JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2004; 35: 261–265Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1145

Differential scanning calorimetric and Ramanstudies of a phase transition in [C3H7NH3]2SiF6

H. Jeghnou,1 A. Ouasri,1 M. Elyoubi,1 A. Rhandour,1∗ M.-C. Dhamelincourt,2

P. Dhamelincourt2 and A. Mazzah2

1 Laboratoire de Physico-Chimie des Materiaux Inorganiques, Faculte des Sciences, Kenitra, Morocco2 Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Centre d’Etudes et de Recherches Lasers et Applications, Universite desSciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France

Received 26 June 2003; Accepted 11 January 2004

A new structural phase transition was detected at low temperatures in [C3H7NH3]2SiF6 by means ofdifferential scanning calorimetry; the basic thermodynamic data were determined for this transition. TheRaman spectra of this compound were recorded from ambient temperature through the phase transitionat 223 K. The observed phase transition may be of the order–disorder type and probably of first-ordercharacter. Copyright 2004 John Wiley & Sons, Ltd.

KEYWORDS: propylammonium hexafluorosilicate; phase transition; differential scanning calorimetry

INTRODUCTION

Propylammonium hexafluorosilicate, [C3H7NH3]2SiF6, be-longs to the large alkylammonium halometallate salt fam-ily with the general formula [RnNH4�n]xMyXz (R D alkyl,M D tetravalent metal, X D halogen; x D 2, 3; y D 1, 2;z D 6, 9), which show several interesting changes in theirproperties due to the phase transitions which appear forthis family of compounds.1 – 10 These phase transitions gen-erally involve hydrogen bonding, in addition to orientationmotions of the alkylammonium cations.11,12 They exhibitimportant reorientation motions in the solid state, whichare consecutively frozen through the phase transitions atdecreasing temperature.3,4 [C3H7NH3]2SiF6 belongs, alongwith other monoalkylammonium salts such as ethylammo-nium and butylammonium hexahalometallate compounds,to an important subgroup of the large family, described bythe formula [CnH2nC1NH3]2MX6, and which has already beenextensively studied.13 – 16 However, the [CnH2nC1NH3]2SiF6

compounds have not been much studied, and only ethy-lammonium and butylammonium hexafluorosilicate werepreviously studied by us.7,9 [C2H5NH3]2SiF6 compoundis hexagonal at room temperature and by using spectro-scopic methods, the structure of this salt has been found tobe disordered.9 [n-C4H9NH3]2SiF6, which is orthorhombic(P321) at room temperature, presents one phase transitionof the order–disorder type at low temperature (268 K when

ŁCorrespondence to: A. Rhandour, Laboratoire de Physico-Chimiedes Materiaux Inorganiques, Faculte des Sciences, Kenitra,Morocco. E-mail: lpcmi@yahoo.fr

heating and 261 K when cooling), detected by both differen-tial scanning calorimetry (DSC) and Raman measurements.7

This important subfamily of monoalkylammonium hexa-halometallate salts was characterized by disordered struc-tures for most of the compounds and thus present sev-eral order–disorder phase transitions.7,9,14 – 18 The structuralphase transitions in the [CnH2nC1NH3]2MX6 family of com-pounds have been studied in recent years using variousexperimental techniques. A particular interest in these com-pounds has arisen mainly from the unusual two-dimensionalproperties and the reorientation motion dynamics of theorganic ions in connection with the phase transitions. Sincethese compounds were generally known owing to theirorder–disorder phase transition, we report in this papera study of the new compound propylammonium hexafluo-rosilicate in order to extend the earlier studies and to obtainmore information about the behaviour of this compoundthrough the transition. These studies were also carried outin order to make some comparisons within this importantfamily of monoalkylammonium compounds, especially con-cerning the role of the substitution of the alkylammoniumcations.

No x-ray structure data are available for [C3H7NH3]2SiF6

and as a consequence no factor group analysis could beperformed. Vibrational spectroscopy is able to give hintsregarding the influence of the size of the cations and theirabilities to form hydrogen bonds, also makes it possible todetect the distortion of the SiF6

2� anions from their octahedralstructure. DSC and Raman spectroscopy at ambient and lowtemperatures were used to study the [C3H7NH3]2SiF6, which

Copyright 2004 John Wiley & Sons, Ltd.

262 H. Jeghnou et al.

was likely to exhibit a phase transition at low temperatures.The structural phase transition found was characterized bydetermination of thermodynamic data.

