Journal of Molecular Structure 697 (2004) 161–171 www.elsevier.com/locate/molstruc
On the structural phase transitions in [n-C4H9NH3]2[SbBr5]: thermal, dielectric and infrared studies J. Tarasiewicza, R. Jakubasa, J. Baranb,*, A. Pietraszkob b
a Faculty of Chemistry, University of Wrocław, F. Joliot Curie 14, Wrocław 50-383, Poland Institute of Low Temperature and Structure Research of the Polish Academy of Science, Oko´lna 2, Wrocław 50-422, Poland
Received 17 February 2004; accepted 15 March 2004
Abstract Differential scanning calorimetry and linear thermal expansion studies reveal seven phase transitions in [n-C4H9NH3]2[SbBr5] crystal. The phase transitions at 261, 280 and 292 K are accompanied by distinct dielectric anomalies. The dielectric relaxation processes were disclosed over the low temperature phases (VI, VII) and over the phase (III). The infrared spectra of powdered sample of [n-C4H9NH3]2 [SbBr5] in Nujol were measured in the region of the internal vibration of the n-butylammonium cation (4000 – 400 cm21) in the temperature range between 26 and 320 K. The temperature changes wavenumbers and width of most of the modes of the n-butylammonium cations were analysed. q 2004 Elsevier B.V. All rights reserved. Keywords: Bromoantimonates(III); Phase transition; Differential scanning calorimetry; Dielectric; IR
1. Introduction Alkylammonium halogenoantimonates(III) and halogenobismuthates(III) of the general formula RaMb X3b þ a (R: organic cations, M: Sb, Bi and X ¼ Cl, Br, I) evoke much interest because of their ferroic properties. The crystals belonging to the R3M2X9 and R5M2X11 subgroups and containing small in size organic cations like methyl-, dimethyl- and trimethyl-ammonium exhibit ferroelectric properties [1 – 4]. In order to investigate in detail the effect of cation size on the dynamical and electric properties, we have extended our investigations to crystals containing bulky alkylammonium cations. Isopropylammonium analogs of the R2MX5 formula show the low temperature phase transitions, which are due to the reorientational motion of the cations, being distributed between two equivalent sites [5]. The n-propylammonium (R2MX5, R3MX6 and R3M2X9) [6,7] and n-butylammonium (R2MX5 and R3MX6) [8] salts disclosed more complex sequence of the solid – solid phase transitions. The X-ray studies of the salts showed that the n-alkylammonium cations perform the flipping motion * Corresponding author. Tel.: þ 48-71-343-5020; fax: þ48-71-441-029. E-mail address:
[email protected] (J. Baran). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.03.050
around their long axis. The dynamical disorder of these cations was confirmed by the dielectric dispersion for the [n-C 3H 7NH 3] 3 [Sb2Cl 9] [9] and [n-C4 H9 NH3 ] 2[BiCl 5] crystals [8]. Debye-like dispersion found in the microwave frequency region clearly indicates the ‘order –disorder’ mechanism of the phase transition in these crystals. Most of the phase transitions in halogenoantimonates(III) and halogenobismuthates(III) family are found to be triggered by a change in dynamics of the alkylammonium groups [10,11]. In this paper, a new representative of the bromoantimonates(III), i.e. bis(n-butylammonium) pentabromoantimonate(III), [n-C4H9NH3]2[SbBr5], is presented, and its thermal (differential scanning calorimetric, dilatometry), dielectric and infrared properties are presented. The mechanism of the phase transitions is discussed.
