Infrared studies of phase transitions in ferroelectric (C5H5NH)5Bi2Br11

Vibrational Spectroscopy 40 (2006) 55–65 www.elsevier.com/locate/vibspec

Infrared studies of phase transitions in ferroelectric (C5H5NH)5Bi2Br11 J. Tarasiewicz a,*, R. Jakubas a, J. Baran b a

Faculty of Chemistry, University of Wrocław, F. Joliot Curie 14, 50-383 Wrocław, Poland Institute of Low Temperature and Structure Research of the Polish Academy of Science, Oko´lna 2, 50-422 Wrocław, Poland

b

Received 18 March 2005; received in revised form 29 June 2005; accepted 1 July 2005 Available online 2 September 2005

Abstract Infrared spectra (3500–500 cm
1) of polycrystalline (C5H5NH)5Bi2Br11 samples were investigated within the temperature range 27– 456 K. The assignments of the observed bands in the spectra measured at 27, 310 and 456 K are proposed. A temperature dependence of the wavenumbers and full width at half maximum (FWHM) of the bands arising from some internal vibrations of pyridinium cations are analysed in order to explain the role of cations in the mechanism of the phase transition at 118 (paraelectric–ferroelectric) and 403 K. It was found that numerous bands arising from the internal modes of the cations exhibit the splitting in the vicinity of both phase transitions, that indicates a distinct changes in the motional state of the pyridinium moieties. # 2005 Elsevier B.V. All rights reserved. Keywords: Infrared; Phase transition; Pyridinium

1. Introduction The crystal of (C5H5NH)5Bi2Br11 belongs to a large family of alkylammonium halogenobismuthate(III) and antimonate(III) crystals [1,2]. Most of the phase transitions found in the crystals of this family exhibit an ‘‘order–disorder’’ mechanism connected with dynamics of organic cations. The pyridinium crystals evoke recently much interest since the ferroelectricity was discovered for a few of them: (C5H5NH)ClO4 [3], (C5H5NH)BF4 [4], (C5H5NH)ReO4 [5]. Numerous halogenoantimonate(III) and bismuthate(III) of pyridinium analogues: (C5H5NH)6Bi4Cl18 [6], (C5H5NH)SbCl4 [7], (C5H5NH)SbBr4 [8], (C5H5NH)5Sb2Br9Br2 [9], (C5H5NH)8Sb4Br12Cl8 [10] and (C5H5NH)BiCl4 [11] were found to be synthesized up to now. Recently, we obtained * Corresponding author. Tel.: +48 71 3757288; fax: +48 71 3282348. E-mail addresses: [email protected] (J. Tarasiewicz), [email protected] (R. Jakubas). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.07.003

a new pyridinium analogue (C5H5NH)5Bi2Br11 [12], which exhibits a ferroelectric properties. The ferroelectricity in this crystal is connected with the dynamics of the organic cations. In the room temperature phase (C5H5NH)5Bi2Br11 crystallizes in the centrosymmetric monoclinic space group, P21/n. The crystal is built up of pyridinium cations and discrete Bi2Br115
anions. The anionic form consists of two octahedra joined by their top ligands. In the unit cell of this crystal one can distinguish four crystallographically independent pyridinium cations exhibiting substantial dynamical disorder. It should be emphasized that this anionic type of structure is quite rare. There are known only two examples of salts with this stoichiometry ([CH3NH3]5Bi2Br11, [CH3NH3]5Bi2Cl11) and both of them revealed ferroelectricity [1]. The differential scanning calorimetry, dilatometric and dielectric studies [10] showed that (C5H5NH)5Bi2Br11 undergoes the following sequence of phase transitions (PTs):

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J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

The (C5H5NH)5Bi2Br11 crystal appears to be a proper ferroelectric over the phase (III) with the spontaneous polarisation of the order of 3  10
3 C/m2 along the b-axis. The spontaneous polarisation is a result of ordering of the dipolar pyridinium cations bestowed significant permanent dipole moment. The paraelectric–ferroelectric PT at 118 K is accompanied by a significant dielectric anomaly ðe0b  220Þ. The fundamental ferroelectric dispersion over the phase (II) appears in the radio-frequency region. The proton magnetic resonance (1H NMR) studies revealed that at the II ! I PT at 405 K the pyridinium cations begin to reorient isotropically, over the intermediate phase (II) all cations are performing the C6 reorientation (the motion about the pseudo-hexagonal symmetry axis), and then approaching the 118 K a freezing of the pyridinium rings takes place. The infrared studies have been undertaken in a wide temperature region to confirm and clarify the cationic motions contribution to the phase transitions mechanism at 118 K (III ! II) and at 405 K (II ! I) in the (C5H5NH)5Bi2Br11 crystals.

