- Email: [email protected]
On structural phase transitions in (i-C4H9NH3)3Bi2Br9: differential scanning calorimetry, dielectric and infrared studies
MOLSTR 10332
Journal of Molecular Structure 450 (1998) 247–257
On structural phase transitions in (i-C 4H 9NH 3) 3Bi 2Br 9: differential scanning calorimetry, dielectric and infrared studies J. Jo´z´ko´w a, G. Bator a, R. Jakubas a, J. Baran b,* a Department 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 15 November 1997; revised 14 January 1998; accepted 14 January 1998
Abstract Two structural phase transitions at 263 and 252 K are detected in a new isobutylammonium crystal (i-C 4H 9NH 3) 3Bi 2Br 9 by means of differential scanning calorimetry (DSC) and dielectric studies. Internal vibrations modes of (i-C 4H 9NH 3) 3Bi 2Br 9 are studied through their phase transitions using the infrared spectrscopy. The infrared studies show that the vibrational state of isobutylammonium cations changes weakly during the phase transition at 252 K. The 263 K phase transition is not reflected in the infrared spectra. The lower temperature phase transition (252 K) is believed to be governed by the reorientational motion of the isobutylammonium cations and may be classified as an ‘order–disorder’ type. q 1998 Elsevier Science B.V.
1. Introduction Metal-halide compounds, containing small in size organic cations such as methyl-, dimethyl- and trimethyl-ammonium with the general formula [(CH 3) nNH 4-n,] 3M 2X 9 (M = Sb, Bi, X = Cl, Br, I) have attracted much interest because of their ferroelectric properties [1]. Recently, we have extended studies to halogenoantimonates(III) and bismuthates(III) containing bulky alkylammonium cations. i-propylammonium analogs of the R 2MX 5 formula (where R = alkylammonium cations) show the low temperature phase transitions which are due to the reorientational motion of the cations. In the disordered phase these cations are distributed between two equivalent positions [2]. The n-propylammonium * Corresponding author. Tel: 00 48 71 343 5021; Fax: 00 48 71 441029; E-mail: [email protected]
(R 2MX 5, R 3MX 6 and R 3M 2X 9) [3,4] and n-butylammonium (R 2MX 5 and R 3MX 6) [5] salts disclosed more complex sequence of solid–solid phase transitions. The X-ray studies for these salts showed that the n-alkylammonium cations perform the flipping motion around the long axis of the molecule [6]. The dynamical disorder of these cations was confirmed by the dielectric dispersion results for the (n-C 3H 7NH 3) 3Sb 2Cl 9 [6] and (n-C 4H 9NH 3) 2BiCl 5 crystals [5]. Debye-like dispersion found in the microwave frequency region clearly indicate the ‘order– disorder’ mechanism of the phase transition in these crystals. Most of the phase transitions found in halogenoantimonates(III) and bismuthates(III) family seems to be triggered by a change in the state of motion of the alkylammonium groups [7]. In order to learn more about the transitional behaviour of halogenobismuthates(III) family of compounds we have undertaken experimental studies on
0022-2860/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 8 )0 0 43 5 -9
248
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
new crystals, containing bulky isobutylammonium cations of the formula (i-C 4H 9NH 3) 3Bi 2Br 9. In this paper we report the calorimetric (DSC), dielectric and IR results around the phase transitions at 263 and 252 K.
2. Experimental details Differential scanning calorimetry (DSC) measurements were carried out using a Perkin-Elmer DSC-7 calorimeter with a scanning speed 108/min on cooling/ heating. The complex electric permittivity, «*, for a single crystal of (i-C 4H 9NH 3) 3Bi 2Br 9 along the a, b and caxis was measured by an HP 4285A Precision LCR Meter at frequency of 100 kHz in the temperature range between 240 and 300 K with a cooling rate of 0.5 K/min. Samples for dielectric measurements were typically of size 4 × 3 × 1 mm 3. The accuracy of the measured, electric permittivity value was about 5%. Infrared spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 (mulls in Nujol) in the temperature range between 14 and 300 K were recorded with a BRUKER IFS-88 FT-IR spectrometer with resolution 1 cm −1. APD Cryogenics with closed cycle helium cryodyne system was used for temperature dependence studies. The temperature of the sample was maintained at an accuracy of 60.1 K.
3. Results
Fig. 1. DSC curves for cooling and heating runs (10 K/min.) for (iC 4H 9NH 3) 3Bi 2Br 9 crystals.
first-order, whereas the one at 263 K is rather close to the second-order type.
