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Raman investigation of the order-disorder phase transitions in the 2[N(C3H7)4]SbCl4 compound
Vibrational Spectroscopy 81 (2015) 90–95
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Raman investigation of the order-disorder phase transitions in the 2[N(C3H7)4]SbCl4 compound N. Weslati* , I. Chaabane, F. Hlel University of Sfax, Condensed Matter Laboratory, Faculty of Sciences BP. 1171, Sfax 3000, Tunisia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 August 2014 Received in revised form 23 October 2015 Accepted 24 October 2015 Available online 30 October 2015
The Raman spectra of bis (tetrapropylammonium tetrachloroantimonate (III)) 2[(C3H7)4N]SbCl4 compound single crystals were studied in the wavenumber range from 3500 to 50 cm1 for temperatures between 300 and 415 K. Two phase transitions occurring at 343 (Ttr1) and 363 K (Ttr2) were observed and characterized. The strong evolutions of the Raman shift, half-widths and intensity of many lines associated with the organic cations were observed with discontinuities in the vicinity of the two phase transitions. The most important changes were noticed for the band at 307 cm1 (at room temperature) assignable to the torsion of CH3 groups of the cations. The spectral characteristics of this band was analyzed and consistently described in the framework of an order–disorder model for the two phase transitions. They allowed us to obtain information relative to the activation energy, the correlation length, and the critical exponent of the mechanism. The decrease of the estimated activation energies for the band 307 cm1 with the increase in temperature has been interpreted in terms of a change in the reorientation motion of cations. The temperature dependence of the reduced peak intensity allowed for the determination of the critical exponents and evolution of the correlation length on approaching the transition. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Bis (tetrapropylammonium tetrachloroantimonate (III)) Raman spectroscopy Phase transition
1. Introduction The prospect of creating new functional materials with tunable properties provides great motivation for investigating organic– inorganic hybrids [1–3]. Recently, the trivalent group metal halide organic–inorganic perovskite-like family of the formula RaMbX3b+a (where M, Sb, Bi) appeared to be very interesting. Some of them exhibit ferroelectric and ferroelastic and low dimensional magnetism properties [4–10]. On the other hand, several halogenometalates compounds with various size of alkylammonium cation exhibit numerous structural phase transitions related to reorientation motion of alkyl chains [11–15] and so are classified as “order–disorder. In fact, recent reports which focus mainly on introduction of tétrapropylammonium chloride amin into metal halides of the Sb, Sn and Bi elements show that these compounds undergoes sequence of phase transitions, which makes the research of this family very attracted [16,17]. In view of this consideration and in order to explore the mechanism of phase transition in this family of compounds, we
have successfully synthesized a new compound of formula bis (tetrapropylammonium tetrachloroantimonate (III)). Structurally, at room temperature, 2[N(C3H7)4]SbCl4 crystals are monoclinic with two cations and two anions per asymmetric unit; the lattice parameters were reported to be a = 18.1973(5) Å, b = 15.7225(4) Å, c = 13.6491(3) Å, b = 91.65(1) and Z = 4 [18]. The atomic arrangement can be described by a stacking of organic– inorganic layers along a direction. Each layer is made up by two types of tetrapropylammonium cations and two types of distorted tetrachloroantimonate (III) anion. The differential scanning calorimetry studies and the dielectric analysis confirm the presence of two order–disorder phase transitions located at 343 K and 363 K, which can be due to the reorientational dynamics of alkyl chains [18]. The objective of the present study is to investigate not only the dynamics of this compound by Raman scattering, but also the mechanism of the two phase transitions at high temperature in relation with the conformation of the alkylammonium chains. 2. Experimental procedure The bis (tetrapropylammonium tetrachloroantimonate (III)) was prepared by slow evaporation of the concentrated HCl (37%)
* Corresponding author. E-mail address: [email protected] (F. Hlel). http://dx.doi.org/10.1016/j.vibspec.2015.10.007 0924-2031/ ã 2015 Elsevier B.V. All rights reserved.