2. Experimental details Deep yellow transparent crystals of [n-C4H9NH3]2 [SbBr5] were prepared by addition of n-butylammine (Aldrich, 99.5%) to a solution of bismuth(III) oxide (Aldrich, 99.9%) in concentrated hydrobromic acid (Aldrich, 48%). The obtained polycrystalline material was recrystallized
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twice from solution. Single crystals were grown by a slow evaporation of an aqueous solution at room temperature. The elementary chemical analysis performance gave: C: 14.13 (theor. 14.78), H: 3.56 (theor. 3.69), N: 4.09 (theor. 4.31). Differential scanning calorimetry (DSC) measurements were carried out with a Perkin – Elmer model DSC-7 between 100 and 400 K at a heating/cooling rate of 2, 5, 10 and 20 K/min. Linear thermal expansion was measured using a thermomechanical analyser—Perkin – Elmer TMA-7. The samples used in the measurements were prepared in the form of thin plates (5 £ 3 £ 1 mm3). The anomalies in dilatation of single crystals in the vicinity of the phase transitions were always reproducible within 10% for each sample. The accuracy of thermal expansion determination was about 3%. The complex electric permittivity, 1p ; for a single crystal of [n-C4H9NH3]2[SbBr5] along the a-axis was measured by a HP 4285A Precision LCR Meter at frequency range between 75 and 12 MHz, in the temperature range between 140 and 320 K at a cooling rate of 0.5 K/min. Samples for dielectric measurements were typically of size 4 £ 3 £ 1 mm3. The accuracy of the measured electric permittivity value was about 5%. The pyroelectric charge was measured with a KEITHLEY 617 electrometer. Infrared spectra of the powder sample [n-C4H9NH3]2 [SbBr5] (in Nujol with NaCl windows) in the temperature range between 26 and 320 K were recorded with a BRUKER IFS-88 FT-IR spectrometer with resolution 1 cm21. FT-Raman spectra were recorded with FRA-106 attachment to the BRUKER IFS-88 spectrometer over the wavenumber range 3600 –50 cm21 at 300 K. APD Cryogenics Displex Closed Cycle Refrigeration System Model CSW-202 was used for temperature dependence studies. The temperature of the sample was maintained at an accuracy of ^ 0.1 K by the Scientific Instruments INC controller Sances 5500.
below 305 K additionally makes the resolving of the room temperature structure (phase II) difficult. In the case of salts with R5MX5 composition their anionic sublattice are built up either of discrete M2X410 moieties consisting of two edge-sharing octahedral or of infinite one-dimensional chains composed of corner-sharing octahedral. It has turned out that the salts containing large in size and bulky cations are usually characterised by discrete M2X42 10 units [8]. 3.2. Differential scanning calorimetry (DSC) The order of the phase transition was determined from the shape of the observed DSC anomaly and from the value of thermal hysteretic extrapolated linearly to the zero scanning rate. The DSC thermograms of the [n-C4H9 NH3]2[SbBr5] crystals (Fig. 1) shows up to seven solid – solid phase transitions below the melting point on cooling scan. The high temperature irreversible phase transition I ! II at 305 K is visible for cooling scan. The subtle
3. Results 3.1. X-ray studies We have not succeeded in the solution in the crystal structure both at 296 K (phase II) and 340 K (phase I). The results indicate, however, that both phases appear to be incommensurate modulated. The lattice parameters of the unit cell of the average structure are as follows: at 340 K, orthorhombic symmetry—Pnma; a ¼ 16:845; b ¼ 54:899; ˚ , and at 300 K, monoclinic symmetry—P2=m; c ¼ 15:697 A ˚ , b ¼ 92:058: Since, a ¼ 15:597; b ¼ 62:854; c ¼ 16:771 A the phase transition at 305 K is associated with the change of the system (orthorhombic ! monoclinic), therefore, it implies that the phase (II) and the lower temperature ones are potentially ferroelastic phases. A splitting of the reflexions
Fig. 1. DSC thermograms of the [n-C4H9NH3]2[SbBr5] crystals for the cooling and heating runs between 160 and 320 K (10 K· min212upper part, 20 K· min212lower part, m ¼ 6:034 mg).
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change in the slope of basic line allows us to classify this phase transition as a second order one. The observed anomalies at 292, 280 and 261 K (on cooling) clearly indicate the first order phase transitions. The entropy change accompanying the phase transition at 261 K is equal to about 9.6 J/mol K. This value of the transition entropy suggests the order – disorder mechanism of these phase transition. Two peaks at 263 and 261 K lie very close each other and the accurate calculation of the enthalpies are impossible. The phase transition at 263 K is irreversible. The two lowest temperature heat anomalies correspond to weak first order transitions. On heating only five phase transitions are detected. The transition temperatures (on cooling and heating), entropies and order (I or II) of the phase transitions in [n-C4H9NH3]2[SbBr5] are collected in Table 1. The temperature dependencies of the linear thermal expansion along three crystallographic directions during the cooling runs are shown in Fig. 2. The dilatometric measurements confirm the complex sequence of the phase transitions in [n-C4H9NH3]2[SbBr5]. The biggest changes in the dilation of the crystal are observed in the vicinity of temperatures close to T5 ¼ 261 K. The crystal dimension contracts along the a-axis whereas expands along the b- and c-axis during cooling. It is also interesting that most of the phase transitions is accompanied by an expansion of the unit cell (during cooling) along the b- and c-axis. The largest structural changes taking place at 262 K correspond to the phase transition accompanied by the huge entropy transition (9.65 J/mol K). One can conclude, finally, that the sequence of phase transitions detected by dilatometric method agrees well with that found by DSC measurements. Fig. 2. Temperature dependence of the linear thermal expansion along the a; b- and c-axis during the cooling scans.