3.1. Differential scanning calorimetry Fig. 1 shows two DSC peaks observed at T c1 = 263 K and T c2 = 252 K, on cooling, and at T c1 = 264 K and T c2 = 252.4 K, on heating (scanning rate 10 K/min and m = 28.3 mg). The respective entropies of the transitions are 6.9 and 1.35 J/mol K. The order of the phase transition was determined from the shape of the observed DSC anomaly and from the thermal hysteresis extrapolated linearly to the zero scanning rate. The lower temperature peak exhibits a wide c p ‘tail’. The temperature hysteresis of this anomaly is quite small, but the shape of the peak is rather strong and the peak positions differ markedly during heating and cooling scan. The phase transition at 252 K is classified as a weak
3.2. Dielectric measurements At room temperature the title crystal belongs to the orthorhombic system and the full crystalographic results will be published elsewhere (J. Zaleski et al., pers. comm.). The temperature dependence of the real part of the complex electric permittivity along the main crystallographic directions for (iC 4H 9NH 3) 3Bi 2Br 9 at 100 kHz is displayed in Fig. 2. The crystal of (i-C 4H 9NH 3) 3Bi 2Br 9 exhibits a distinct anisotropy of the dielectric properties. The temperature dependence of the static electric permittivity along the b-axis reveals a distinct peak at 252 K which confirms the presence of the phase transition disclosed by DSC measurements. The Curie–Weiss
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
249
Fig. 2. Temperature dependence of the real part, «9, of the complex electric permittivity along the main crystallographic directions for the (iC 4H 9NH 3) 3Bi 2Br 9 crystals at 100 kHz.
relation « − «0 = TC−+T0 is fulfilled over the temperature range 252–258 K (see Fig. 3) with a constant value C + = 39 K (above T c2 = 252 K) and C − = 2.4 K (below T c2). The ratio of these Curie constant C +/C - > 16 is considerably larger than that expected for ferroelectric crystals (C +/C - = 2 or 4 for second- or first-order ferroelectric phase transition). The 252 K phase transition may be considered as a antiferroelectric or incommensurate one. The second phase transition is seen as a subtle change in the slope of «9 b vs temperature at 263 K. The dielectric characteristics, particularly in the intermediate phase (between T c1 and T c2) is unusual. A quite considerable increase of permittivity (of the order of 8 units) is observed in the range of a few Kelvins. Such dielectric response is typical for materials in which the significant dipole–dipole interactions with a weak ferroelectric or antiferroelectric order occur. No polar properties are, however, detected by means of the pyroelectric effect measurements in this crystal in the vicinity of T c2. A preliminary studies of dielectric dispersion indicate the relaxation process in the megahertz frequency region (75 kHz–30 MHz) which additionally confirms the ‘order–disorder’ mechanism of the phase transition to be related to a reorientation of the i-butylammonium cations. The origin of the dielectric anomaly, especially
close to 252 K, seems to be connected with the ordering of the isobutylammonium cations. As a possible relaxation process one can consider the 1808 flip motion of one of three cations. Such a motion is acceptable based on the calorimetric results (DS teor = Rln2 = 5.7 J/mol.K for the two positions model, whereas DS exp = 6.9 J/mol K). 3.3. Infrared and Raman studies The infrared spectra of the polycrystalline (iC 4H 9NH 3) 3Bi 2Br 9 in the frequency range between 3500 and 500 cm -1 and for temperature between 14 and 300 K are presented in Fig. 4. This frequency region is related to the internal vibrations of the isobutylammonium cations. The spectra in the frequency range 3000–2700 cm -1, 1500–1350 cm -1 and 750– 700 cm -1 are not shown (broken line in Fig. 4) because the bands of Nujol appear in these regions. The assignment of the infrared and Raman bands presented in the Table 1 is essentially based on comparison with n-propylamine, n-butylamine [8,9] and bromobismuthates(III) [10] spectral studies. The temperature dependent I.R. studies allow to check out the influence of the change in the dynamical state of the i-butylammonium cations on the internal
250
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 3. The Curie–Weiss law for (i-C 4H 9NH 3) 3Bi 2Br 9 along the b-axis.
vibrations of them. Usually the splitting of the bands related to the ammonium NH3+ group vibrations as well as the changes in their intensities and width are expected. The analysis was performed also for the other bands for which the distinct temperature changes are observed. The temperature evolution of the infrared spectra in the region of the asymmetric na (NH3+ ) and symmetric ns (NH3+ ) stretching vibrations, deformation d(NH3+ ) vibrations and those for stretching skeleton n a(CCC) or n(CH 3) and rocking r(CH 3) vibrations are shown in Figs. 5, 7, and 9, respectively. In the roomtemperature phase these bands are broad and they can be resolved only into few components. The temperature dependence of the frequencies of the stretching [ns (NH3+ ) and na (NH3+ )] modes is shown in Fig. 6. Nine components of these vibrations are observed in the low-temperature phase. Four of them disappear and five components for the n(NH3+ ) vibrations can be seen above T c2 = 252 K. The temperature dependence of the bending d(NH3+ ) mode is shown in Fig. 8. At low temperatures five components can be distinguished in this region. Two of them at 1595 and 1587 cm -1 disappear far below T c2 and only two broad bands in this region are found above in the vicinity of T c2. In Fig. 10 there are displayed the temperature dependencies of the frequencies for the
Fig. 4. The powder IR spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 at 14 and 300 K (in Nujol).
