.9

I

252.9

i

(a.u.)

285.9

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

350 400 Wavenumber (cm-1 )

450

500

550

600

1 500

1 600

(a) 1453.5

15.0 14.0 13.0 12.0

1029.4

11.0

1314.6

10.0

8.0

1350.8

1267.8

1175.4

971.9

1377.9

1329.1

1155.1

940.2

842.7

2.0

901.3 918.1

3.0

870.0 879.7

795.8

4.0

827.3

753.0

5.0

1050.2

6.0

1133.6

1099.4

7.0

778.8

Intensity (a.u.)

9.0

1.0 0.0 700

800

900

1 000

1 100 1 200 Wavenumber (cm-1)

1 300

1 400

2938.3

(b)

16

2877.7

12

10

2903.4

8

3001.9

Intensity (a.u.)

2978.7

14

2743.4

6

4

2

0 2 700

2 750

2 800

2 850

2 900 Wavenumber (cm-1 )

2 950

3 000

3 050

3 100

(c) Fig. 6. Deconvolution of the Raman spectrum at T ¼ 418 K; (a): Spectral range from 140 to 600 cm1. (b): Spectral range from 600 to 2000 cm1. (c): Spectral range from 2000 to 3800 cm1.

24

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

312

1314

310

υ(Bi-Cl)

395 290

405

410

415

420

425

430

435

1310 395 1032

440

1031

υ(Bi-Cl)

395 254 252

400

405

410

415

420

425

430

435

440

1029 395

υ(Bi-Cl)

250 395 155 150 145 140 395

400

405

410

415

420

425

430

435

440

976

400

405

410

415

420

425

430

435

440

405

410

415

420

425

430

435

440

415 T 420 T (K)

425

430

435

440

υ (C-C)

972

405

410

415

420

425

430

435

395 754

440

υ(Bi-Cl)

752

405

410

415 T 420 T (K)

425

430

435

440

400

υ (NC ) + ρ (CH )

750

400

400

υ (N-C-C) + γ (C-N-C)

1030

286

υ (cm )

υ (cm )

288

400

ω(CH ) + τ(CH )

1312

308

395

400

405

410

(a)

(b)

Fig. 7. Temperature dependence of the position of some bands observed in the spectral ranges; (a): 140e320 cm1, (b): 700e1400 cm1.

24 20 ω(CH ) + τ(CH ) 16 12 395 400 405 21.0

12 10 8 395

υ(Bi-Cl)

400

405

410

415

420

425

430

435

440

20.5

18 395 32

υ(Bi-Cl)

400

405

410

415

420

425

430

435

440

405

410

415

420

425

430

435

440

υ(Bi-Cl)

30 28 395 44 40

400 υ(Bi-Cl)

υ (cm )

-1

υ 1/2(cm )

20

410

415

420

425

430

435

440

405

410

415

420

425

430

435

440

405

410

415

420

425

430

435

440

410

415 T 420 T (K)

425

430

435

440

υ (N-C-C) + γ (C-N-C)

20.0 395 400 22 21 υ (C-C) 20 19 395 400 60 40

υ (NC ) + ρ (CH )

20

395

400

405

410

415 TC 420 T (K)

425

430

435

440

(a)

395

400

405

(b)

Fig. 8. Temperature dependence of the half-widths of some bands observed in the spectral ranges; (a): 140e320 cm1, (b): 700e1400 cm1.

g(3) ¼ 3.48 104 K1, g(4) ¼ 2.52 104 K1, g(22) ¼ 8.93 105 K1 for T > Tc. The corresponding “hard-core wavenumber” n0 are 252.5, 286 and 1029.5 cm1, respectively. The decreasing of the thermal coefficient related to the changes of these bands positions indicates an increase of the volume of the crystal [29]. This behavior is related to an increase of the molecular motion of the cations and the anions. Since the asymmetric unit of the title compound present two different conformation of cation, the cation geometry can be changed from symmetric crosses to broken crosses by reorientational motion of the alkylammonium chains between two sites [30]. On the other hand, the observed variations in the internal modes of the anion proves the contribution of the isolated [Bi3Cl12]3 groups in the phase transition. Up to now there are known only three alkylammonium chlorobismuthates (III); [CH3NH3]3Bi2Cl9 [24], [CH3NH3]5Bi2Cl11 [31] and [C5H5NH]6Bi4Cl18 [25]; for which the low-frequency Raman have been studied in the vicinity of the order-disorder phase transitions. In those cases, the coupling

between the cationic and anionic sublattices indicated that the internal vibrations of the anions are involved in the phase transitions process.

