Infrared studies of structural phase transitions in [NH2(CH3)2]3SbBr6

Journal of Molecular Structure, 274 (1992) l-8 Elsevier Science Publishers B.V., Amsterdam

Infrared studies of structural phase transitions in ENH2vw213SbB% G. Bator, R. Jakubas, I. Majerz and Z. Malarski Institute

of Chemistry,

University

of Wroclaw F. Joliot-Curie

14, 50-383 Wroctaw (Poland)

(First received 12 February 1992; in final form 25 May 1992)

Abstract The infrared spectra of [NH*(CH~)~]~SbBr~(DMAHBA) are reported in the internal mode region in the temperature range 99345 K. The studies show that the vibrational states of the dimethylammonium cations (the v, and V~ bands) change markedly through the 338 K phase transition. The temperature variation of the analyzed bands corroborates the first-order nature of this transition.

INTRODUCTION

In the last few years various investigations (see eqn. 1 and refs. cited therein) have been reported of alkyla~onium halogenoantimonates(II1) and halogenobismuthates(III), with the general formula [NH*..~(CH~)~]~M~~ and (CH~NH~)~Bi~X~~(M = Bi, Sb; X = Cl, Br, I). These compounds show structural phase transitions and some are reported to be ferroelectric and ferroelastic [2,3]. Recently, a new material of different stoichiometry, [NH2(CH,),],SbBrG (DMAHBA), was reported [4], also showing structural phase transitions. DSC studies indicate two high-temperature phase transitions of first order, at 390 and 334K, which are related to the disordering of the dimethylammonium (DMA) cations. 8’Br NQR measurements have not revealed any temperature anomalies in the low-temperature region, down to 77 K [5]. No structural or spectroscopic studies of DMAHBA have been reported so far. In this paper, the temperature dependence of the IR spectra of a polycrystalline sample of DMAHBA in the vicinity of the phase transition (T, = 334 K) is investigated in order to elucidate the mechanism of the transition. Correspondence to: Professor Z. Malarski, Institute of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland.

0022-2860/92/$05.00 0 1992 Elsevier Science Publishers

B.V. All rights reserved.

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EXPERIMENTAL

DMAHBA was prepared as described earlier [4]. The IR absorption in the frequency region 4000-300cm-’ was recorded for a suspension in Nujol using KBr plates on a Perkin-Elmer 180 spectrophotometer. Polyethylene plates were used in the far-IR region (below 300 cm-‘). The vibrational frequency measurements are accurate to within 1 cm-‘. The higher temperature spectra in the range 270-345 K were obtained with a variable temperature cell, a vacuum thermostat of our own construction and a self-made temperature controller which kept the sample temperature constant to within ? 1 K. A copper-constantan thermocouple junction was embedded in a hole drilled into the plate in close contact with the sample. It should be pointed out that the phase transition at 334 K is followed by cracking of the crystal, so that only measurements on a polycrystalline sample were possible. Preliminary X-ray studies of DMAHBA, carried out at the estimated relatively large absorption coefficient p x 160 cm-l, did not provide a reliable estimation of the crystallographic system (the symmetry is undoubtedly lower than tetragonal). From the morphology and optical observations, a symmetry not higher than orthorhombic or monoclinic may be assumed at room temperature (Zmin= 2). Overlapping bands were resolved graphically assuming gaussian band shapes. RESULTS

The major vibrational frequencies of the DMA ion in DMAHBA for two phases are presented in Table 1. Table 2 lists the far-IR frequencies for DMAHBA at room temperature. Using the assignments of Bellanato [6] for the DMA halides as guides, the assignments for DMAHBA are fairly straightforward, with the exception of the N-H stretching region (35003000cm-l). This region contains a wealth of absorption bands and it is evident that extensive Fermi resonances exist between the N-H stretching fundamentals, and overtones and combinations arising from the N-H bending modes. Figure 1 shows the spectra for DMAHBA in the frequency region 1500700cm-’ at 300 and 90K. Both spectra refer to the low-temperature phase of DMAHBA. In this phase no significant changes in the spectra are observed below the 338K phase transition. The spectra referring to the internal vibration of the SbBri- anions, in the far-IR (below 300 cm-l), are presented in Fig. 2. Full temperature analysis was confined to the v,,(pNH,+), v,(6NH,) and

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TABLE 1 Vibrational frequencies” (cm-‘) of the dimethylammonium ion in [(CH,),NH,],SbBr, (DMAHBA) at 90, 300 and 350 K and full width at half maximum (FWHM) at 300 K Vibration

Description

90K

300K

350 K

FWHM

“4

6NH;

“20 “I

PCH, PCH, PCH,

“21

vCN

1563s 1244m 1226~ 1081m 1098m 1011 1015s

1572s 1243m 1226~ 1081m 1098m 1011 1015s

29 12 17

“25

“8

“C-N

1558s 1244m 1226~ 1080m 1lOOm 1010 1012s 1026? 889s 878s

886s

886s

878s

880s

PNH,

826s

“26

816s

“9

6NC,

“27

823s 805s

14

15 21

398w 267m

tCH,

aAbbreviations: s = strong, m = medium, w = weak. v,(vCN) bands, for which distinct temperature effects were observed in the vicinity of the 338K phase transition. The temperature dependences of the vzs(pNHi ) and v4(6NH,) modes are presented in Figs. 3 and 4, respectively. The temperature evolution of the vZ6@NH,’ ) band is shown in Fig. 5. The v4(GNHz) and v,(vCN) modes exhibit a stepwise change in frequency in the vicinity of the phase transition point (2’ = 330 K, on cooling). The temperature hysteresis on cooling and heating is due to a first-order phase transition. It was found that the full widths at half maximum (FWHM) of the analyzed bands (vq,v8) do not change significantly through the phase transition. The temperature variation of the infrared frequencies in Fig. 3 TABLE 2 Far-IR vibrational frequencies (cm-‘) of the SbB$

anion” at 300 K

Vibration

Description

300K

“,(-%,) “Q(T1”) “,(T,“)? UT,“)

SbBr SbBr Sb-Br SbBr

166sh 150s br 114sh 64m

“Abbreviations:

stretching stretching bending bending

sh = shoulder, s = strong, m = medium, br = broad.