EXPERIMENTAL

[C3H7NH3]2SiF6 crystals were prepared by slow evaporationfrom a hydrofluoric acid solution containing stoichiometricamounts of H2SiF6 and C3H7NH3F. Many efforts have beenmade to obtain a single crystal under different preparationconditions, leading only to the formation of crystals whichwere never adequate for a structural determination.

Elemental microanalysis of C, H, N, F and Si was carriedout at the microanalysis central service of CNRS, Vernaison,France. The results were is in good agreement with calculatedvalues derived from the formula. C6H20N2SiF6: calculated C27.48, H 7.63, N 10.68, Si 10.68, F 43.51; found C 27.63, H 7.82,N 10.74, Si 10.59, F 43.46%.

DSC measurements were carried out with a Seteram DSC-141 calorimeter, using a 10 K min�1 scanning rate duringheating and cooling. Linear extrapolation of the hysteresisto a zero scanning rate was performed. Raman spectrawere recorded between 298 and 223 K using a LABRAMRaman microspectrometer equipped with an internal He–Nelaser (632.8 nm) as the exciting source and a liquid N2-cooled charge-coupled derive (CCD) detector. The spectralresolution was 4 cm�1 and the band position were accurate toš1 cm�1. A LINKAM cooling stage with a 1 °C temperatureaccuracy was used to record the spectra as a function oftemperature. An 80ð Olympus long working distance (0.8numerical aperture) objective was used both to select theparticle to be analysed and to record the spectrum.

RESULTS AND DISCUSSION

Differential scanning calorimetry (DSC)One thermal abnormality was detected in the 173–315 Ktemperature range first during cooling and second duringheating. Figure 1 shows the DSC scans for [C3H7NH3]2SiF6.It is worth noting the splitting of the peak observed ataround 229 K on heating. This effect is reproducible andreversible on heating, with an important splitting occurringat around 216 K which leads to a doublet. The phasetransition temperatures observed during both cooling andheating (229 and 216 K) correspond to the centre of gravityof the observed cooling and heating doublets. The order ofthis phase transition was determined from the shape of theobserved DSC peak and also from the thermal hysteresisextrapolated linearly to zero scanning rate. The sharp peaksand the non-zero scanning hysteresis imply probably afirst-order transition. The measured large transition entropy�S D 40.8285 J mol�1 K�1�, in the case of the endothermiceffect, also indicates the first-order character of this phasetransition.

End

oE

xo

Temperature (K)

173 223 273

Figure 1. DSC curves corresponding to cooling and heatingcycles (10 °C min�1 for heating and cooling, m D 23.6 mg) for[C3H7NH3]2SiF6.

Raman spectrum of [C3H7NH3]2SiF6 at roomtemperatureThe free propylammonium cation can be considered to havethe Cs symmetry point group and, therefore, possesses36 internal vibrational modes, which can be classified as21A0 (IR, R) C 15A00 (IR, R). The free SiF6

2� anion possessesthe Oh symmetry and the internal vibrational modes areclassified as 1A1g(R) C 1Eg(R) C 1F2g(R) C 2F1u(IR) C 1F2u (–).For the isolated ions, all the cation vibrational modes are bothRaman and infrared active, whereas for the anions only theA1g, Eg and F2g modes are Raman active.

The Raman band assignment, presented in Table 1,was essentially made by comparison with work previouslycarried out on other alkylammonium compounds19 – 23 and onhexafluorosilicate salts.7 – 10,24 The Raman spectra at varioustemperatures were divided into two wavenumber regions(200–1800 and 2600–3400 cm�1), which are presented inFigs 2 and 3, respectively.

C3H7NH3+ internal vibrational modes

The two broad and weak bands located around 3231 and3159 cm�1 in the Raman spectrum of [C3H7NH3]2SiF6 wereassigned to the NH3 asymmetric stretching modes, whereasthe medium band observed at 3011 cm�1 was assigned tothe NH3 symmetric vibrational modes. The broadening andthe shift towards the high wavenumbers of these bands maybe indicative of the formation of hydrogen bonds inside thecrystal, as reported for several halometallates.25 – 27 In the2880–3000 cm�1 spectral region, four bands were observedwhich were assigned to the CH3 and CH2 asymmetric andsymmetric stretching modes.