3.6. Temperature dependence of the reduced intensities The relative intensities of the vibrational lines in Raman spectrum depend upon the population energy levels involved in the scattering process which is related to temperature variations. In order to be able to distinguishes that the changes in the relative intensities of the vibrational lines was derivates from pure structural changes and eliminate the influence of temperature, we use the reduced Raman intensity determined by the following expression [32]:

Ired ðuÞ ¼ ðu0  uÞ4 u½nðu; TÞ þ 11 I exp ðuÞ

(4)

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

253.5

289 P4

P3

253.0

-1

-1

Wavenumber (cm )

252.5

Wavenumber (cm )

25

252.0 251.5

288

287

251.0

286

250.5 250.0 395

400

405

410

415 420 T (K)

425

430

395

435

400

405

410

415

420

425

430

435

440

T (K)

(a)

(b)

1031.1 P22

-1

Wavenumber (cm )

1030.8

1030.5

1030.2

1029.9

1029.6

395

400

405

410

415 420 T (K)

425

430

435

(c) Fig. 9. Temperature dependence of the Raman wavenumbers of the P3 (a), P4 (b) and P22 (c) modes; the lines represent the theoretical fits.

nðu; TÞ ¼ ½expðZu=kB TÞ  11

(5)

where nðu; TÞ is the Bos-Einstein population factor, u is the Raman shift in cm1, u0 is the wavenumber of the incident radiation and I exp is the experimental Raman intensity. Fig. 10(a and b) show the spectra of both experimental and reduced Raman intensity at 398 K. As can be seen from this figures there is a remarkable difference between the two spectra. According to Dultz and all the reduced intensity of the Raman

scattering modes associated with the disorder mechanism is given by Refs. [33,34]:

  arctanðq0 xÞ Ip ¼ ai þ bi 1  q0 x

where q0 ¼ 2p=l, for a given geometrical configuration of the Raman scattering: ai and bi are phenomenological parameters independent of temperature and the correlation length describing the order of the fluctuations of the clusters is determined by:

 Table 2 Fitted values of the thermal coefficient of the wavenumber variation vs. temperature of the bands P3, P4 and P22. Peak

Temperature range

P3

T T T T T T

P4 P22

< > < > < >

TC Tc TC Tc TC Tc

Thermal coefficient, g (K1) g g g g g g

¼ ¼ ¼ ¼ ¼ ¼

1.58 3.48 1.39 2.52 6.22 8.93

     

104 104 104 104 105 105

(6)

x ¼ x0

T  Tc Tc

d (7)

For T > Tc, the reduced intensities of the modes P3, P4 and P22 are fitted using Eq. (6). The reduced intensities of these bands are normalized by the intensity of the peak of das (CH3) (at 1452 cm1). This peak is not degenerated and does not undergoes any evolution on heating the crystal. Fig. 11(aec) present the results of the theoretical fits. The obtained values of the critical exponents are d(P3) ¼ 0.473, d(P4) ¼ 0.453 and d(P22) ¼ 0.465, while the values of the product

7.0x10

250

6.0x10 R educed intensity (a.u)

Reduced intensity (a.u)

200

150

100

5.0x10 4.0x10 3.0x10 2.0x10

50

1.0x10 0

0.0 500

1000

1500 2000 2500 -1 Wavenumber (cm )

3000

3500

500

1000

1500 2000 2500 Wavenumber (cm )

3000

3500

(b)

(a)

Fig. 10. Raman spectra of experimental (a) and reduced Raman intensity (b) at 398 K.

0.055

0.21 P4

P3 0.18 Reduced intensity

Reduced intensity

0.050

0.045

0.040

0.035 395

0.15

0.12

400

405

410

415 420 T (K)

425

430

435

440

395

400

405

410

415 420 T (K)

(a)

425

430

435

440

(b) 0.055 0.050

P22

Reduced intensity

0.045 0.040 0.035 0.030 0.025 395

400

405

410

415 420 T (K)

425

430

435

440

(c) Fig. 11. Temperature dependence of the reduced intensities of P3 (a), P4 (b) and P22 (c) modes in the temperature domain (T > Tc); the red lines represent the theoretical fits. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