2. Experimental details The crystals of (C5H5NH)5Bi2Br11 were synthesized by reaction of (BiO)2CO3 and C5H5N in concentrated HBr. Large single crystals were grown by a slow evaporation at constant temperature (24 8C). The deuterated salt (C5H5ND)5Bi2Br11, was prepared by repeated recrystallization (three-fold) from 99.9% D2O (Aldrich) with an excess of DBr. The infrared spectra for (C5H5NH)5Bi2Br11 crystal were measured in the wide temperature range (from 27 to 456 K) for the mulls in Nujol (KBr windows) with a Bruker IFS-88 spectrometer. The powder infrared spectrum for the (C5H5ND)5Bi2Br11 crystal was measured at room tempera-

ture only. The low temperature spectra were measured with the APD Cryogenics Inc. Displex Model CS-202 closed cycle cryogenic helium system (350–10 K) and with the variable temperature cell SPECAC P/N 21.500 (77–456 K). The measurements were performed over the wavenumber range 3500–500 cm
1 with resolution 1 cm
1. The FTRaman spectra were taken with the FRA-106 attachment to the IFS-Bruker 88 using Nd:YAG diode pump laser. The measurements were performed over the wavenumber range 3500–80 cm
1 with resolution better than 2 cm
1. The program GRAMS/368 Galactic Industries was used for numeral fitting of the experimental data. The Gaussian functions were used for fitting of the infrared bands.

3. Results and discussion 3.1. The selection rules for the (C5H5NH)5Bi2Br11 crystals in the room temperature phase The site symmetry of the ‘‘isolated’’ Bi2Br115
anion is Ci in the (C5H5NH)5Bi2Br11 crystal. The total number of its vibrational modes appears to be 39. They are distributed between the irreducible representations of the Ci site group as follows: 21Au + 18Ag. Taking into account that their formal external (translational and librational) modes transform as 3Au + 3Ag one gets the following internal modes for the ‘‘isolated’’ anion: 18Au + 15Ag. As the unit cell contains two (Z = 2) such anions, therefore, each internal mode splits into two components of the unit cell (unit cell group is C2h). As follows from Table 1 the infrared allowed modes (Au) split into Au + Bu fundamental modes of the unit cell; the Raman allowed modes (Ag) are split into Ag + Bg fundamental modes of the unit cell. One should mention that the translational modes (T) contain three acoustic modes of the crystal.

Table 1 Analysis of the vibrations of the Bi2Br115
anion and C5H5NH+ cation (only of those occupying C1 sites) in the (C5H5NH)5Bi2Br11 crystal for the room temperature phase C2h

N Ag Au Bg Bu

C5H5NH+

Bi2Br115
18 21 18 21

T

ni

N

T

L

ni

IR

Raman

3

15 18 15 18

36 36 36 36

3 3 3 3

3 3 3 3

30 30 30 30

i Y I X, Y

xx, yy, zz, zx i yx, zy i

3 3 3

Selection rules

L

Abbreviations: N, total number of modes; T, translational modes (may contain acoustic modes); L, librational modes; ni, internal modes.

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

It is impossible to classify all internal modes of the pyridinium cations, as three cations exhibit disorder occupying the Ci sites. The fourth independent pyridinium cation occupies the general C1 (4) sites. The formal classification of their modes is listed in Table 1. 3.2. Temperature analysis of the IR spectra of the (C5H5NH)5Bi2Br11 crystals The IR spectra for (C5H5NH)5Bi2Br11 crystal measured in Nujol for three temperatures (the lowest (27 K), room temperature (300 K) and the highest one (456 K)) are presented in Fig. 1. There is also shown the IR spectrum measured at room temperature in fluorolube mull. The IR spectrum for partly deuterated crystal was measured at room temperature (Fig. 2). There are observed some bands assigned for ND vibrations in the wavenumber range 2600– 2100 cm
1. The Raman spectra for (C5H5NH)5Bi2Br11 and its deuterated analogue measured at room temperature are presented in Fig. 3. The wavenumbers of the bands arising from the internal vibrations of the pyridinium cations and their tentative assignments are collected in Table 2. The assignments were proposed on the basis of calculated normal vibrations of pyridine and d5-pyridine [13,14] and on the comparison of the assignments proposed for the pyridine, d5-pyridine [15–24], pyridinium, d5-pyridinium cation [25] and metal complexes with pyridine [26,27]. The Herzberg notation of the pyridine internal modes was used for assignement of the bands. Despite a large number of publications, some assignments of normal modes are still controversial. The work of Uren˜a et al. [20] clarify the assignments of these normal modes. Besides the fundamentals, a number of overtones and combination bands and