251
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257 Table 1 Wavenumbers (cm -1), relative intensities and tentative assignement of the IR bands arising from the internal vibrations of isobuthylammonium cations in the (i-C 4H 9NH 3) 3Bi 2Br 9 at 30 and 300 K 30 K 3242w 3202m 3178m 3157m 3137m 3101m 3083s 3063m 3036w
IR 300 K
Raman 300 K
3.197m
928vw 925vw 914w
3108w
ns (NH3+ )
856w 851m 847w 845w 789w 787w
IR 300 K
Raman 300 K
Tentative assignment
844w
846vw
n s(CCC)
788vw
790w
q(NH3+ )
554w 468w 441vw
471vw 458w
424
*
1585vw 1575s
1478vs * 1468vs * 1400m * 1373vw *
1040s 1035m 1029m 1007m 1004s 1003m 990sh 987s 967 963vw 958vw
na (NH3+ )
30 K
3082m 3044m
2876m
1319w 1309w 1300w 1204w 1199vw 1180vw 1155w
3175w 3134m
2962m *
1595w 1587sh 1584s 1575m 1566m
Tentative assignment
Table 1 continued
1317vw 1310w 1288vw
2993w 2975vw 2962s 2930w 2897m 2875w 1580vw
n a(CH 3) n a(CH 2) n s(CH 3) n s(CH 2) n(CH)
d(CH 2) d a(CH 3) d s(CH 3)
1312vw
g(CH 2)
1034m
1045vw 1035vw
combination of r(NH3+ ) and r(CH 3)
n a(CN)
1000m
n a(CCC)
988w
r(NH3+ )
961vw
963vw
913vw
n s(CN)
r(CH 3)
929w 914vw
184vs 162w 134m
105 89 71
84w 67s 48w 30vw
*
n s Bi–Br external n a Bi–Br external n s Bi–Br bridge n a Bi–Br bridge bending mode lattice modes
Assignment for the spectrum of the pellet in KBr.
q(CH 2)
t(CH 2)
1182vw 1154vw
164
da (NH3+ ) ds (NH3+ )
1482vw 1468vw 1459vw 1378vw 1349vw
1197vw 1181vw 1152w
369 264
425w 381vw 353w
chain deformation d(CCN) and d(CCC)
r(CH 2)
stretching n a(CCC), n(CN) and rocking r(CH 3)modes. Below T c2 three components for the n a(CCC), five components for the n(CN) and three components for r(CH 3) bands are observed. At room temperature single, broad band assigned to r(CH 3) is found, whereas in the region of n(CN) two bands are visible. The bands below 600 cm -1 can be assigned to a combination of (CCC) deformation with a contribution from the d(CCN) mode. The temperature changes of the other modes are quite small. The important changes observed in the IR spectrum at 252 K for most internal modes of cations clearly indicates alteration in the motional state of these cations when the phase transition is crossed. The hydrogen bonds are believed to be responsible for the ordering process in the cationic sublattice. It is interesting to note that only the 252 K phase transition is detected by the IR spectrum. The higher temperature phase transition at 263 K is less visible
252
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 5. Temperature evolution of the asymmetric and symmetric NH3+ stretching, na (NH3+ ) and ns (NH3+ ), modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
probably due to the fact that it is subtle taking into account its small entropy change (DS < 1.35 J/mol K). The Raman spectra in the frequency range (3400– 5 cm −1) at room temperature for the b(aa)c configuration are shown in Fig. 11.The Raman bands at 184 and 162 cm −1 correspond to the symmetric and asymmetric Bi–Br stretching modes of the external bromines, respectively. The stretching frequencies for the bridged Bi–Br–Bi stretches give Raman bands at 134 and 84 cm −1. Two lattice modes are observed below 50 cm −1. The temperature dependence of the IR intensity of the 1040 cm -1 mode shows a distinct variation in the temperature range between 140 and 240 K. The reduced peak intensity I p = I p9[q p/(n(q) + 1)], where q p and I p9 are peak frequency and intensity,
respectively, and n(q) is a Bose–Einstein distribution of oscillator, is plotted as a function of temperature in Fig. 12. According to Bruce et al. [11] the rate at which I p decreases in the ordered state depends on the long range interactions and is proportional to square of the order parameter h. Here, we consider the variations in the reduced peak intensity so that the dispersion effects would be minimised [12]. We plot a log–log graph of intensity changes against (T c − T), with T c = 252 K, in the temperature range 140–240 K. The plot is shown as the inset in Fig. 12. From the least square approximation the estimated slope is equal to 0.75 6 0.05. This value of 2b is significantly different from the mean field exponent, but is quite close to that observed in other similar systems [12–14].