N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
solution containing a stoichiometric mixture of the [N(C3H7)4]Cl (purity 97%) and Sb2O3 (purity 99%) at room temperature. Raman scattering investigations were performed using the 514.5 nm radiation of an argon-krypton ion laser for excitation, and a Horiba-Jobin-Yvon T64000 spectrometer in the wavenumber range of 3500–50 cm1. The laser power in the samples was limited to 4 mW with a 600 r/mm grating and 100 s exposition time. The measurements were conducted under a microscope (X50 objective with long working distance) with polarized excitation on a good quality transparent single crystal. A Linkam heating stage was used for temperature control and measurements. The Raman shift, widths, and intensities of the Raman lines were determined by fitting, using the LabSpec5 software with a combined Lorentzian– Gaussian band shape. 3. Results and discussion 3.1. Temperature variation in Raman spectrum Fig. 1 shows the Raman spectrum of the bis (tetrapropylammonium tetrachloroantimonate (III)) compound, at room temperature. The strong Raman bands at 2988, 2975, 2946, 2880 and 2836 cm1 are assigned to the stretching vibrations of the CH3 and CH2 groups. Concerning the bands observed in the wavenumber range between 1460 and 1300 cm1, they are assigned to the bending vibration of the CH2 and CH3 groups. As for the bands observed in the range of 1034–787 cm1, they emanate from the C N C and N C stretching vibrations and the bending of the CC groups. The set of Raman bands of the 2[N(C3H7)4]SbCl4 compound at room temperature are reported in Table 1. The tentative assignment is proposed on the basis of calculated vibrational modes using the density functional theory (DFT) method [19]. The aim of this work is to find the modes sensitive to the phase transition at 343 and 363 K in the 2[N(C3H7)4]SbCl4 compound. The Raman spectra at several temperatures are illustrated in Fig. 2. In Fig. 2 the bands observed in the 400–50 cm1 region are shown where the internal vibrations of tetrachloroantimonate (III) (SbCl4) anions, the rocking and torsion modes of the CH2, CH3 and C C skeleton arise. The bands observed at 125 cm1, 166 cm1 and 247 cm1 are assigned to the vibration modes of the [SbCl4] anion. With respect to the band located at 143 cm1, which appears in the Raman spectrum at room temperature, it is assigned to the rocking vibration of CH3, CH2 and CC groups. Below the second phase transition Ttr2 (363 K), only one band is observed, whereas above
91
Ttr2, two new bands appear in the left and right for the band at 143 cm1 and their position show considerable change exceeding 20 cm1 during the second phase transition. This behavior may be understood as displacement of the cationic sublattice with respect to the anionic one. In Fig. 2, we observed the appearance of a new Raman band at 245 cm1 above 363 K, which can be due to the change in the symmetry of the cationic sublattice with respect to the anionic one. The band assigned to the torsional vibration of CH3 groups appears in the wavenumber region of 310–290 cm1. Below Ttr2, only one band is observed at 307 cm1, whereas above Ttr2 this band decreases up to a Raman shift at 290 cm1 with a variation of about 17 cm1 during the second phase transition. The bands formed by three peaks appear at 333, 339 and 345 cm1 . They are assigned to the rocking modes of CH3, CH2 and the stretching SbCl vibrations, respectively. For the three modes, the Raman shift and linewidth shifts are less than 0.5 cm1 and 1 cm1. The vibrational modes in the spectral range of 330– 350 cm1 are shown, illustrating that the peak shifts to the higher wavenumber side at 383 K with respect to 373 K. It is worthy to note that the observed shift can be due to the coupling effect between the organic and the anionic groups in the 2[N(C3H7)4] SbCl4 compound. Fig. 3 shows the temperature dependence of the Raman shift of the bands in the 400–50 cm1 wavenumber region. In the same order, it is obviously seen that the distinct changes are in the vicinity of 343 and 363 K, where these temperatures correspond to the first and second order–disorder phase transition (Ttr1 and Ttr2) shown by the thermal analysis [18]. It is clear from Fig. 2 that the bands observed at high wavenumbers of 3500–1400 cm1 do not change significantly with temperature. Actually, they exhibit a shifting of their position (about 1.5 cm1) around the order–disorder phase transition (Ttr1 = 343 K and Ttr2 = 363 K), which means that these modes are not directly connected to the phase transition. The temperature dependence of the position for a few bands in the 3500–1400 cm1 region is presented in Fig. 4. From these observations, the weak contribution of the anionic parts to the mechanism of the phase transition can be concluded. Indeed, the cation motions [N(C3H7)4]+ can be the only responsible ones for the disorder between the wavenumber region of 400– 50 cm1 at various temperatures. In order to get a more quantitative view of the two order–disorder phase transitions and the dynamical state of the cations, a quantitative study of the band located at 307 cm1 has been undertaken. In fact, the clear variation associated with the torsion of CH3 groups of the cation [N (C3H7)4]+ is confirmed (Fig. 2). 3.2. Temperature dependence of the Raman shift The temperature dependence of the Raman shift of a phonon connected to an order–disorder mechanism can be described by [20–24]:
n2 ¼ n20 ½1 þ g ðT T tr Þ
ð1Þ
where g is a thermal coefficient and n0 is the “hardcore Raman shift” at T = Ttr. Owing to the fact that the values of g are small, the Raman shift variation can be expressed as i h g ð2Þ n ¼ n0 1 þ ðT T tr Þ 2 The thermal coefficient depends on the volume of crystals according to the following expression:
gi ¼ Fig. 1. Raman spectrum of the 2[N(C3H7)4]SbCl4 compound at room temperature.
