Study of the structural phase transitions of (CH3NH3)3Sb2Cl9 (MACA) and (CH3NH3)3Bi2Cl9 (MACB) by infrared spectroscopy

Journal of Molecular Structure, 246 (1991) 193-202

193

Elsevier Science Publishers B.V., Amsterdam

Study of the structural phase transitions of (CH3NH3)3Sb2C19 (MACA) and (CH3NH3)3Bi2C19 (MACB) by infrared spectroscopy G. Bator, R. Jakubas and Z. Malarski Institute of Chemistry, University of Wroctaw, F. Joliot-Curie 14, 50-383 Wroctaw (Poland) (Received 16 May 1990; in final form 18 October 1990)

Abstract

Infrared spectra of polycrystalline (CH3NH 3)38b2C19 and (CH3NH3)3Bi2Cl9 have been studied in the temperature range 90-300 K. A systematic temperature dependence study of the internal modes has been carried out. We discuss the effects of the dynamic state of methylammonium (MA) cations on their vibrational spectra. The results show that the dynamics of MA cations in both compounds is similar in higher (about 300 K) and lower temperature (in the vicinity of 100 K) regions. Substantial differences are revealed in the intermediate temperature interval. The results are in good agreement with earlier dielectric, calorimetric and 1H NMR studies.

INTRODUCTION

Methylammonium salts are interesting systems for studying the effects of the crystalline environment on the vibrational spectrum of a molecule or ion. Important factors affecting the observed spectrum are the symmetry around the molecule in the crystal (site group) and the number of non-equivalent molecules in the unit cell, which determines the factor group splitting of the vibrational levels. Hydrogen bonding plays an important role in many of the methylammonium salts, particularly in the halides. In many of these salts some phase transitions in the solid state exist. The phase transitions have been discussed by lattice dynamical models [ 1-3 ] although attempts to interpret the entropy of transitions by orientational and positional disorder of the ions and atoms have also been made [4,5]. Among various alkylammonium halogenoantimonates(III) and bismuthanes (III) forming a large class of polar crystals, those containing methylammonium cations of general formula ( C H a N H 3 ) 3 M 2 X 9 (where M - S b , Bi; X = C1, Br, I) deserve special attention. It has been shown that the structure of anions M2X93- and the physical properties of these compounds are affected by the type of halogen atoms. 0022-2860/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

194 The chlorine compounds, (CH3NH3)3Sb2C19 (MACA) and (CH3NH3) 3Bi2C19 (MACB) characterized by one- dimensional double chains of polyanions undergo only one structural phase transition at 208 K [6] and 385 K [7], respectively. The bromine analogues with the two-dimensional corrugated layer structure of polyanions revealed interesting polar properties (pyroelectricity (Sb) and improper ferroelectricity (Bi)) and finally iodine compounds with simple double face-sharing octahedra in the crystalline lattice exhibit low temperature antiferroelectric (Sb) or ferroelectric (Bi) phases. The general feature of the methylammonium crystals is that at room temperature they show considerable freedom for motions of the CH3NH3 + cations, and the freezing of this rotational motion yields various types of ordered phases. Recently a number of investigations have been performed on the dielectric and calorimetric properties of (CH3NH3)3M2X9 crystals [6-10]. Relatively little, however, has been known about their spectroscopic properties. In this paper we report IR spectra for the polycrystalline samples of MACA and MACB in a wide temperature range to investigate the influence of various factors related to phase transitions in both crystals on vibrational spectra. EXPERIMENTAL (CH3NH3) 3Sb2C19 and (CH3NH3) 3Bi2C19were prepared as described earlier [6,7]. The IR absorption in the 4000-300 cm-1 region was recorded for a suspension in Nujol using KBr plates and for a pellet in KBr on a Perkin-Elmer 180 spectrophotometer. The vibrational frequencies are believed to be accurate to 1 cm -1. The low temperature spectra in the range 300-90 K were obtained with a variable temperature cell using liquid nitrogen as the coolant. A vacuum cryostat of our own construction and a self-made temperature controller were used to keep the sample temperature within + 1 K. A copper-constantan thermocouple junction was embedded in a hole drilled in the plate in close contact with the sample. All IR spectra were measured on polycrystalline samples because the single crystals of MACA are destroyed at the phase transition point, which makes it difficult to obtain single crystals of the low temperature phase. The numerical separation of the v9 and v12 bands was performed by using an original computer program RSP200 of Szostak on the IBM PC AT microcomputer. RESULTS The vibrational frequencies of the methylammonium (MA) ion in (CH3NH3)3Sb2C19 (MACA) and (CH3NH3)3Bi2C19 (MACB) at 300 and 100 K are presented in Table 1. Using the assignments of Sandorfy and co-workers