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 6. Temperature dependence of the frequencies of the na (NH3+ ) and ns (NH3+ ) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 7. Temperature evolution of the asymmetric NH3+ bending, d(NH3+ ), mode for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
253
254
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 8. Temperature dependence of the frequencies of the d(NH3+ ) mode for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 9. Temperature evolution of the n(CN) and r(CH 3) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
255
Fig. 10. Temperature dependence of the frequencies of the n(CN) and r(CH 3) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 13 presents the temperature dependence of the bandwidths of the 987 and 914 cm −1 modes. It is clearly seen that the 914 cm -1 band undergoes the stepwise change at T c. Below T c its width first decreases however it is practically temperature
independent below 150 K. In contrary to this behaviour we observed only linear width of 987 cm -1 mode. We can state that the linewidth of this modes is weakly temperature dependence, which may indicate weak anharmonic forces.
Fig. 11. The Raman spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 at room temperature for the b(aa)c configuration. In the wavenumber range between 3400 and 2800 cm −1 the values of intensity for all spectra were divided by 10 (the 1:10 scale) with respect to the spectra taken between 1600 and 400 cm -1. The intensity value of the spectra between 0 and 300 cm -1 was divided by 200 (the 1:200 scale) due to its high intensity value.
256
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 12. Reduced peak intensity I p for the 1040 cm −1 mode. Inset shows a plot of log (I p) vs log(T c2 − T) for this mode.
Fig. 13. Linewidth variations of the 987 cm −1 mode (a) and the 914 cm −1 mode (b).
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
4. Conclusions 1. Two reversible solid–solid phase transitions of first-order type are disclosed at 252 and 263 K in the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals. 2. Temperature changes of the IR spectra corresponding to the internal vibrations of the i-butylammoniumn cations were observed only in the vicinity of 252 K phase transition. 3. Dynamics of the i-butylammonium cation (two positions model) seems to contribute to the mechanism of the 252 K phase transition of the ‘order–disorder’ type.
References [1] R. Jakubas, L. Sobczyk, Phase Transitions 20 (1990) 163.
257
[2] R. Jakubas, G. Bator, P. Ciapała, J. Zaleski, J. Baran, J. Lefebvre, J. Phys. Condens. Matter 7 (1995) 5335. [3] R. Jakubas, P. Ciapała, G. Bator, Z. Ciunik, J. Baran, J. Lefebvre, Physica B 217 (1996) 67. [4] N. Pis´lewski, J. Tritt-Goc, R. Jakubas, Phys. Stat. Sol. (b) 193 (1996) 67. [5] P. Ciapała, R. Jakubas, G. Bator, J. Zaleski, A. Pietraszko, M. Drozd, J. Baran, J. Phys. Condens. Matter 9 (1997) 627. [6] P. Ciapała, J. Zaleski, G. Bator, R. Jakubas, A. Pietraszko, J. Phys. Condens. Matter 8 (1996) 627. [7] L. Sobczyk, R. Jakubas, J. Zaleski, Polish J. Chem. 71 (1997) 265. [8] M. Hamada, Chem. Phys. 125 (1988) 55. [9] J.J.C. Teixeira-Dias, L.A.E. Batista de Carvalho, A.M. Amorim da Costa, I.M.S. Lampeira, E.F.G. Barbosa, Spectrochimica Acta 42A (5) (1986) 589. [10] J. Laane, P.W. Jagodzinski, Inorg. Chem. 19 (1986) 44. [11] A.D. Bruce, W. Taylor, A.F. Murray, J. Phys. C: Solid State Phys. 13 (1980) 483. [12] W. Dultz, J. Chem. Phys. 65 (1976) 2812. [13] P.S.R. Prasad, H.D. Bist, J. Phys. Chem. Solids 50 (1989) 1033. [14] P.S.R. Prasad, H.D. Bist, Phys. Stat. Sol. (a) 116 (1989) 275.