Dni V n i DV
ð3Þ
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N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
Table 1 Tentative assignments of the bands observed in the Raman spectrum of 2[N(C3H7)4] SbCl4 compound at room temperature. Raman wavenumber (cm1) at room temperature and assignment 2989 s 2971 vs 2953 vs 2935 s 2903 w 2879 vs 2836 m 1458 s 1357 vw 1327 m 1169 vw 1142 m 1106 m 1034 m 930 m 846 w 787 m 345 vs 339 vs 333 s 307 m 247 m 166 m 143 m 125 w 52 s
nas (CH3) nas (CH3) nas (CH2) nas (CH2) nas (CH2) ns (CH2) ns (CH3) das (CH3) ds (CH3) v (CH2) t (CH2) d (skeletal) d (skeletal) d (C–C–C) + t (CH2) + v (CH2) ns (NC) + d (C–N–C) d (C–N–C)+d (C–C–C) n (C–N–C) + d (C–C–C) ns (Sb–Cl) rr (CH3) rr (CH2) t (CH3) ns(Sb–Cl) + rr (CH3) + rr (CH2) d (SbCl4) rr (C–C–C) + rr (CH3) + rr (CH2) d (SbCl4) latice mode
where Dni and DV are the variations of the Raman shift position and the volume compressibility, respectively, ni is the band position and V is the initial volume at room temperature. The experimental values of the Raman shift of the 307 cm1 mode at various temperatures were fitted to the above equation and the results are summarized in Table 2. Fig. 5 shows the Raman shift versus temperature for the mode 307 cm1; these values were calculated on the basis of Eq. (2). Fitting by Eq. (2) confirms the values Ttr1 = 343 K, Ttr2 = 363 K and provides values of n01 = 306.63 cm1,n02 = 306.24 cm1 and n03 = 301.06 cm1 for the hardcore Raman shift, respectively. The relative change of the vibrational frequency is directly proportional to the relative change in the volume [25]. It is worthy to note that the thermal coefficient related to the changes of the Raman shift positions increases, which can be explained by the increase of the volume of the crystal [26]. Indeed, an important weakening of the Vander Waals interaction and the CH3 groups involved in the CH Cl Vander Waals interaction gain emotional freedom. Since the asymmetric unit of the title compound presents two different conformations of cations, the cation geometry can be changed from symmetric crosses to broken crosses below the phase transition. This geometric conversion is already reported in the case of organic–inorganic compounds based on the tetraalkylammonium, which has a key role on the mechanism of the phase transition [27,28].
Fig. 2. Temperature evolution of the Raman spectra in the wavenumber range of 3500–0 cm1.
(T > Ttr); the reduced intensity of the Raman scattering is then given by arctanðq0 jÞ IðvÞ ¼ A þ B 1 ð4Þ q0 j
3.3. Temperature dependence of the reduced Raman intensities In order to study the temperature dependence of the Raman intensity, the theoretical considerations will follow [20–24,29–31]. In this context, the phase transition is assigned to the fluctuations of an order parameter. Dultz [20–24,29–31] proposed that the strong temperature dependence of the intensity is directly connected to the correlation length in the disordered phase
Fig. 3. Temperature dependence of the position of the modes in the wavenumber range of 450–0 cm1.
Reduced Intensity (arb. units)
N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
6.0x10
-11
5.0x10
-11
4.0x10
-11
3.0x10
-11
2.0x10
-11
1.0x10
-11
93
0.0 3500
3000
2500
2000
1500
1000
500
0
-1
Wavenumber (cm ) Fig. 4. Temperature dependence of the position of the modes in the wavenumber range of 3500–1400 cm1.
Table 2 Summary of the fitted values of the thermal coefficient of the wavenumber variation versus temperature of the mode at 307 cm1. Wavenumber 1
307 cm
Temperature range
Thermal coefficient (K1)
T < Ttr1 T > Ttr1 T < Ttr2 T > Ttr2
g = 4.25 105 g = 4.60 104 g = 4.68 104 g = 6.09 104
Fig. 6. Reduced Raman intensity spectrum at room temperature.
modes are normalized by the intensity of the largest band of d(CH3) located at 1458 cm1, as this band is not degenerated and does not show any variation by heating of the crystal. Fig. 7 shows the rapid decrease of the intensity for the torsion mode of the CH3 groups at 307 cm1, in particular in the range from 300 to 415 K. The changes in the Raman intensity near the transition at Ttr1 = 343 K and Ttr2 = 363 K can be attributed to fluctuations in the order parameter. Fig. 7 shows the result of the fit using Eq. (4) for T > Ttr1 (343 K) and T > Ttr2 (363 K) for the torsion mode of the CH3 groups. The values of the critical exponents are found to be d1 = 0.67 for T > Ttr1 (343 K) and d2 = 1.16 for T > Ttr2 (363 K), while those of the j0 products are j0 (343 K) = 90.09 Å and j0 (363 K) = 17.2 Å. To determine the evolution of the correlation length in the disordered state, j is plotted using Eq. (5) versus temperature, which is shown in Fig. 8 for the band at 307 cm1. It is therefore possible to determine the correlation length. For the first phase transition (T > Ttr1), the correlation length extends from 1700 to 13000 elementary cells for the second phase transition (T > Ttr2). In the ordered state, the Raman intensity is governed by the longrange interactions. It shows a decrease on approaching the transition temperature, which has been shown to be proportional to the order parameter [32].
Fig. 5. Raman shift versus temperature of the mode at 307 cm1. The lines represent the theoretical fit according to Eq. (1).