0.8

1.1 1.0

P3

0.7

P4

0.9

Correlation lenght

0.6

Correlation lenght

27

0.5 0.4

0.8 0.7 0.6 0.5

0.3

0.4

0.2 420

425

430 T (K)

435

0.3

440

420

425

(a)

430 T (K)

435

440

(b) 0.9 P22

Correlation lenght

0.8 0.7 0.6 0.5 0.4 0.3 420

425

430 T (K)

435

440

(c) Fig. 12. Correlation lengths in the disordered state expressed in terms of number of cells along the c crystallographic axis of the P3 (a), P4 (b) and P22 (c) modes.

x0 ¼ 24.3 nm, x0 ¼ 24.5 nm and x0 ¼ 24.7 nm, respectively. Fig. 12(aec) shows the evolution of the correlation length calculated using Eq. (7). One can note a strong correlation at low temperatures approaching the ordered phase. The correlation lengths increases near the Tc temperature and becomes very long, while by heating the crystal the correlation lengths decrease and the structure becomes more and more disordered [35]. This behavior is presumably governed by the change of the conformation of the cations coupled with changes of electrostatic interactions with the anionic groups.

(FWHM) associated to the order-disorder mechanism given by the generalized Langevin equation [36]:

t ¼ a þ bT þ c

tR 1 þ u2 t2R

(8)

where tR is the correlation time defined as the reorientational time of the atoms necessary to jump from one potential well to another, it is given by:

  Ea tR ¼ t∞ exp RT

(9)

3.7. Temperature dependence of the half-width In order to determine the activations energy of the modes P3, P4 and P22, we have use analysis of the full width at half maximum

where t∞ : is the relaxation time at infinite temperature, Ea: is the activation energy for the mode connected to the order-disorder transition and R ¼ 8.314472 J K1 mol1: is gas constant.

28

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

20.0

P3 32

P4

19.8

-1

Δν 1/2 (cm )

-1

Δν 1/2 (cm )

19.6 31

30

19.4 19.2 19.0 18.8

29 TC 395

400

405

410

415

TC

18.6

420 T (K)

425

430

435

440

395

400

405

410

(a)

415

420 T (K)

425

430

435

440

(b) 21.0

-1

Δν1/2 (cm )

20.8

P22

20.6

20.4

20.2 TC

20.0 395

400

405

410

415 420 T (K)

425

430

435

(c) Fig. 13. Variation of half-width of the P3 (a), P4 (b) and P22 (c) modes in function of temperature. The red lines represent the theoretical fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Generally (utR)2>>1 (where u ¼ 2pn), so that the Eq. (8) can be written as [37]:

  Ea FWHMðTÞ ¼ ða þ bTÞ þ c exp  RT

bands can be explained by the increase of the disorder in the crystal resulting from the increase of the dynamic of the cations and the anions [24,25,31].

(10) 4. Conclusions

where the first term represents the influence of the vibrational relaxation and the second term represents the thermal orientational mechanism of diffuse character. The experimental values of full width at half maximum (FWHM) of the chosen lines P3, P4 and P22 at various temperatures were fitted according to Eq. (10), the results are shown in Fig. 13(aec). As the temperature increases the half-width of the bands associated to the y(BieCl) (P3 and P4) and the ys(NeCeC)þds(CeNeC) (P22) increase at high temperature. The obtained activations energy values for P3, P4 and P22 are Ea(I) ¼ 119 kJ, Ea(I) ¼ 76 kJ and Ea(I) ¼ 80 kJ, at T〈 Tc, respectively. While the activations energies for these bands are Ea(II) ¼ 100 kJ, Ea(II) ¼ 64 kJ and Ea(II) ¼ 69 kJ, at T〉 Tc, respectively. The decreases of the activation energy with the increases of temperature for these

The tri-tetrapropylammonium dodeca chlorobismuthate (III) compound that crystallizes in a centrosymmetric triclinic system (P1 space group) undergoes an irreversible phase transition in the vicinity of 414 K. The Raman spectra as a function of temperature show a significant changes of several bands except for the internal modes of the inorganic anion. Several of these temperature evolutions have been analyzed in the framework of order-disorder models. The temperature dependence analysis of the frequency, the half-width and the reduced intensity of these modes can be attributed to an order-disorder phase transitions detected at 414 ± 5 K. The temperature dependence of the correlation lengths of the P3, P4 and P22 bands agrees with a disordered state at high temperature. The study of the half-width gives a value of activation

W. Trigui et al. / Journal of Molecular Structure 1106 (2016) 19e29

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