57

other very weak bands are observed as well. It can result from a presence of four crystallographically independent pyridinium cations in the crystal lattice. A detailed analysis of the band shapes was made for majority of the bands assigned to the internal vibrations of the pyridinium cation. For analysed bands the changes are observed in the vicinity of the phase transitions at 118 and 403 K. Some of these bands exhibit anomalous changes in the intensity and in the full width at half maximum (abbreviated as FWHM hereafter) around the temperatures of the phase transitions, which could confirm a key role of the organic cations in the molecular mechanism of the phase transition. The temperature evolution of the infrared bands which reveal the most important changes are shown in Figs. 4a, 5 and 7a. At high temperatures one can observe, generally, a single or few wide overlapping bands. The splitting of these bands and increasing of their intensity take place with decreasing temperature. The temperature evolution of the band at 1532 cm
1 (at T = 465 K) corresponding to the deformation d(NH+) modes (see Table 2) are presented in Fig. 4a. The temperature dependencies of wavenumbers of this band and its components are presented in Fig. 4b. The band at 1543 cm
1 (observed at T = 27 K) strongly changes its position towards lower wavenumbers on approaching the ferroelectric–paraelectric phase transition at 118 K (Tc2) from below. This tendency is similar over the (II) phase, but the changes are smaller. The band at 1521 cm
1 (27 K) shifts linearly with temperature over the phase III and II, whereas in the vicinity of 403 (Tc1) its frequency increases step-wisely. Over the phase I a single wide band is observed. Fig. 5 shows the temperature evolution of the bands in the region of some of the n(ring) + d(CH) vibrations. In this

Fig. 1. The IR spectra of polycrystalline sample of (C5H5NH)5Bi2Br11 measured at 27, 300 and 456 K in Nujol and at 300 K in fluorolube). Only the some bands are marked at the IR spectra.

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J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

Fig. 2. The IR spectra of polycrystalline samples of (C5H5NH)5Bi2Br11 and (C5H5ND)5Bi2Br11 measured in Nujol at room temperature. Only some bands are marked at the spectra.

frequency region the positions of the observed bands change little with temperature (see Fig. 6a). The band at 1321 cm
1 (at 27 K) shifts towards higher wavenumbers over the (III) and (II) phases. The band at 1330 cm
1 (at 27 K) shift slightly to lover wavenumbers and disappears in the phase (II) close to Tc1. The changes in the position of the bands at 1249 and 1262 cm
1 (at 27 K) around the Tc2 are quite small

(Fig. 6b). Near below Tc1 one of the band disappears and at the high temperature phase only one weak and broad band exists. The band at 1228 cm
1 (at 27 K) (Fig. 6c) shift to higher wavenumbers. This tendency is smaller in the (I) phase than in the (II) and (III) phases. The changes around Tc1 and Tc2 temperatures of the phase transitions are quite visible.

Fig. 3. The Raman spectra of polycrystalline sample of (C5H5NH)5Bi2Br11 and its deuterated analogue measured at room temperature. In the wavenumber range between 3500 and 2000 cm
1 the values of intensity for all spectra were multiplied by 5 (the 5:1 scale), between 1700 and 500 cm
1 were multiplied by 2 (the 2:1 scale) respect to the spectra taken 500 and 80 cm
1, due to its intensity value.