The quantities A and B are phenomenological parameters that are independent of the temperature. The parameter j is the correlation length given by:
j¼
d
j0 ðT T tr Þ Tt
ð5Þ
The comparison between the spectra of the experimental (Fig. 1) and reduced Raman intensity at room temperature (Fig. 6) shows a difference between the two spectra. To omit the phenomena due to the change of the position of the crystal and the opacification of the windows, the reduced intensities of the vibration band relative to the CH3 torsional
Fig. 7. Temperature dependence of the reduced peak intensities for T > Ttr1 and T > Ttr2 of the Raman bands at 307 cm1. The lines represent the theoretical fit according to Eq. (4).
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N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95 Table 3 Values of the fit according to Eq. (8) and calculated values of Eaat room temperature. Parameters
a (cm1) b (cm1 K1) c (cm1) Ea (kJ mol1)
307 cm1 T < Ttr1
Ttr1 < T < Ttr2
T > Ttr2
9.39 0.0027 1.41014 90.7
11.18 0.01837 3.21013 86.9
16.15 0.00573 2.41013 76.7
where t 1 is the relaxation time at infinite temperature, Ea is the activation energy for the mode connected with the order–disorder transition and R = 8.314472 J K1 mol1 is the perfect gas constant. Generally $ 2t R2 >> 1 where$ = 2pn is the frequency of a particular phonon mode. Therefore, the temperature dependence of the band width is described by [33,23,35,36]. Ea ð8Þ FWHM ¼ ða þ bT Þ þ cexp RT Fig. 8. Correlation length versus temperature in the disordered state for T > Ttr1 and T > Ttr2.
3.4. Temperature dependence of the full width at half maximum To determine whether the observed phase transitions are correlated with the changes in the reorientational dynamics of the CH3 group of the [N(C3H7)4]+ cation, the analysis of the full width at half maximum (FWHM) as described by Carabatos–Nedelec and Becker [33,23] was followed. Such analysis, which is based on the theory used for the full width at half maximum associated with an order–disorder mechanism [23]. The linewidth of the phonon associated with the disorder mechanism is given by the generalized Langevin equation [21–34] as a function of the correlation time t R by:
GðvÞ ¼ ða þ bT Þ þ c
tR 1 þ v2 t 2R
ð6Þ
Where 1 << (vt R)2 with v the phonon Raman shift and the orientation correlation time (t R) is the mean reorientational time of the atoms to jump from one potential well to another and it is given by the relation. E t R ¼ t 1 exp a ð7Þ RT
where a, b, c and Ea are the fitting parameters. The linear part of Eq. (8) corresponds to the vibrational relaxation and the exponential term corresponds to the reorientational relaxation. The latter is connected to the thermal molecular reorientational motions of a diffusion nature. Fig. 9 presents the temperature dependence of the FWHM of the Raman band at n = 307 cm1, associated with the CH3 vibrational mode. At low temperature, the variation of the FWHM is less pronounced and does not show an important change with increasing temperature until Ttr1 = 343 K (temperature characteristic of the first phase transition), indicating that only the vibrational relaxation process is involved. Beyond Ttr2 = 363 K, the FWHM increases exponentially. The fitted parameters listed in Table 3 have shown the very low value of the slope b. The latter indicates that the torsion mode of CH3 is weakly anharmonic. The estimated activation energy values for the reorientation of CH3 are Ea1 = 90.9 kJ/mol for T < Ttr1, Ea2 = 86.9 kJ/mol for Ttr1 < T < Ttr2 and Ea3 = 76.7 kJ/mol for T > Ttr2. In fact, the decrease of the activation energy values, for the Raman band at 307 cm1, is probably due to the decrease of the population involved in this vibration, which can be due to the change of the conformation of the [N(C3H7)4]+ cation. These behaviors suggest that the observed phase transitions are related to the reorientational dynamical disorder of the tetrapropylammonium cations 4. Conclusion
Fig. 9. Linewidth versus temperature of the mode at 307 cm1. The lines represent the theoretical fit according to Eq. (8).
In this work, the Raman spectra at different temperatures have been studied and shown the presence of very thermosensitive modes (torsion of the CH3 group at 307 cm1). The detailed analysis of wavenumber positions, intensities and bands widths of this mode is consistent with a dynamic reorientation of the tetrapropylammonium cations at the order– disorder phase transitions. The temperature dependence of the correlation lengths of the t (CH3) modes at 307 cm1 shows the decrease of the correlation lengths j0 = 90.9 Å for T > Ttr1 (343 K) to j0 = 17.2 Å for T > Ttr2 (363 K). This result indicates the predominance of the disordered state in the 2[N(C3H7)4]SbCl4 compound, in which the temperature increases. The study of the full width at half maximum of the selected band 307 cm1 gives an activation energy of Ea1 = 90.9 kJ/mol, Ea2 = 86.9 kJ/mol and Ea3 = 76.7 kJ/mol for T < Ttr1, Ttr1 < T < Ttr2 and T > Ttr2, respectively. The decrease of Ea values below the order– disorder phase transition can be understood as a change in the dynamical states of cations.