Three bands with weak to medium intensities wereobserved at around 2759 cm�1, which can be assigned tonon-fundamental modes, probably overtones and harmonicsof the CH3 and CH2 bending modes. These bands gained

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 261–265

DSC and Raman studies of a phase transition in [C3H7NH3]2SiF6 263

Table 1. Observed Raman wavenumbers �cm�1� and band assignments for [C3H7NH3]2SiF6a

Ambient Q�/cm�1 223 K Q�/cm�1 173 K Q�/cm�1 Assignment

331 w, b 331 w, b 331 w, b ��NH3�

403 m 403 m 403 m �5�υs F–Si–F)449 w, b 449 w, b — υs(C–C–N)/υs(C–C–C)/�2 (�as Si–F)649 vs 649 vs 649 vs �1(�s Si–F)770 vw — 770 w CH2 rocking826 s 826 s 826 s �s(C–N)862 m 862 m 862 m �s(C–C)

870 sh955 w

961 m 961 m 961 m �as (C–N)/CH3 rocking1023 m 1023 m 1023 s �as (C–N)1050 vw 1050 w 1050 w �as (C–C)1080 w 1080 w 1080 m �as (C–C)1180 vw 1180 vw 1180 vw NH3 and CH3 rocking1195 w 1195 w 1195 w NH3 and CH3 rocking1295 m 1295 m 1295 m CH3 rocking1324 w 1324 w 1324 w CH2 twisting1357 w 1357 w 1357 w CH2 wagging

1450 m 1450 m υs�CH3�

1465 m 1465 m υs�CH2�

1480 m, b 1480 m 1480 s υas (CH2)/υas (CH3)1608 w, b 1608 w, b 1608 w, b υas�NH3�

2373–2850 w 2373–2850 w — Non-fundamental modes2880 s 2880 s 2880 m �s(C–H)2911 m 2911 m 2911 w �s�CH2�

2936 s 2936 s 2936 m �s�CH3�

2973 m 2973 m 2973 m �as(CH2)/�as (CH3)3011 w 3011 vw 3011 w �s�NH3�

3159 w, b 3159 w, b 3125 w,b �as�NH3�

3231 w, b 3231 w, b — �as�NH3�

a s, Strong; m, medium; w, weak; b, broad; v, very.

Ram

an In

tens

ity

173 K

273 K

223 K

298 K

Wavenumber (cm−1)

200 600400 1000 1400800 1200 1600

Figure 2. Temperature evolution of the Raman spectra�200–1700 cm�1� of [C3H7NH3]2SiF6 between ambienttemperature and 173 K.

173 K

223 K

273 K

298 K

Ram

an In

tens

ity

Wavenumber (cm−1)

2600 2800 3000 3200 3400

Figure 3. Temperature evolution of the Raman spectra�2600–3400 cm�1� of [C3H7NH3]2SiF6 between ambienttemperature and 173 K.

considerably in intensity because of a Fermi resonance dueto the formation of medium to strong hydrogen bonds

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264 H. Jeghnou et al.

�N–H Ð Ð Ð F� within the crystal.7 – 10,25 The assignment of theother bands is as follows. The medium and broad bandlocated near 1608 cm�1 can be assigned to the NH3 groupasymmetric bending modes; the broadening of this band is inaccordance with the NH3 polar group being involved in theformation of hydrogen bonds connecting the SiF6

2� anions tothe propylammonium cations. The bands observed between1480 and 1357 cm�1 were assigned to the asymmetric andsymmetric deformation modes of the CH3 and CH2 groups;the feature observed at 1357 cm�1 may be assigned to theCH2 wagging modes. The bands located near 1324 cm�1

correspond to the CH2 twisting vibrational modes, and thoseappearing at 1295, 1195, 961 and 770 cm�1 were assigned tothe NH3, CH3 and CH2 rocking modes. The bands observedat 1080, 1050, 1023, 961, 862, 826 and 449 cm�1 are probablydue to the skeletal stretching and bending modes of thebutylammonium cation. One weak and broad band observedat 331 cm�1 may originate from NH3 group torsion motions.Finally, the appearance of some broad features in the spectralrange corresponding to cation vibrational modes in theRaman spectrum of [C3H7NH3]2SiF6 could be indicative of apossible C3H7NH3

C cation disorder within this compound.

SiF62− internal vibrational modes

For the SiF62� anions, the very strong band observed at

649 cm�1 was assigned to the �1 (�s Si–F) stretching mode.The weak and broad feature located at around 449 cm�1 wasconsidered to be a composite of the bands correspondingto the �2 (�as Si–F) asymmetric stretching mode with thosecorresponding to the propylammonium skeletal bendingmodes. The band of medium intensity observed at 403 cm�1

presenting one shoulder on the lower wavenumber side canbe assigned to the �5 (υas F–Si–F) deformation mode. Thesplitting of this band, which corresponds to a degeneratemode, may indicate that the anions are distorted from theirOh free symmetry. The band observed at 331 cm�1, assignedto an NH3 group torsion mode, could also be due to asplitting of the band corresponding to the �5 (υas F–Si–F)triply degenerate deformation mode of the free SiF6

2� anion.