3.3. Dielectric measurements The temperature dependencies of the real and imaginary parts of the complex electric permittivity, 1pa ; Table 1 Phase transitions in alkylammonium bromoantimonate(III) (CnH2nþ1 NH3)2SbBr5 salts
n¼2
155
n¼3
188.7 165 137
n¼4
a
305 292 280 263a 261 191 178
DT (K)
TðheatingÞ (K)
TðcolingÞ (K)
DS (J/mol K)
Order of the PT I
26.8 1.6 T1 T2 T3 T4 T5 T6 T7
– 307 292 – 285 193 184
T1 T2 T3 T4 T5
15 12 – 24 2 6
Observed only at the scans of 20 K/min.
– 1.22 0.9 0.61 9.65 0.53 0.38
II I I II I I I I I I
between 75 kHz and 12 MHz along the a- direction performed on cooling for [n-C4H9NH3]2[SbBr5] are shown in Fig. 3(a) and (b). Some of the phase transitions are inactive in dielectric measurements. The values of 10a observed in this crystal are quite large. The electric permittivity increases rapidly by about 4 units at 292 K (II ! III PT) and then in the vicinity of T3 ¼ 280 K (III ! IV PT) a comparable decrease in 10a is observed. In the vicinity of the phase transition at T5 ¼ 261 K, the 10 falls abruptly from 24 to 18 units. The source of the dielectric anomaly, especially close to 261 K, is undoubtedly connected with the freezing of dipolar groups, i.e. both of the rigid n-butylammonium cations or some fragment of the alkyl group. It is interesting that over the phases VI and VII (see insert in Fig. 3a) a clear relaxation process is disclosed. It should be noted that a relaxation process in megahertz frequency region is also observed in the higher temperature phase (III). It was shown that the dielectric relaxation in the [n-C4H9NH3]2[SbBr5] crystal over the low temperature
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Fig. 3. Temperature dependence of the real (a) and imaginary (b) parts of 1p measured along the a-axis for the frequency range between 75 kHz and 12 MHz (on cooling).
phases (VII) and (VI) is well described by Cole – Cole relation 1p ðvÞ ¼ 10 ðvÞ 2 i100 ðvÞ ¼ 11 þ
D1 1þðivtÞ12a
ð1Þ
where D1 ¼ 10 2 11 ; 10 and 11 denote the low- and highfrequency limits of the electric permittivity, v is an angular frequency, t is the macroscopic relaxation time and a is a parameter which represents a measure of the distribution of the relaxation times. 10 ; 11 ; t and a are the fit parameters
for each temperature independently. The Argand diagrams at selected temperatures of the low temperature phases (VII and VI) are displayed in Fig. 4(b). The macroscopic relaxation times t changes significantly with temperature from 3.38 £ 10 27 s at 182 K to 3.94 £ 1028 s at 210 K. The parameters a estimated with the Eq. (1) changes from 0.27 (at 182 K) to 0.06 (at 210 K). The temperature dependence of the macroscopic relaxation time in the low temperature phases fulfils the Arrhenius
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Fig. 4. The Argand diagrams at selected temperatures; (a) for the phases (III) and (IV) and (b) for the phases (VI) and (VII).
relation, with the activation energy Ea ¼ 23 kJ/mol. The dielectric response in the phases (III) and (IV) is well described by the Cole – Cole relation, as well. The estimated macroscopic relaxation time for the phase (III) and (IV) changes weakly with temperature, and at about 277 K it amounts 3.45 £ 1027 s. The activation energy, estimated over the phase (III) amounts to approximately 8 kJ/mol. The pyroelectric charge was measured after poling the crystal, on cooling, from 300 K down to , 275 K (below T3) by a DC electric field of about þ 100 V. After the poling field has been removed the electrodes of the specimen were
shorted at temperature 275 K during 3 h. The pyroelectric charge was then measured on heating. The spontaneous polarization as a function of temperature is shown in Fig. 5. Relatively small change in the spontaneous polarization value of the order of 8 £ 1025 C m22 appears in the vicinity of III0 ! II0 phase transition (292 K). No visible anomaly on the spontaneous polarization is visible through the II0 ! I phase transition at 307 K. The pyroelectric current is not reversed under the external DC electric field. It can be concluded that [n-C4H9NH3]2[SbBr5] may be classified as a pyroelectric crystal below 292 K.
Fig. 5. Spontaneous polarization changes, lDPs l; as a function of temperature obtained on warming by the pyroelectric measurements after poling the crystal by a DC electric field 100 V.