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

59

Table 2 Wavenumbers (cm
1), intensity and tentative assignments of the bands observed in the IR and Raman spectra of (C5H5NH)5Bi2Br11 at 27, 300 and 456 K and its deuterated analogue IR

Raman

IR deut

Raman deut

300 K

300 K

300 K

27 K

300 K

456 K

3245 vw 3235 vw

3238 vw

3242 m

3214 m 3172 m sh

3217 m 3174 m sh

3175 m sh

3162 m

3160 m

3160 m

3125 m

3125 m

3212 3179 3171 3166 3159 3133 3125 3114 3107 3095 3089 3074 3067 3061 3053 3046 3032 3027

m m m m m w w m w w w w w w m w w w

3131 3126 3114 3107 3098

w sh w m w w

3070 w

vw vw vw vw vw vw vw vw vw vw vw vw

3105 s 3095 s

3069 w

3078 vw

3068 vs

3060 m

3033 w

3037 w

2972a 2948a 2917a 2882a 2845a 2814 2791 2780 2769 2759 2724 2714 2702 2690 2682 2662 2639

3104 m sh 3091 m

3057 w

w w w w w

2791 vw

}

3237 vw sh 3214 m

3101 m

3036 w

3106 w sh 3097 w 3081 m

1 2 3

}

3056 vs

3052 w

19

3037 s

3031 w

20

2944 vw

+

n(NH )

n(CH)

2952 vvw

2793 vw

}

2771 vw 2728 vw

Assignment

ni

2730 vw

Overtones

2600 vw 2554 vw 2506 2418 2358 2278 2223 2185 2115 2049 2042 1982 1957

vw vw vw vw

1898 1886 1876 1853 1836

vw vw vw vw vw

1996 vw 1956 vw

1991 vw 1960 vw

vw w m w vw vw vw

2036 vw 1996 vw 1937 vw

1883 vw 1845 vw

1839 vw 1845 vw 1837 vw

2423 vvw 2367 vvw 2203 vvw

}

+

n(ND )

60

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

Table 2 (Continued ) IR 27 K 1801 1730 1638 1635 1632 1630 1617 1612 1609 1601 1598

vw vw w w m m w m w s m

1543 1528 1521 1509

w m vs vw

1353 1336 1330 1321 1313

w vw w m vw

1287 1262 1249 1242 1230 1228 1222 1198 1185 1180 1170

vw vw w vw w w vw vw s w vw

300 K

456 K

1732 vw

1737 vw

Raman

IR deut

Raman deut

300 K

300 K

300 K

1746 vw 1643 sh

1635 w sh 1631 m

1632 m

1633 m

1612 w

1611 w sh

1614 vw

1603 s

1603 m

1604 m 1589 vw

1533 w sh 1527 vw 1525 vs 1503 vw

1630 w 1622 w

1603 m

1602 w

4

1583 vw 1535 w 1527 vs

1574 w

21

1532 s 1504 w

1479a vs 1463a w 1433a vw 1412a vw 1383a vw 1374a w 1358 w 1329 w sh 1323 m

1630 s 1620 w 1612 w

1495 w 1482 vw

1483 vw

23

1327 w

1324 vw

1324 w 1302 m

1295 w

1254 w

1250 vw

1255 w

1236 vw

1236 w

1235 vw

1235 w

1193 m

1189 m

1189 m

1156 vw

1153 w

1259 vw

1234 w

1190 m

24

1192 w

1155 w

1126 vw

1123 vw

1101 1092 1079 1073 1064 1056 1053 1047 1044 1040 1031 1027 1025 1022 1018

1102 vw

}

d(NH+)

}

n(Ring) + d(CH)

1054 vw

1045 m

1047 w

1027 w

1026 vw

1053 w

26 1051 w

1045 vw

1026 w

1025 vw

} }

1038 w sh 1024 m

n(Ring) + d(CH)

n(Ring) + d(CH)

d(ND+)

7} 1061 vw

1054 w

n(Ring)

1132 w 1121 w 1113 vw

1071 vw

1066 w

1018 w sh

1163 w 1157 w 25

1110 vw

1073 w sh

6

1162 w 1131 vw

vw vw vw w w m s m s m w m m w w

5

}

n(Ring) + d(CH)

22

1287 w 1261 w 1249 w

1155 w 1148 w 1146 w

Assignment

ni

8

} }

}

n(Ring) + d(CH)

n(Ring) + d(CH)