195 TABLE 1 Vibrational frequencies of the methylammonium ion in (CH3NH3)3Sb2C19 (MACA) and (CH~NH3)~Bi2CI9 (MACB) at 300 and 100 K Vibration

Symmetry

Description

MACA

MACB

300K

100K

300 K

100K

vl

A1

Sym. NH3 + stretch

3160

3160

3160

3160

v2

A1

Sym. CH3 stretch

2930

a

2940

--"

v3

A~

Sym. NH3 + bend

1480

1469

1480

1469

V4

AI

Sym. CH3 bend

1428

1428

1425

1430

v5

A~

C-N stretch

976

979 971

975

976 979

vs

A2

v7

E

Torsional Asym. stretchNH3 +

3205

3205

3205

3205

vs

E

Asym. CH3 stretch

2975

--"

2980

--"

v9

E

Asym. NH3 + bend

1592

1605 1596 1576

1591

1605 1588

V~o

E

Asym. CH3 bend

1458

1458

1460

1458

v~

E

Rocking

1255

1260 1265

1255

1260

923

931 926 923 917

919

927 919

v~2

E

Rocking

"Impossible to assign due to overlapping of Nujol bands. [11,12] for t h e m e t h y l a m m o n i u m halides as guides, t h e a s s i g n m e n t s are fairly s t r a i g h t f o r w a r d , w i t h t h e e x c e p t i o n of t h e N - H s t r e t c h i n g region. T h i s region, e s p e c i a l l y a t 90 K, c o n t a i n s a w e a l t h o f a b s o r p t i o n b a n d s a n d it is e v i d e n t t h a t extensive Fermi resonances exist between the N - H stretching fundamentals and overtones and combinations arising from the N-H bending modes. I n Fig. 1 t h e r e are p r e s e n t e d s p e c t r a for M A C A a n d M A C B in t h e regions 3 5 0 0 - 2 5 0 0 c m - l a n d 1700-900 c m - 1 . Full t e m p e r a t u r e a n a l y s i s w a s d o n e in the frequency ranges of the asymmetric ammonium group bending vibration, v9, a n d o f t h e a m m o n i u m g r o u p r o c k i n g v i b r a t i o n , v12, w h e r e d i s t i n c t t e m p e r a t u r e effects w e r e o b s e r v e d . I n Figs. 2 a n d 3 we s h o w t h e t e m p e r a t u r e e v o l u t i o n o f t h e b a n d o f t h e a s y m -

196

%T

\

30100

25i00

~( 16'00

14~)0

12'00

1000 cm-,

Fig. 1. I R s p e c t r a for polycrystalline M A C A a n d M A C B in t h e regions 3500-2000 c m -1 a n d 1700900 c m -1 at 300 K.

o/!

303 K ~218

K

K

2<

1- 1

K

K

650

1600

1550 cm-1

Fig. 2. Temperature evolution of the band of the asymmetric ammonium group bending vibration, vg, for MACA.

197 K

/•303 ~ 218 K

208 K

%T

203 K

172 K

140 K

122 K

95O

9OO

850 c m -1

Fig. 3. Temperature evolution of the band of the ammonium group rocking vibration, ~12, for MACA.

1600 T E

1580 930

92~

I

100

L

2OO

J

T/K 3 0

Fig. 4. Temperature dependence of the u9 and ~2 frequencies for MACA.

198

? E 980

.?.

/ 970

~Tc

,60

'

2{~()

' T/K 360

Fig. 5. Temperature dependence of the C-N stretching vibration, vs, frequency for MACA.

16150

16h00

15150

cm-i

Fig. 6. Temperature evolution of the band of the asymmetric ammonium group bending vibration,

vg,for MACB. metric ammonium group bending vibration, g9, and of the ammonium group rocking vibration, v12, for MACA. The temperature variation of the IR frequencies in Fig. 4 shows that the band at 1592 cm -1 (v9) in phase I of MACA is nearly independent of temperature and then in the vicinity of Tc (structural phase transition at 208 K) exhibits a sharp splitting into two components. Similarly the band at 923 cm -1 (v12) shows the same behaviour at the phase transition point, where at Tc a stepwise splitting of v~2 takes place. Below 170 K the upper branch of the v12 for MACA gradually splits at first into two and then into three components. The phase transition I-*II is also accompanied by stepwise splitting of the C-N stretching vibration, vs, band (Fig. 5). The components observed became sharper and better resolved on cooling to 90 K. Figures 6 and 7 show the region of the antisymmetric NH3 + bending mode

199 303 K 218 K %T

168 K

148 K

136 K

115 K

I

t

950

900

I

850 c m -1

Fig. 7. Temperature evolution of the band of the ammonium group rocking vibration, 912, for MACB.