Journal of Molecular Structure 450 (1998) 247–257
On structural phase transitions in (i-C 4H 9NH 3) 3Bi 2Br 9: differential scanning calorimetry, dielectric and infrared studies J. Jo´z´ko´w a, G. Bator a, R. Jakubas a, J. Baran b,* a Department 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 15 November 1997; revised 14 January 1998; accepted 14 January 1998
Abstract Two structural phase transitions at 263 and 252 K are detected in a new isobutylammonium crystal (i-C 4H 9NH 3) 3Bi 2Br 9 by means of differential scanning calorimetry (DSC) and dielectric studies. Internal vibrations modes of (i-C 4H 9NH 3) 3Bi 2Br 9 are studied through their phase transitions using the infrared spectrscopy. The infrared studies show that the vibrational state of isobutylammonium cations changes weakly during the phase transition at 252 K. The 263 K phase transition is not reflected in the infrared spectra. The lower temperature phase transition (252 K) is believed to be governed by the reorientational motion of the isobutylammonium cations and may be classified as an ‘order–disorder’ type. q 1998 Elsevier Science B.V.
1. Introduction Metal-halide compounds, containing small in size organic cations such as methyl-, dimethyl- and trimethyl-ammonium with the general formula [(CH 3) nNH 4-n,] 3M 2X 9 (M = Sb, Bi, X = Cl, Br, I) have attracted much interest because of their ferroelectric properties [1]. Recently, we have extended studies to halogenoantimonates(III) and bismuthates(III) containing bulky alkylammonium cations. i-propylammonium analogs of the R 2MX 5 formula (where R = alkylammonium cations) show the low temperature phase transitions which are due to the reorientational motion of the cations. In the disordered phase these cations are distributed between two equivalent positions [2]. The n-propylammonium * Corresponding author. Tel: 00 48 71 343 5021; Fax: 00 48 71 441029; E-mail: [email protected]
(R 2MX 5, R 3MX 6 and R 3M 2X 9) [3,4] and n-butylammonium (R 2MX 5 and R 3MX 6) [5] salts disclosed more complex sequence of solid–solid phase transitions. The X-ray studies for these salts showed that the n-alkylammonium cations perform the flipping motion around the long axis of the molecule [6]. The dynamical disorder of these cations was confirmed by the dielectric dispersion results for the (n-C 3H 7NH 3) 3Sb 2Cl 9 [6] and (n-C 4H 9NH 3) 2BiCl 5 crystals [5]. Debye-like dispersion found in the microwave frequency region clearly indicate the ‘order– disorder’ mechanism of the phase transition in these crystals. Most of the phase transitions found in halogenoantimonates(III) and bismuthates(III) family seems to be triggered by a change in the state of motion of the alkylammonium groups [7]. In order to learn more about the transitional behaviour of halogenobismuthates(III) family of compounds we have undertaken experimental studies on
0022-2860/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 8 )0 0 43 5 -9
248
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
new crystals, containing bulky isobutylammonium cations of the formula (i-C 4H 9NH 3) 3Bi 2Br 9. In this paper we report the calorimetric (DSC), dielectric and IR results around the phase transitions at 263 and 252 K.
2. Experimental details Differential scanning calorimetry (DSC) measurements were carried out using a Perkin-Elmer DSC-7 calorimeter with a scanning speed 108/min on cooling/ heating. The complex electric permittivity, «*, for a single crystal of (i-C 4H 9NH 3) 3Bi 2Br 9 along the a, b and caxis was measured by an HP 4285A Precision LCR Meter at frequency of 100 kHz in the temperature range between 240 and 300 K with a cooling rate of 0.5 K/min. Samples for dielectric measurements were typically of size 4 × 3 × 1 mm 3. The accuracy of the measured, electric permittivity value was about 5%. Infrared spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 (mulls in Nujol) in the temperature range between 14 and 300 K were recorded with a BRUKER IFS-88 FT-IR spectrometer with resolution 1 cm −1. APD Cryogenics with closed cycle helium cryodyne system was used for temperature dependence studies. The temperature of the sample was maintained at an accuracy of 60.1 K.
3. Results
Fig. 1. DSC curves for cooling and heating runs (10 K/min.) for (iC 4H 9NH 3) 3Bi 2Br 9 crystals.
first-order, whereas the one at 263 K is rather close to the second-order type.