Ring breatching

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

61

Table 2 (Continued ) Raman

IR deut

Raman deut

27 K

IR 300 K

456 K

300 K

300 K

300 K

1014 w 1008 w 1002 w

1012 w 1009 w

1011 vw

1013 sh 1008 vs

1014 vw

1012 m sh 1007 m

997 vw 992 984 979 975

915 904 891 876 873 867 859 852 819 800

vw vw w w

vw vw vw vw vw w w w sh w vw

976 w

976 vw

932 vw

927 vw

888 w

892 vw

9 11 15

945 w 927 w

860 vw

802 w

805 vw

761 w

677 675 671 664

677 676 671 665

809 vw

734 vw m m sh vs vs

865 vw

868 vw

790 782 761 748 705

805 vw

w s vw vs vw

12

16

676 m sh 669 s

637 vw

637 w

636 w

637 w

611 w

610 w

610 w

610 w

173 vs 147 vs sh 101 vs

674 667 649 635 621 608 604 472

vs m sh w m s w sh vw vw

}

g(CH)

g(NH+)

867 vw 863 w

763 vw 751 s m vs sh vs vs

973 vw

14

994 s 984 w

Assignment

ni

17

635 w 623 w 604 vw

397 377 173 146 107

vw vw vs s s

} }

g(CH) + t(ring)

g(ND+) n(Ring)

27 10

g(CH)

}

n(Ring)

n(Bi–Br)terminal n(Bi–Br)bridge d(BrBiBr)

vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. a Spectra in fluorolube.

Fig. 7a shows an evolution of the bands arising from the g(CH) + t(ring) vibrations. In the lowest temperature phase four bands are observed therein (Fig. 7b). One (675 cm
1 at 27 K) of the band disappears at phase (II). The two other components (671 and 664 cm
1, see Table 2) show only a weak anomaly in the band positions at Tc2, whereas just above Tc1 they exhibit an important drop by several cm
1. The temperature dependencies are different for various modes in the vicinity of the ferroelectric–paraelectric phase transition temperature (Tc2) and structural transition at Tc1. Most of the bands change continuously with temperature around the Tc2, which is in agreement with the continuous character of this transition. The most sensitive to the ferroelectric transition appears to be the highest frequency component (1543 cm
1 at 27 K) of the strong band assigned to the d(NH+) deformation mode. Its frequency decreases significantly over the phase (III) approaching ca. 1536 cm
1

at Tc2. A similar behaviour but with smaller changes, reveals the medium band observed at the 675 cm
1 (in 27 K). The temperature variations of positions for the analysed modes change like an order parameter (the spontaneous polarization) of the title crystal. Most of the analysed bands corresponding to the internal vibrations of the pyridinium cation show anomalous changes of intensity and of the FWHM in the vicinity of the temperature of the phase transitions at 118 and 403 K. This behaviour confirms that the changes of the dynamical states of the organic cations play the main role in the mechanism of the phase transitions. However, the observed changes of the FWHM can be described by self-diffusion theory [28,29] for few bands only. This behaviour is characteristic for the ‘‘order–disorder’’ phase transition. The reorientational correlation time is the mean reorientational time of the cations to jump from one potential well to another and it is

62

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

Fig. 4. (a) The temperature evolution of the infrared spectra of (C5H5NH)5Bi2Br11 of the deformation modes d(NH+) and (b) temperature dependence of the bands in this wavenumber range.

Fig. 5. The infrared spectra of (C5H5NH)5Bi2Br11 at several temperatures of the deformation modes, n(ring) + d(CH).

Fig. 6. Temperature dependence of the deformation modes, n(ring) + d(CH), appearing between 1340 and 1210 cm
1.

J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

63

Fig. 7. (a) The temperature evolution of the infrared spectra of (C5H5NH)5Bi2Br11 of the deformation modes, g(CH) + t(ring), in the frequency region 690– 645 cm
1, and (b) temperature dependence in the 678–662 cm
1 wavenumber range.

given by:

semi-diffusion approximation): 

tR ðTÞ ¼ t0 exp

U kB T

 (1)

where t0 is given by h/kBT, U the height of the potential barrier and kB is the Boltzman constant. The temperature dependence of the band width G(v, T) (i.e., FWHM) is described by the equation [28,29] (in a

G ðv; TÞ ¼ ða þ bTÞ þ c0

tR 1 þ v2 t 2R

(2)

where v is the frequency of a particular phonon mode. The first linear part of these Eq. (2) corresponds to the vibrational relaxation or the anharmonicity and the second term represents the thermal orientational mechanism of diffuse nature. In our case, the t
1 R at Tc appears in the

Fig. 8. Temperature dependence of the FWHM of the 3053 (a), 1321 (b) and 664 cm
1 (c) modes.