G. Bator et ah/J. Mol. Struct., 274 (1992) 1-8

800

1200 2,/cm4

1600

Fig. 1. 150(r’7OOcm~’ region in the IR spectra of [NH,(CH,),],SbBr,

at (a) 300K and (b) 90K.

shows that the band at 820 cm-‘, when cooling down to 330 K, is nearly independent of temperature; in the vicinity of T,it exhibits a sharp splitting into two components. The frequencies of these components in the lowtemperature phase are also independent of temperature. The position of the v,, and v,(NH: ) bands (the region below 3200 cm-‘) indicates rather weak N-H . . . Br hydrogen bonds. Our studies were limited to the investigations of the internal modes of the DMA cations close to the lower temperature phase transition (at 338 K). It should be noted that the crystal also undergoes a high-temperature phase

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G. Bator et al.lJ. Mol. Struct., 274 (1992) 1-8 %T

I

I

100

300

200

Fig. 2. Far-IR spectrum of DMAHBA at 3OOK.

830

820T

1 L-_-_-..___ p 0’0

E

<

----o_

z -810

800’

-

--h

300

_

320

340

T/K

Fig. 3. Frequency variation of the vS band of DMAHBA as a function of temperature.

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Fig. 4. Frequency variation of the vpband of DMAHBA as a function of temperature.

transition (at 397 K), but it does not seem to be stable for a long time at these temperatures. DISCUSSION

From the expected crystallographic system of DMAHBA (lower than tetragonal) at room temperature and its stoichiometry, at least several non-equivalent DMA cations may exist in the elementary cell. However, no strong splitting at room temperature in the analyzed bands is observed. This suggests that the force constants of the corresponding vibrations for all DMA cations do not differ markedly. The observed changes in the spectra are evidently connected with the internal vibrations of the DMA cations. This indicates that the phase transition is related to changes in the dynamics and/or environment of the DMA cation (the stepwise change of the frequencies v,(6NH,) and v,(vCN), and the splitting of vZ6(pNH2+)). The temperature coefficients of the NH, vibrational frequencies and FWHM suggest a rigid packing of the DMA cations below T, (see Table 1). The splitting of the vX6(6NH2)band is connected with the differentiation of the DMA cations in the low-temperature phase. On heating, when the crystal undergoes phase transition, an unexpected temperature change with frequency of the v,(GNH,) band is observed. Usually, for this type of vibration, the stepwise decrease in frequency, on heating, is due to the weakening of the interactions. In our case, the increase of the frequency with temperature indicates the strengthening of N-H . . . Br interactions, as a result of a shortening of the N * * - Br distance. Hence, it is probable that the 338K phase transition is also accompanied by a drastic change in the anion sublattice.

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c c

41K 29K

I

760

I

800

840

880 +26/m'

Fig. 5. Typical IR spectral evolution of the p(NH,) mode (v& in the temperature range 34.-321 K. The numbers above each curve represent the corresponding temperatures.

From the DSC studies, a phase transition of the order-disorder-type was found. A large heat anomaly accompanying this transition [4] may be explained by a freezing of motion of the DMA cations on cooling. However, the rotational motion of the NH,CH,),+ group is hindered by the formation ofaweakN-H-s* Br hydrogen bond in the high-temperature phase. For the undistorted octahedral structure of the SbBri- anions (assumed symmetry O,), the only IR active Sb-Br stretching mode vg (T,,) is usually observed as a very intense band. In our case, this band, at about 150 cm-‘, is quite broad with half-bandwidth of the order of 70-80cm-‘. The large breadth of the v3 band in DMAHBA may be related to the presence of

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several crystallographically different octahedra. In general, the bands observed in the far-IR spectra of our crystal are similar to those found for the SbBq- complexes, having a highly distorted octahedral environment

[7&31. CONCLUSIONS

The distinct splitting of the vZ6band at T,(Av = 18cm-l) and the significant stepwise change of v4are the obvious evidence that the mechanism of the phase transition is related to the change in disordering of the DMA cations (order-disorder mechanism). The stepwise change of the analyzed parameters and the temperature hysteresis of v(T)indicate a first-order phase transition. The IR activity of v1(A,,) and v,(T,,) of the SbBri- anions reflects the distortion present in the octahedral structure caused by N-H . . . Br hydrogen bonds. REFERENCES 1 2 3 4 5 6 7 8

R. Jakubas and L. Sobczyk, Phase Transitions, 20 (1990) 163. J. Zaleski, R. Jakubas and L. Sobczyk, Phase Transitions, 27 (1990) 25. R. Jakubas, L. Sobczyk and J. Matuszewski, Ferroelectrics, 74 (1987) 139. Z. Galewski, R. Jakubas and L. Sobczyk, Acta Phys. Pol. A, 78 (1990) 645. T. Okuda, N. Tanaka, S. Ichiba and K. Yamada, Z. Naturforsch., Teil A, 41 (1988) 319. J. Bellanato, Spectrochim. Acta, 16 (1960) 1344. G.C. Allen and R.F. McMeeking, Inorg. Chim. Acta, 23 (1977) 185. P.W. Jagodzinski and J. Laane, J. Raman Spectrosc., 9 (1980) 1.