3.1. Differential scanning calorimetry Fig. 1 shows two DSC peaks observed at T c1 = 263 K and T c2 = 252 K, on cooling, and at T c1 = 264 K and T c2 = 252.4 K, on heating (scanning rate 10 K/min and m = 28.3 mg). The respective entropies of the transitions are 6.9 and 1.35 J/mol K. The order of the phase transition was determined from the shape of the observed DSC anomaly and from the thermal hysteresis extrapolated linearly to the zero scanning rate. The lower temperature peak exhibits a wide c p ‘tail’. The temperature hysteresis of this anomaly is quite small, but the shape of the peak is rather strong and the peak positions differ markedly during heating and cooling scan. The phase transition at 252 K is classified as a weak
3.2. Dielectric measurements At room temperature the title crystal belongs to the orthorhombic system and the full crystalographic results will be published elsewhere (J. Zaleski et al., pers. comm.). The temperature dependence of the real part of the complex electric permittivity along the main crystallographic directions for (iC 4H 9NH 3) 3Bi 2Br 9 at 100 kHz is displayed in Fig. 2. The crystal of (i-C 4H 9NH 3) 3Bi 2Br 9 exhibits a distinct anisotropy of the dielectric properties. The temperature dependence of the static electric permittivity along the b-axis reveals a distinct peak at 252 K which confirms the presence of the phase transition disclosed by DSC measurements. The Curie–Weiss
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
249
Fig. 2. Temperature dependence of the real part, «9, of the complex electric permittivity along the main crystallographic directions for the (iC 4H 9NH 3) 3Bi 2Br 9 crystals at 100 kHz.
relation « − «0 = TC−+T0 is fulfilled over the temperature range 252–258 K (see Fig. 3) with a constant value C + = 39 K (above T c2 = 252 K) and C − = 2.4 K (below T c2). The ratio of these Curie constant C +/C - > 16 is considerably larger than that expected for ferroelectric crystals (C +/C - = 2 or 4 for second- or first-order ferroelectric phase transition). The 252 K phase transition may be considered as a antiferroelectric or incommensurate one. The second phase transition is seen as a subtle change in the slope of «9 b vs temperature at 263 K. The dielectric characteristics, particularly in the intermediate phase (between T c1 and T c2) is unusual. A quite considerable increase of permittivity (of the order of 8 units) is observed in the range of a few Kelvins. Such dielectric response is typical for materials in which the significant dipole–dipole interactions with a weak ferroelectric or antiferroelectric order occur. No polar properties are, however, detected by means of the pyroelectric effect measurements in this crystal in the vicinity of T c2. A preliminary studies of dielectric dispersion indicate the relaxation process in the megahertz frequency region (75 kHz–30 MHz) which additionally confirms the ‘order–disorder’ mechanism of the phase transition to be related to a reorientation of the i-butylammonium cations. The origin of the dielectric anomaly, especially
close to 252 K, seems to be connected with the ordering of the isobutylammonium cations. As a possible relaxation process one can consider the 1808 flip motion of one of three cations. Such a motion is acceptable based on the calorimetric results (DS teor = Rln2 = 5.7 J/mol.K for the two positions model, whereas DS exp = 6.9 J/mol K). 3.3. Infrared and Raman studies The infrared spectra of the polycrystalline (iC 4H 9NH 3) 3Bi 2Br 9 in the frequency range between 3500 and 500 cm -1 and for temperature between 14 and 300 K are presented in Fig. 4. This frequency region is related to the internal vibrations of the isobutylammonium cations. The spectra in the frequency range 3000–2700 cm -1, 1500–1350 cm -1 and 750– 700 cm -1 are not shown (broken line in Fig. 4) because the bands of Nujol appear in these regions. The assignment of the infrared and Raman bands presented in the Table 1 is essentially based on comparison with n-propylamine, n-butylamine [8,9] and bromobismuthates(III) [10] spectral studies. The temperature dependent I.R. studies allow to check out the influence of the change in the dynamical state of the i-butylammonium cations on the internal
250
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 3. The Curie–Weiss law for (i-C 4H 9NH 3) 3Bi 2Br 9 along the b-axis.
vibrations of them. Usually the splitting of the bands related to the ammonium NH3+ group vibrations as well as the changes in their intensities and width are expected. The analysis was performed also for the other bands for which the distinct temperature changes are observed. The temperature evolution of the infrared spectra in the region of the asymmetric na (NH3+ ) and symmetric ns (NH3+ ) stretching vibrations, deformation d(NH3+ ) vibrations and those for stretching skeleton n a(CCC) or n(CH 3) and rocking r(CH 3) vibrations are shown in Figs. 5, 7, and 9, respectively. In the roomtemperature phase these bands are broad and they can be resolved only into few components. The temperature dependence of the frequencies of the stretching [ns (NH3+ ) and na (NH3+ )] modes is shown in Fig. 6. Nine components of these vibrations are observed in the low-temperature phase. Four of them disappear and five components for the n(NH3+ ) vibrations can be seen above T c2 = 252 K. The temperature dependence of the bending d(NH3+ ) mode is shown in Fig. 8. At low temperatures five components can be distinguished in this region. Two of them at 1595 and 1587 cm -1 disappear far below T c2 and only two broad bands in this region are found above in the vicinity of T c2. In Fig. 10 there are displayed the temperature dependencies of the frequencies for the
Fig. 4. The powder IR spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 at 14 and 300 K (in Nujol).