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J. Tarasiewicz et al. / Vibrational Spectroscopy 40 (2006) 55–65

Table 3 The fitted parameters, a, b, c and U, for temperature dependence of the damping coefficient G Parameter
1

a (cm ) b (cm
1 K
1) c (cm
1) U (eV)

3053 cm
1

1333 cm
1

664 cm
1

3.65 2.49  10
2 49.0 0.032

2.66 2.84  10
3 115.1 0.060

3.18 4.35  10
3 859.1 0.069

microwave frequency region. Therefore, v2 t 2R  1 and Eq. (2) is reduced to [28,29]:  
U G ðTÞ ¼ ða þ bTÞ þ c exp (3) kB T Fig. 8 presents the temperature dependence of the FWHM for the 3053 cm
1 (19 mode in Table 2), 1321 and 664 cm
1 band. The temperature coefficient of the FWHM of all these bands change distinctly around of the both phase transitions temperatures. The FWHM of band at 3053 cm
1 (19 mode) increases with increasing temperature over the low temperature (III) and intermediate (II) phase. Then, it decreases stepwisely in the vicinity of Tc1. The FWHM of 1321 cm
1 mode increases over the all measured temperature region. The temperature coefficients of the bandwidths change visibly close to the Tc2 and Tc1. The FWHM of 664 cm
1 mode, in general, show a similar temperature characteristic as that of 1321 cm
1. The method of least squares was used for fitting the experimental data to Eq. (3). The fitted a, b, c and U parameters for the analysed bands are listed in Table 3. It should be emphasized the relatively low value of the estimated energy barrier (U  0.030–0.070 eV). These values are significantly smaller than those obtained for various alkylammonium salts characterised by an order– disorder mechanism of the PT. Using the same treatment (Carabatos-Nedelec and Becker [30]) the energy barriers in [(CH3)2NH2]3Sb2Cl9 [31] (Raman studies) and (CH3ND3)3Sb2Br9 [32] (infrared studies) for the selected modes of the organic cations were found to be about 0.1 eV. Similarly, in the case of [(CH3)2NH2]3Cd3Cl11 from Raman studies, the U is estimated to be 0.16 eV [33]. It is interesting that the energy barrier calculated with the CarabatosNedelec and Becker [30] and de Andrade and Porto [29] approach for (C5H5NH)5Bi2Br11 is comparable with that found from the dielectric dispersion measurements [12]
0.05 eV based on the phenomenological Vogel-Fulcher law and assigned to the motion of dipolar pyridinium moieties. The good agreement for the U values from various spectroscopic methods clearly confirms that the dynamics of the pyridinium cations should be involved in the mechanism of the ferroelectric PT at 118 K. Our infrared studies allows us to conclude that the character of the temperature changes of the bandwidths (FWHM) of all analysed modes indicate a significant increase of freedom of rotational motions of the organic cations contributing to the mechanism of both phase transitions.

It is interesting to note that a similar temperature effects in the infrared spectra are encountered in the case of other ionic pyridinium salts exhibiting structural phase transitions governed by the dynamics of the cationic sublattice, e.g., (PyH)6Bi4Cl18 [34], (PyH)IO4 [35]. In the case of the bismuthate(III) salts, the most sensitive to the phase transitions at 122 and 154 K appear to be the n(C–H), n(ring) and g(C–H) modes. Whereas, in the case of (PyH)IO4 ferroelectric crystal [34], the symmetric ring-breathing mode is the most sensitive. Nevertheless, the observed temperature changes of the positions and FWHM of the bands in the case of (PyH)IO4 were rather subtle in a comparison to those observed in title crystal.

4. Conclusions 1. The most important changes of the positions are observed for the bands arising from the n(ring) + d(CH), g(CH) + t(ring) and g(NH+) vibrations. 2. Substantial changes observed at 118 and 403 K for most of the internal modes of pyridinium cations clearly indicates that the dynamics of these cations contributes to the mechanism of both phases transitions. 3. The analysis of several internal modes (3053 cm
1— mode 19, 1321 and 664 cm
1) based on the ‘‘selfdiffusion’’ theory revealing relatively small value of the activation energy U suggests that the pyridinium cations and Bi2Br115
anions are connected by quite weak hydrogen bonds.

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