Raman spectra of [(C3H7NH3]2SiF6 as a function oftemperatureSeveral alkylammonium halometallates show order–dis-order phase transitions at low and high temperatures.8,14,28,29

For compounds containing �CH3�4NC and C2H5NH3C

cations, it was stated that the methyl groups were involvedin these transitions. However, in salts containing C3H7NH3

C

and C4H9NH3C cations, which have been less studied, disor-

der may also imply different parts of the cation. Therefore, wethought that [C3H7NH3]2SiF6 was very interesting to exam-ine near the phase transition temperature, specifically themodifications of the bands assigned to the C3H7NH3

C cationvibrational modes in the Raman spectra. The recording ofthese spectra versus temperature allowed us to observe the

influence of changes in the dynamic state of the propylam-monium cations through their internal vibrational modes.

The evolution of the bands corresponding to the NH3,CH3 and CH2 bending and rocking modes, and of thosecorresponding to the stretching and deformation modes ofthe propylammonium cation skeleton, are presented in Fig. 2.The bands corresponding to the SiF6

2� internal vibrationalmodes and those corresponding to the skeletal deformationmodes of the cations are presented in Fig. 2.

No changes were observed for the Raman spectrarecorded between room temperature and 273 K (Figs 2 and3). This is in good agreement with the DSC measurements,which indicate that the compound did not show any phasetransition in this temperature range.

Between 273 and 223 K, the evolution of the Raman spec-trum versus temperature in the spectral range correspondingto the NH3, CH3 and CH2 stretching modes (Fig. 3) showsthat one new band appears near 3175 cm�1 at 223 K. Nogreat changes were observed in the positions, intensitiesand widths of the bands occurring in this region when thetemperature changed from 273 to 223 K.

In the spectral region 200–1700 cm�1 (Fig. 2), the split-tings of some bands corresponding essentially to the cationmodes were observed, for example the broad band observednear 1480 cm�1, at room temperature and at 273 K, splitand gave three components when the temperature wasdecreased to 223 K. These splittings became clearer inthe spectra recorded at temperatures lower than 223 Kand were clearly observed in the spectra recorded at173 K in the range 200–1700 cm�1, and also in the range2600–3400 cm�1 (Fig. 3). Moreover, important modificationsconcerning intensities were observed in the C–H and N–Hstretching spectral region (Fig. 3). The bands observed in theRaman spectrum at 298 and 273 K have higher intensitiesthan those observed weakly in the spectrum recorded at173 K.

It is worth noting that the band intensities changedmarkedly across the phase transition temperature. Weinterpreted these observations as the result of a phasetransition occurring at around 223 K, consistent with theDSC measurements. It can be seen that, when the crystalundergoes the phase transition, the number, intensities andwidths of the bands corresponding to the NH3, CH3 and CH2

deformation and stretching modes change significantly.The important modifications observed in the Raman spec-

tra for most of the cation internal vibrational modes indicate achange in the state of propylammonium motions. More par-ticularly, these modifications indicate that the C3H7NH3

C

cations, which probably are dynamically disordered andexhibit some reorientation motions at high temperatures,become ordered at low temperature since their motions arelikely to be frozen below the phase transition temperature.This structural phase transition is probably of first orderand might be driven by an order–disorder mechanism orig-inating from the freezing of the reorientational motions of

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 261–265

DSC and Raman studies of a phase transition in [C3H7NH3]2SiF6 265

propylammonium cations when the temperature becomeslower than the phase transition temperature (around 223 K).

CONCLUSION

A reversible and reproducible phase transition of possi-bly first-order type was revealed by DSC measurementsat around 223 K in [C3H7NH3]2SiF6. Results of C, H, N, Fand Si elemental analysis of this compound are in goodagreement with calculated values derived from this for-mula [C3H7NH3]2SiF6. Changes for the bands correspondingto the majority of the propylammonium cation internalvibrational modes were observed in the Raman spectrarecorded as a function of the temperature near and belowthe phase transition temperature. The dynamics of thesecations seem to contribute to the mechanism of the phasetransition. The Raman spectroscopic study seems to indi-cate that reorientational motions of the propylammoniumcations may be responsible of this order–disorder phasetransition.

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