251
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257 Table 1 Wavenumbers (cm -1), relative intensities and tentative assignement of the IR bands arising from the internal vibrations of isobuthylammonium cations in the (i-C 4H 9NH 3) 3Bi 2Br 9 at 30 and 300 K 30 K 3242w 3202m 3178m 3157m 3137m 3101m 3083s 3063m 3036w
IR 300 K
Raman 300 K
3.197m
928vw 925vw 914w
3108w
ns (NH3+ )
856w 851m 847w 845w 789w 787w
IR 300 K
Raman 300 K
Tentative assignment
844w
846vw
n s(CCC)
788vw
790w
q(NH3+ )
554w 468w 441vw
471vw 458w
424
*
1585vw 1575s
1478vs * 1468vs * 1400m * 1373vw *
1040s 1035m 1029m 1007m 1004s 1003m 990sh 987s 967 963vw 958vw
na (NH3+ )
30 K
3082m 3044m
2876m
1319w 1309w 1300w 1204w 1199vw 1180vw 1155w
3175w 3134m
2962m *
1595w 1587sh 1584s 1575m 1566m
Tentative assignment
Table 1 continued
1317vw 1310w 1288vw
2993w 2975vw 2962s 2930w 2897m 2875w 1580vw
n a(CH 3) n a(CH 2) n s(CH 3) n s(CH 2) n(CH)
d(CH 2) d a(CH 3) d s(CH 3)
1312vw
g(CH 2)
1034m
1045vw 1035vw
combination of r(NH3+ ) and r(CH 3)
n a(CN)
1000m
n a(CCC)
988w
r(NH3+ )
961vw
963vw
913vw
n s(CN)
r(CH 3)
929w 914vw
184vs 162w 134m
105 89 71
84w 67s 48w 30vw
*
n s Bi–Br external n a Bi–Br external n s Bi–Br bridge n a Bi–Br bridge bending mode lattice modes
Assignment for the spectrum of the pellet in KBr.
q(CH 2)
t(CH 2)
1182vw 1154vw
164
da (NH3+ ) ds (NH3+ )
1482vw 1468vw 1459vw 1378vw 1349vw
1197vw 1181vw 1152w
369 264
425w 381vw 353w
chain deformation d(CCN) and d(CCC)
r(CH 2)
stretching n a(CCC), n(CN) and rocking r(CH 3)modes. Below T c2 three components for the n a(CCC), five components for the n(CN) and three components for r(CH 3) bands are observed. At room temperature single, broad band assigned to r(CH 3) is found, whereas in the region of n(CN) two bands are visible. The bands below 600 cm -1 can be assigned to a combination of (CCC) deformation with a contribution from the d(CCN) mode. The temperature changes of the other modes are quite small. The important changes observed in the IR spectrum at 252 K for most internal modes of cations clearly indicates alteration in the motional state of these cations when the phase transition is crossed. The hydrogen bonds are believed to be responsible for the ordering process in the cationic sublattice. It is interesting to note that only the 252 K phase transition is detected by the IR spectrum. The higher temperature phase transition at 263 K is less visible
252
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 5. Temperature evolution of the asymmetric and symmetric NH3+ stretching, na (NH3+ ) and ns (NH3+ ), modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
probably due to the fact that it is subtle taking into account its small entropy change (DS < 1.35 J/mol K). The Raman spectra in the frequency range (3400– 5 cm −1) at room temperature for the b(aa)c configuration are shown in Fig. 11.The Raman bands at 184 and 162 cm −1 correspond to the symmetric and asymmetric Bi–Br stretching modes of the external bromines, respectively. The stretching frequencies for the bridged Bi–Br–Bi stretches give Raman bands at 134 and 84 cm −1. Two lattice modes are observed below 50 cm −1. The temperature dependence of the IR intensity of the 1040 cm -1 mode shows a distinct variation in the temperature range between 140 and 240 K. The reduced peak intensity I p = I p9[q p/(n(q) + 1)], where q p and I p9 are peak frequency and intensity,
respectively, and n(q) is a Bose–Einstein distribution of oscillator, is plotted as a function of temperature in Fig. 12. According to Bruce et al. [11] the rate at which I p decreases in the ordered state depends on the long range interactions and is proportional to square of the order parameter h. Here, we consider the variations in the reduced peak intensity so that the dispersion effects would be minimised [12]. We plot a log–log graph of intensity changes against (T c − T), with T c = 252 K, in the temperature range 140–240 K. The plot is shown as the inset in Fig. 12. From the least square approximation the estimated slope is equal to 0.75 6 0.05. This value of 2b is significantly different from the mean field exponent, but is quite close to that observed in other similar systems [12–14].