N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
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Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Raman investigation of the order-disorder phase transitions in the 2[N(C3H7)4]SbCl4 compound N. Weslati* , I. Chaabane, F. Hlel University of Sfax, Condensed Matter Laboratory, Faculty of Sciences BP. 1171, Sfax 3000, Tunisia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 August 2014 Received in revised form 23 October 2015 Accepted 24 October 2015 Available online 30 October 2015
The Raman spectra of bis (tetrapropylammonium tetrachloroantimonate (III)) 2[(C3H7)4N]SbCl4 compound single crystals were studied in the wavenumber range from 3500 to 50 cm1 for temperatures between 300 and 415 K. Two phase transitions occurring at 343 (Ttr1) and 363 K (Ttr2) were observed and characterized. The strong evolutions of the Raman shift, half-widths and intensity of many lines associated with the organic cations were observed with discontinuities in the vicinity of the two phase transitions. The most important changes were noticed for the band at 307 cm1 (at room temperature) assignable to the torsion of CH3 groups of the cations. The spectral characteristics of this band was analyzed and consistently described in the framework of an order–disorder model for the two phase transitions. They allowed us to obtain information relative to the activation energy, the correlation length, and the critical exponent of the mechanism. The decrease of the estimated activation energies for the band 307 cm1 with the increase in temperature has been interpreted in terms of a change in the reorientation motion of cations. The temperature dependence of the reduced peak intensity allowed for the determination of the critical exponents and evolution of the correlation length on approaching the transition. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Bis (tetrapropylammonium tetrachloroantimonate (III)) Raman spectroscopy Phase transition
1. Introduction The prospect of creating new functional materials with tunable properties provides great motivation for investigating organic– inorganic hybrids [1–3]. Recently, the trivalent group metal halide organic–inorganic perovskite-like family of the formula RaMbX3b+a (where M, Sb, Bi) appeared to be very interesting. Some of them exhibit ferroelectric and ferroelastic and low dimensional magnetism properties [4–10]. On the other hand, several halogenometalates compounds with various size of alkylammonium cation exhibit numerous structural phase transitions related to reorientation motion of alkyl chains [11–15] and so are classified as “order–disorder. In fact, recent reports which focus mainly on introduction of tétrapropylammonium chloride amin into metal halides of the Sb, Sn and Bi elements show that these compounds undergoes sequence of phase transitions, which makes the research of this family very attracted [16,17]. In view of this consideration and in order to explore the mechanism of phase transition in this family of compounds, we
have successfully synthesized a new compound of formula bis (tetrapropylammonium tetrachloroantimonate (III)). Structurally, at room temperature, 2[N(C3H7)4]SbCl4 crystals are monoclinic with two cations and two anions per asymmetric unit; the lattice parameters were reported to be a = 18.1973(5) Å, b = 15.7225(4) Å, c = 13.6491(3) Å, b = 91.65(1) and Z = 4 [18]. The atomic arrangement can be described by a stacking of organic– inorganic layers along a direction. Each layer is made up by two types of tetrapropylammonium cations and two types of distorted tetrachloroantimonate (III) anion. The differential scanning calorimetry studies and the dielectric analysis confirm the presence of two order–disorder phase transitions located at 343 K and 363 K, which can be due to the reorientational dynamics of alkyl chains [18]. The objective of the present study is to investigate not only the dynamics of this compound by Raman scattering, but also the mechanism of the two phase transitions at high temperature in relation with the conformation of the alkylammonium chains. 2. Experimental procedure The bis (tetrapropylammonium tetrachloroantimonate (III)) was prepared by slow evaporation of the concentrated HCl (37%)
* Corresponding author. E-mail address: [email protected] (F. Hlel). http://dx.doi.org/10.1016/j.vibspec.2015.10.007 0924-2031/ ã 2015 Elsevier B.V. All rights reserved.