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Fig. 6. The infrared spectra of the powdered [n-C4H9NH3]2[SbBr5] sample in Nujol in the wavenumber range between 3500 and 500 cm21 for 26 and 320 K (the wavenumber regions 3000–2750 and 1500– 1350 cm21 for Nujol are replaced by parts of the spectrum in Fluorolube).
3.4. Infrared and Raman studies The infrared studies at different temperatures have been undertaken to clarify the cationic dynamics contribution to the phase transition mechanisms in the [n-C4H9NH3]2 [SbBr5] crystals. The infrared spectra of the polycrystalline [n-C4H9NH3]2[SbBr5] in the frequency range between 3500 and 500 cm21 at two temperatures, 26 and 320 K, are presented in Fig. 6. The FT-Raman spectrum at 300 K between 3500 and 50 cm21 is shown in Fig. 7. The assignments of the observed bands at 26 and 320 K for IR spectrum and at
300 K for Raman one presented in the Table 2 are essentially based on comparison with the IR spectra of n-propylamine [12], n-butylamine [13] and alkylammonium bromobismuthates(III) [14 – 16] salts. The low frequency vibrational spectra of bromo- and iodoantimonates(III) anions are reported by and assignments of the MX22 5 Jagodzin´ski et al. [17]. The temperature dependent IR studies allow checking out the influence of the change in the dynamical state of the n-butylammonium cations on the infrared internal vibrations. Usually, the splitting of the bands related to the ammonium NHþ 3 group vibrations as well as the changes
Fig. 7. The powder Raman spectra of the [n-C4H9NH3]2[SbBr5] crystal at 300 K. In the wavenumber range between 3500 and 2250 cm21 and between 2050 and 550 cm21, the values of intensity for the spectrum were multiplied by 10 and 20 with respect to the spectrum taken between 50 and 550 cm21.
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Table 2 Wavenumbers, intensity and tentative assignments of the bands observed in the IR spectra in (n-C4H9NH3)2SbBr5 at 26 and 320 K and Raman at 300 K
vs, Very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. p Spectra in fluorolube.
in their intensities and widths are expected. The analysis was performed also for the other bands for which the distinct temperature changes are observed. A number of the bands exhibit the anomalous changes in the bandwidth around 184, 292 and 307 K (heating cycle), which could confirm the main role of the organic cation in the mechanism of the phase transitions. The temperature evolution of the infrared spectra in the region of the v(CH2) and t(CH2) deformation vibrations, temperature dependence of these bands and temperature dependence of full width at half maximum (FWHM) of
1328 cm21 mode are shown in Fig. 8. The changes of the wavenumbers of these modes are quite small. At T1 (307 K) and T2 (292 K) some anomalies are observed (Fig. 8(c)). The FWHM of 1328 cm 21 band is weakly temperature dependent below T5 (285 K). The biggest changes of width of this band are observed at T4 (193 K) and T2 (Fig. 8(b)). Fig. 9(a) presents the temperature evolution of the infrared spectra in the region of the deformation vibrations r(NHþ 3 ), Fig. 9(b) temperature dependence of FWHM of 1155 cm21 mode and Fig. 9(c) temperature dependence of
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Fig. 8. (a) Temperature evolution of the IR spectra in the v(CH2) and t(CH2) vibrations region, (b) temperature dependence of the full width at half maximum (FWHM) for 1328 cm21 mode, (c) temperature dependence of the v(CH2) and t(CH2) modes.
21 Fig. 9. (a) Temperature evolution of the IR spectra in the r(NHþ mode, 3 ) vibrations region, (b) temperature dependence of (FWHM) for 1155 cm ) modes. (c) temperature dependence of the position of the r(NHþ 3
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Fig. 10. (a) Temperature evolution of the IR spectra in the d(NHþ 3 ) vibrations region, (b) temperature dependence of the wavenumbers of the maximum for the d(NHþ 3 ) modes.