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 6. Temperature dependence of the frequencies of the na (NH3+ ) and ns (NH3+ ) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 7. Temperature evolution of the asymmetric NH3+ bending, d(NH3+ ), mode for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
253
254
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 8. Temperature dependence of the frequencies of the d(NH3+ ) mode for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 9. Temperature evolution of the n(CN) and r(CH 3) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
255
Fig. 10. Temperature dependence of the frequencies of the n(CN) and r(CH 3) modes for the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals.
Fig. 13 presents the temperature dependence of the bandwidths of the 987 and 914 cm −1 modes. It is clearly seen that the 914 cm -1 band undergoes the stepwise change at T c. Below T c its width first decreases however it is practically temperature
independent below 150 K. In contrary to this behaviour we observed only linear width of 987 cm -1 mode. We can state that the linewidth of this modes is weakly temperature dependence, which may indicate weak anharmonic forces.
Fig. 11. The Raman spectra of (i-C 4H 9NH 3) 3Bi 2Br 9 at room temperature for the b(aa)c configuration. In the wavenumber range between 3400 and 2800 cm −1 the values of intensity for all spectra were divided by 10 (the 1:10 scale) with respect to the spectra taken between 1600 and 400 cm -1. The intensity value of the spectra between 0 and 300 cm -1 was divided by 200 (the 1:200 scale) due to its high intensity value.
256
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
Fig. 12. Reduced peak intensity I p for the 1040 cm −1 mode. Inset shows a plot of log (I p) vs log(T c2 − T) for this mode.
Fig. 13. Linewidth variations of the 987 cm −1 mode (a) and the 914 cm −1 mode (b).
J. Jo´z´ko´w et al./Journal of Molecular Structure 450 (1998) 247–257
4. Conclusions 1. Two reversible solid–solid phase transitions of first-order type are disclosed at 252 and 263 K in the (i-C 4H 9NH 3) 3Bi 2Br 9 crystals. 2. Temperature changes of the IR spectra corresponding to the internal vibrations of the i-butylammoniumn cations were observed only in the vicinity of 252 K phase transition. 3. Dynamics of the i-butylammonium cation (two positions model) seems to contribute to the mechanism of the 252 K phase transition of the ‘order–disorder’ type.
References [1] R. Jakubas, L. Sobczyk, Phase Transitions 20 (1990) 163.
257
[2] R. Jakubas, G. Bator, P. Ciapała, J. Zaleski, J. Baran, J. Lefebvre, J. Phys. Condens. Matter 7 (1995) 5335. [3] R. Jakubas, P. Ciapała, G. Bator, Z. Ciunik, J. Baran, J. Lefebvre, Physica B 217 (1996) 67. [4] N. Pis´lewski, J. Tritt-Goc, R. Jakubas, Phys. Stat. Sol. (b) 193 (1996) 67. [5] P. Ciapała, R. Jakubas, G. Bator, J. Zaleski, A. Pietraszko, M. Drozd, J. Baran, J. Phys. Condens. Matter 9 (1997) 627. [6] P. Ciapała, J. Zaleski, G. Bator, R. Jakubas, A. Pietraszko, J. Phys. Condens. Matter 8 (1996) 627. [7] L. Sobczyk, R. Jakubas, J. Zaleski, Polish J. Chem. 71 (1997) 265. [8] M. Hamada, Chem. Phys. 125 (1988) 55. [9] J.J.C. Teixeira-Dias, L.A.E. Batista de Carvalho, A.M. Amorim da Costa, I.M.S. Lampeira, E.F.G. Barbosa, Spectrochimica Acta 42A (5) (1986) 589. [10] J. Laane, P.W. Jagodzinski, Inorg. Chem. 19 (1986) 44. [11] A.D. Bruce, W. Taylor, A.F. Murray, J. Phys. C: Solid State Phys. 13 (1980) 483. [12] W. Dultz, J. Chem. Phys. 65 (1976) 2812. [13] P.S.R. Prasad, H.D. Bist, J. Phys. Chem. Solids 50 (1989) 1033. [14] P.S.R. Prasad, H.D. Bist, Phys. Stat. Sol. (a) 116 (1989) 275.