N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
solution containing a stoichiometric mixture of the [N(C3H7)4]Cl (purity 97%) and Sb2O3 (purity 99%) at room temperature. Raman scattering investigations were performed using the 514.5 nm radiation of an argon-krypton ion laser for excitation, and a Horiba-Jobin-Yvon T64000 spectrometer in the wavenumber range of 3500–50 cm1. The laser power in the samples was limited to 4 mW with a 600 r/mm grating and 100 s exposition time. The measurements were conducted under a microscope (X50 objective with long working distance) with polarized excitation on a good quality transparent single crystal. A Linkam heating stage was used for temperature control and measurements. The Raman shift, widths, and intensities of the Raman lines were determined by fitting, using the LabSpec5 software with a combined Lorentzian– Gaussian band shape. 3. Results and discussion 3.1. Temperature variation in Raman spectrum Fig. 1 shows the Raman spectrum of the bis (tetrapropylammonium tetrachloroantimonate (III)) compound, at room temperature. The strong Raman bands at 2988, 2975, 2946, 2880 and 2836 cm1 are assigned to the stretching vibrations of the CH3 and CH2 groups. Concerning the bands observed in the wavenumber range between 1460 and 1300 cm1, they are assigned to the bending vibration of the CH2 and CH3 groups. As for the bands observed in the range of 1034–787 cm1, they emanate from the C N C and N C stretching vibrations and the bending of the CC groups. The set of Raman bands of the 2[N(C3H7)4]SbCl4 compound at room temperature are reported in Table 1. The tentative assignment is proposed on the basis of calculated vibrational modes using the density functional theory (DFT) method [19]. The aim of this work is to find the modes sensitive to the phase transition at 343 and 363 K in the 2[N(C3H7)4]SbCl4 compound. The Raman spectra at several temperatures are illustrated in Fig. 2. In Fig. 2 the bands observed in the 400–50 cm1 region are shown where the internal vibrations of tetrachloroantimonate (III) (SbCl4) anions, the rocking and torsion modes of the CH2, CH3 and C C skeleton arise. The bands observed at 125 cm1, 166 cm1 and 247 cm1 are assigned to the vibration modes of the [SbCl4] anion. With respect to the band located at 143 cm1, which appears in the Raman spectrum at room temperature, it is assigned to the rocking vibration of CH3, CH2 and CC groups. Below the second phase transition Ttr2 (363 K), only one band is observed, whereas above
91
Ttr2, two new bands appear in the left and right for the band at 143 cm1 and their position show considerable change exceeding 20 cm1 during the second phase transition. This behavior may be understood as displacement of the cationic sublattice with respect to the anionic one. In Fig. 2, we observed the appearance of a new Raman band at 245 cm1 above 363 K, which can be due to the change in the symmetry of the cationic sublattice with respect to the anionic one. The band assigned to the torsional vibration of CH3 groups appears in the wavenumber region of 310–290 cm1. Below Ttr2, only one band is observed at 307 cm1, whereas above Ttr2 this band decreases up to a Raman shift at 290 cm1 with a variation of about 17 cm1 during the second phase transition. The bands formed by three peaks appear at 333, 339 and 345 cm1 . They are assigned to the rocking modes of CH3, CH2 and the stretching SbCl vibrations, respectively. For the three modes, the Raman shift and linewidth shifts are less than 0.5 cm1 and 1 cm1. The vibrational modes in the spectral range of 330– 350 cm1 are shown, illustrating that the peak shifts to the higher wavenumber side at 383 K with respect to 373 K. It is worthy to note that the observed shift can be due to the coupling effect between the organic and the anionic groups in the 2[N(C3H7)4] SbCl4 compound. Fig. 3 shows the temperature dependence of the Raman shift of the bands in the 400–50 cm1 wavenumber region. In the same order, it is obviously seen that the distinct changes are in the vicinity of 343 and 363 K, where these temperatures correspond to the first and second order–disorder phase transition (Ttr1 and Ttr2) shown by the thermal analysis [18]. It is clear from Fig. 2 that the bands observed at high wavenumbers of 3500–1400 cm1 do not change significantly with temperature. Actually, they exhibit a shifting of their position (about 1.5 cm1) around the order–disorder phase transition (Ttr1 = 343 K and Ttr2 = 363 K), which means that these modes are not directly connected to the phase transition. The temperature dependence of the position for a few bands in the 3500–1400 cm1 region is presented in Fig. 4. From these observations, the weak contribution of the anionic parts to the mechanism of the phase transition can be concluded. Indeed, the cation motions [N(C3H7)4]+ can be the only responsible ones for the disorder between the wavenumber region of 400– 50 cm1 at various temperatures. In order to get a more quantitative view of the two order–disorder phase transitions and the dynamical state of the cations, a quantitative study of the band located at 307 cm1 has been undertaken. In fact, the clear variation associated with the torsion of CH3 groups of the cation [N (C3H7)4]+ is confirmed (Fig. 2). 3.2. Temperature dependence of the Raman shift The temperature dependence of the Raman shift of a phonon connected to an order–disorder mechanism can be described by [20–24]:
n2 ¼ n20 ½1 þ g ðT T tr Þ
ð1Þ
where g is a thermal coefficient and n0 is the “hardcore Raman shift” at T = Ttr. Owing to the fact that the values of g are small, the Raman shift variation can be expressed as i h g ð2Þ n ¼ n0 1 þ ðT T tr Þ 2 The thermal coefficient depends on the volume of crystals according to the following expression:
gi ¼ Fig. 1. Raman spectrum of the 2[N(C3H7)4]SbCl4 compound at room temperature.