the wavenumbers of the modes observed between 1135 and 1195 cm21. At 26 K seven bands can be distinguished. Two of them disappear below T5 ; one of bands at 1140 cm21 (26 K) disappear at T3 (Fig. 9(a) and (c)). In the roomtemperature phase these bands are broad and they can be resolved only into four components. The width of the 1155 cm21 band increases in over all temperature region, but at T4 and T2 the increase is stronger than in other phases (Fig. 9(b)). The temperature evolution and the temperature dependence of the frequencies of the d(NHþ 3 ) deformation modes are shown in Fig. 10(a) and (b). Six components of these vibrations are observed in the low-temperature phase. Three of them disappear and three components for the r(NHþ 3 ) vibrations are seen above T4 ¼ 193 K, whereas above T3 ¼ 285 K only two bands are observed. Fig. 11(a) and (b) displays the temperature evolution and temperature dependence of the rocking r(CH3) and r(CH2) modes. At low temperatures, nine components can be distinguished in this region. One of them at 900 cm21 disappears far below T5 ; three disappear around T5 and four bands in this region are found above T3 : The temperature wavenumbers changes of the other modes are quite small. The important changes in the IR spectrum at 193 and 285 K for most internal modes of cations clearly indicate that the dynamics of the n-butylammonium cations contributes significantly to
the mechanism of phase transitions (at T5 and T4 ) in [n-C4H9NH3]2[SbBr5].
4. Discussion In order to throw more light on the mechanism of the phase transitions in [n-C4H9NH3]2[SbBr5] it is instructive to summarize the information on the phase transitions in homologous compounds of the R2SbBr5 composition. Table 1 gives the thermodynamic parameters characterizing the phase transitions in [C2H5NH3]2[SbBr5] [11], [n-C3H7 NH3]2[SbBr5] [18] and [n-C4H9NH3]2[SbBr5] crystals. The examination of Table 1 indicates a general trend to an increased number of phase transitions as the alkyl chain is lengthened. In principle, there are various possible sources of structural phase transitions in these compounds; (i) reorientational motion of the rigid alkylammonium chains about their long axis triggered by flipping of NH3 polar groups between several sites, (ii) the ‘hindered rotation’ of the CH2 units with respect to the long axis of the alkyl chains (the chains ‘melt’) and (iii) changes in the H-bond configuration. The ‘melting’ of the chains is possible mainly in the homologous possessing long alkyl chains as n-butyl. The first two sorts of motion, ((i) and (ii)), are treated as a primary contributors to the entropy transition in contrast to the third (iii) possibility. It should be noted
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Fig. 11. (a) Temperature evolution of the IR spectra in the r(CH3) and r(CH2) vibrations region, (b) temperature dependence of the wavenumbers of the maximum for the r(CH3) and r(CH2) modes.
that [n-C3H7NH3] 2[SbBr5] and [n-C4H9NH 3] 2[SbBr5] exhibit only one major transition (DS larger than R ln 2 ¼ 5:76 J/mol K) and remaining ones are characterised by quite small entropy effect (close to 1 J/mol K or smaller). The minor transitions are undoubtedly associated with the changes in the H-bond configurations whereas the major transitions involve the all three types of motion. It is also interesting that the major transitions in n-propyl and n-butyl homologous (at 165 and 261 K, respectively) are accompanied by a significant step-wise change in the electric permittivity that is characteristic for crystals with high temperature ‘rotator’ phase. In our opinion the phase transition at 261 K in [n-C4H9NH3]2[SbBr5] is associated with onset of rotation of the CH2 units or/and rotation of rigid alkyl chains around long axis of the cations. Such types of motions involve an important change in the resultant dipole moment of the unit cell, and in consequence it leads to a change in the electric permittivity. The dilatometric studies showed that the corresponding major phase transitions in both analogous are associated with a drastic change in the volume of the unit cell. They should be accompanied by a significant change in the structure of both analogues. The calorimetric and dilatometric studies suggest, therefore, a similar transition mechanism for the major transitions (at 165 for n-propyl and at 261 K for n-butyl analogue). The presence of the slow relaxation process below 261 may be explained in terms of the reorientation of the rigid alkyl chain of the cation.
Relatively long macroscopic relaxation time, 1027 –1028 s, is justified by both a significant size of the disordered cations and large steric effects. A similar dielectric responce is found in n-propylammonium analogue [9].
5. Conclusions (i)
(ii)
(iii)
(iv)
(v)
(vi)
DSC and dilatometric studies on the [n-C4H9NH3]2 [SbBr5] crystal disclose the existence of seven solidsolid phase transitions. A rich sequence of phase transitions is more likely related to the rotational motion of the alkyl chains around the long axis of the n-butylammonium cations and changes in the hydrogen bond configurations. Large entropy change (DS < 9:6 J·mole 21 · K 21) accompanying the phase transition V ! VI at 261 K indicates an ‘order – disorder’ mechanism. The step-wise dielectric anomaly, that appears at the phase transition at 261 K is characteristic of crystals with plastic crystal phase. The low frequency dielectric relaxation process is disclosed over in the phases (III), (VI) and (VII) of [n-C4H9NH3]2[SbBr5]. The infrared studies confirmed the contribution of the n-butylammonium cations to the mechanism of most phase transitions.
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