Dni V n i DV
ð3Þ
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N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
Table 1 Tentative assignments of the bands observed in the Raman spectrum of 2[N(C3H7)4] SbCl4 compound at room temperature. Raman wavenumber (cm1) at room temperature and assignment 2989 s 2971 vs 2953 vs 2935 s 2903 w 2879 vs 2836 m 1458 s 1357 vw 1327 m 1169 vw 1142 m 1106 m 1034 m 930 m 846 w 787 m 345 vs 339 vs 333 s 307 m 247 m 166 m 143 m 125 w 52 s
nas (CH3) nas (CH3) nas (CH2) nas (CH2) nas (CH2) ns (CH2) ns (CH3) das (CH3) ds (CH3) v (CH2) t (CH2) d (skeletal) d (skeletal) d (C–C–C) + t (CH2) + v (CH2) ns (NC) + d (C–N–C) d (C–N–C)+d (C–C–C) n (C–N–C) + d (C–C–C) ns (Sb–Cl) rr (CH3) rr (CH2) t (CH3) ns(Sb–Cl) + rr (CH3) + rr (CH2) d (SbCl4) rr (C–C–C) + rr (CH3) + rr (CH2) d (SbCl4) latice mode
where Dni and DV are the variations of the Raman shift position and the volume compressibility, respectively, ni is the band position and V is the initial volume at room temperature. The experimental values of the Raman shift of the 307 cm1 mode at various temperatures were fitted to the above equation and the results are summarized in Table 2. Fig. 5 shows the Raman shift versus temperature for the mode 307 cm1; these values were calculated on the basis of Eq. (2). Fitting by Eq. (2) confirms the values Ttr1 = 343 K, Ttr2 = 363 K and provides values of n01 = 306.63 cm1,n02 = 306.24 cm1 and n03 = 301.06 cm1 for the hardcore Raman shift, respectively. The relative change of the vibrational frequency is directly proportional to the relative change in the volume [25]. It is worthy to note that the thermal coefficient related to the changes of the Raman shift positions increases, which can be explained by the increase of the volume of the crystal [26]. Indeed, an important weakening of the Vander Waals interaction and the CH3 groups involved in the CH Cl Vander Waals interaction gain emotional freedom. Since the asymmetric unit of the title compound presents two different conformations of cations, the cation geometry can be changed from symmetric crosses to broken crosses below the phase transition. This geometric conversion is already reported in the case of organic–inorganic compounds based on the tetraalkylammonium, which has a key role on the mechanism of the phase transition [27,28].
Fig. 2. Temperature evolution of the Raman spectra in the wavenumber range of 3500–0 cm1.
(T > Ttr); the reduced intensity of the Raman scattering is then given by arctanðq0 jÞ IðvÞ ¼ A þ B 1 ð4Þ q0 j
3.3. Temperature dependence of the reduced Raman intensities In order to study the temperature dependence of the Raman intensity, the theoretical considerations will follow [20–24,29–31]. In this context, the phase transition is assigned to the fluctuations of an order parameter. Dultz [20–24,29–31] proposed that the strong temperature dependence of the intensity is directly connected to the correlation length in the disordered phase
Fig. 3. Temperature dependence of the position of the modes in the wavenumber range of 450–0 cm1.
Reduced Intensity (arb. units)
N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95
6.0x10
-11
5.0x10
-11
4.0x10
-11
3.0x10
-11
2.0x10
-11
1.0x10
-11
93
0.0 3500
3000
2500
2000
1500
1000
500
0
-1
Wavenumber (cm ) Fig. 4. Temperature dependence of the position of the modes in the wavenumber range of 3500–1400 cm1.
Table 2 Summary of the fitted values of the thermal coefficient of the wavenumber variation versus temperature of the mode at 307 cm1. Wavenumber 1
307 cm
Temperature range
Thermal coefficient (K1)
T < Ttr1 T > Ttr1 T < Ttr2 T > Ttr2
g = 4.25 105 g = 4.60 104 g = 4.68 104 g = 6.09 104
Fig. 6. Reduced Raman intensity spectrum at room temperature.
modes are normalized by the intensity of the largest band of d(CH3) located at 1458 cm1, as this band is not degenerated and does not show any variation by heating of the crystal. Fig. 7 shows the rapid decrease of the intensity for the torsion mode of the CH3 groups at 307 cm1, in particular in the range from 300 to 415 K. The changes in the Raman intensity near the transition at Ttr1 = 343 K and Ttr2 = 363 K can be attributed to fluctuations in the order parameter. Fig. 7 shows the result of the fit using Eq. (4) for T > Ttr1 (343 K) and T > Ttr2 (363 K) for the torsion mode of the CH3 groups. The values of the critical exponents are found to be d1 = 0.67 for T > Ttr1 (343 K) and d2 = 1.16 for T > Ttr2 (363 K), while those of the j0 products are j0 (343 K) = 90.09 Å and j0 (363 K) = 17.2 Å. To determine the evolution of the correlation length in the disordered state, j is plotted using Eq. (5) versus temperature, which is shown in Fig. 8 for the band at 307 cm1. It is therefore possible to determine the correlation length. For the first phase transition (T > Ttr1), the correlation length extends from 1700 to 13000 elementary cells for the second phase transition (T > Ttr2). In the ordered state, the Raman intensity is governed by the longrange interactions. It shows a decrease on approaching the transition temperature, which has been shown to be proportional to the order parameter [32].
Fig. 5. Raman shift versus temperature of the mode at 307 cm1. The lines represent the theoretical fit according to Eq. (1).
The quantities A and B are phenomenological parameters that are independent of the temperature. The parameter j is the correlation length given by:
j¼
d
j0 ðT T tr Þ Tt
ð5Þ
The comparison between the spectra of the experimental (Fig. 1) and reduced Raman intensity at room temperature (Fig. 6) shows a difference between the two spectra. To omit the phenomena due to the change of the position of the crystal and the opacification of the windows, the reduced intensities of the vibration band relative to the CH3 torsional
Fig. 7. Temperature dependence of the reduced peak intensities for T > Ttr1 and T > Ttr2 of the Raman bands at 307 cm1. The lines represent the theoretical fit according to Eq. (4).
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N. Weslati et al. / Vibrational Spectroscopy 81 (2015) 90–95 Table 3 Values of the fit according to Eq. (8) and calculated values of Eaat room temperature. Parameters
a (cm1) b (cm1 K1) c (cm1) Ea (kJ mol1)
307 cm1 T < Ttr1
Ttr1 < T < Ttr2
T > Ttr2
9.39 0.0027 1.41014 90.7
11.18 0.01837 3.21013 86.9
16.15 0.00573 2.41013 76.7
where t 1 is the relaxation time at infinite temperature, Ea is the activation energy for the mode connected with the order–disorder transition and R = 8.314472 J K1 mol1 is the perfect gas constant. Generally $ 2t R2 >> 1 where$ = 2pn is the frequency of a particular phonon mode. Therefore, the temperature dependence of the band width is described by [33,23,35,36]. Ea ð8Þ FWHM ¼ ða þ bT Þ þ cexp RT Fig. 8. Correlation length versus temperature in the disordered state for T > Ttr1 and T > Ttr2.
3.4. Temperature dependence of the full width at half maximum To determine whether the observed phase transitions are correlated with the changes in the reorientational dynamics of the CH3 group of the [N(C3H7)4]+ cation, the analysis of the full width at half maximum (FWHM) as described by Carabatos–Nedelec and Becker [33,23] was followed. Such analysis, which is based on the theory used for the full width at half maximum associated with an order–disorder mechanism [23]. The linewidth of the phonon associated with the disorder mechanism is given by the generalized Langevin equation [21–34] as a function of the correlation time t R by:
GðvÞ ¼ ða þ bT Þ þ c
tR 1 þ v2 t 2R
ð6Þ
Where 1 << (vt R)2 with v the phonon Raman shift and the orientation correlation time (t R) is the mean reorientational time of the atoms to jump from one potential well to another and it is given by the relation. E t R ¼ t 1 exp a ð7Þ RT
where a, b, c and Ea are the fitting parameters. The linear part of Eq. (8) corresponds to the vibrational relaxation and the exponential term corresponds to the reorientational relaxation. The latter is connected to the thermal molecular reorientational motions of a diffusion nature. Fig. 9 presents the temperature dependence of the FWHM of the Raman band at n = 307 cm1, associated with the CH3 vibrational mode. At low temperature, the variation of the FWHM is less pronounced and does not show an important change with increasing temperature until Ttr1 = 343 K (temperature characteristic of the first phase transition), indicating that only the vibrational relaxation process is involved. Beyond Ttr2 = 363 K, the FWHM increases exponentially. The fitted parameters listed in Table 3 have shown the very low value of the slope b. The latter indicates that the torsion mode of CH3 is weakly anharmonic. The estimated activation energy values for the reorientation of CH3 are Ea1 = 90.9 kJ/mol for T < Ttr1, Ea2 = 86.9 kJ/mol for Ttr1 < T < Ttr2 and Ea3 = 76.7 kJ/mol for T > Ttr2. In fact, the decrease of the activation energy values, for the Raman band at 307 cm1, is probably due to the decrease of the population involved in this vibration, which can be due to the change of the conformation of the [N(C3H7)4]+ cation. These behaviors suggest that the observed phase transitions are related to the reorientational dynamical disorder of the tetrapropylammonium cations 4. Conclusion
Fig. 9. Linewidth versus temperature of the mode at 307 cm1. The lines represent the theoretical fit according to Eq. (8).
In this work, the Raman spectra at different temperatures have been studied and shown the presence of very thermosensitive modes (torsion of the CH3 group at 307 cm1). The detailed analysis of wavenumber positions, intensities and bands widths of this mode is consistent with a dynamic reorientation of the tetrapropylammonium cations at the order– disorder phase transitions. The temperature dependence of the correlation lengths of the t (CH3) modes at 307 cm1 shows the decrease of the correlation lengths j0 = 90.9 Å for T > Ttr1 (343 K) to j0 = 17.2 Å for T > Ttr2 (363 K). This result indicates the predominance of the disordered state in the 2[N(C3H7)4]SbCl4 compound, in which the temperature increases. The study of the full width at half maximum of the selected band 307 cm1 gives an activation energy of Ea1 = 90.9 kJ/mol, Ea2 = 86.9 kJ/mol and Ea3 = 76.7 kJ/mol for T < Ttr1, Ttr1 < T < Ttr2 and T > Ttr2, respectively. The decrease of Ea values below the order– disorder phase transition can be understood as a change in the dynamical states of cations.
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