1600 E

1580

925

920

100

200

T/K 300

Fig. 8. Temperature dependence of the 129and v12 frequencies for MACB.

200 99 and the a m m o n i u m group rocking v~2 vibration at temperatures between 300 and 100 K for MACB. In contrast to antimonate derivatives in the (CH3NH3)2Bi2C19 crystals, the structure of the 99 band, starting from room temperature, is continuously evident over a wider temperature range. The temperature dependences of the 99 and ~2 frequencies for MACB are shown in Fig. 8. The strong overlapping of the components of the 99 band made it impossible to determine accurately the frequency of the respective components. There is no doubt, however, that the splitting occurs in a temperature range at least in the order of tens of degrees (see Fig. 6). A similar temperature dependence is observed for the v~2band. In contrast to the antimonate derivative the structure of this band at the lowest temperature is less complicated, although by further decrease of temperature an additional splitting may be expected since the observed bands are still asymmetric (close to the liquid nitrogen temperature. ) DISCUSSION The results of X-ray diffraction, calorimetric, dielectric and proton magnetic resonance studies [6] showed that (CH3NH3)3Sb2C19 crystals undergo a first order phase transition at To = 208 K related to a freezing of reorientational motion of CH3NH3 + cations. Three types of methylammonium cations were found. The cations of type (1) possess the greatest freedom of reorientational motions which are not restrained down to To. For the cations of type (2), isotropic rotation diminishing down to the transition point was assumed. The cations of type (3) are seized in the crystalline lattice in both phases retaining only a rotation around its long axis, with possible small angle precession. The studies performed to date prove that the substitution of antimony by bismuth (Sb-~Bi) can lead to substantial changes of the physical properties in spite of similarities in the crystal structure [ 7 ]. In contrast to MACA crystals the (CH3NH3) 3Bi2C19crystals do not show the low temperature first-order phase transition, and a continuous change of the dielectric permittivity e' in the temperature range 300-100 K was found; for MACA the phase transition at 208 K is characterized by a stepwise change of permittivity. Thus the continuous freezing of orientational motion of CH3NH3 + cations was assumed for the former crystal. At room temperature the dynamics (or ordering) of MA cations giving a contribution to the electric permittivity are similar in both compounds, which show the same values of e and its anisotropy. At the lowest temperatures (near 100 K) the mentioned properties are also very similar. Substantial differences are revealed in the intermediate temperature interval. The presented situation, postulated from X-ray, 1H N M R and dielectric results, is well reflected in our IR studies. The observed structure of the 99 and v12bands and of all the other non-analyzed ones in the IR spectrum for MACA

201

and M A C B (Table I), seems to indicate the significantsimilarityin disorder of the methylammonium cations for both analogues. At room temperature in both compounds the M A cations reorientvery rapidly to average out any splitting due to a differentsitesymmetry of three non-equivalent M A cations (two of three cations occupy sitesof nearly C3v symmetry and the lastone Cs). This dynamic disorder is confirmed by the halfwidths of the IR bands in the C - N stretchingvibration region (A vi;2= 9 c m - i and 10 c m - ~ for M A C A and M A C B , respectively) as well as in the asymmetric a m m o n i u m group bending vibration, v9, (Av~;2=33 c m -~ and 47 cm -~ for M A C A and M A C B , respectively) and in the a m m o n i u m group rocking vibration, v12, (A Vl;2= 22 c m - ~ and 27 c m - ~ for M A C A and M A C B , respectively) regions. The strongest temperature effectsare observed on bands attributed to the N H 3 group (vs, v9, P~2),which undoubtedly confirms the changes in the disordering of the M A cations with temperature and their diminishing site symmetry at least from C3v to Cs. In the case of M A C A we observed the nearly stepwise splittingof the vs, v9 and P12 bands at the phase transitionpoint. For M A C B the splittingoccurs over a wider temperature range. The splittingof ~5, v9, P~2 observed with decrease of temperature isthe result of the partial limitation of the freedom of rotation of the N H 3 group in the methylammonium cation (changes in the dynamic state of this cation ) as well as the additional effects related to a differentiationof the three types of M A cations. The separation of both these effectsfrom the value of splittingseems to be impossible. It should be stressed that all M A cations in the high temperature phase are indistinguishablefrom each other giving only single bands for all internal modes of C H 3 N H 3 + ions. The temperature dependence of v9 (more exactly of the lower component of v9 in M A C B ) mentioned above m a y be due to an asymmetrization of one of the types of the large cavitiesoccupied by the methylammonium cations which leads to a decrease of interactions between the a m m o n i u m group and surrounding chlorine anions. The splittingof Ps (C-N stretchingvibration) in M A C A is a scarce example not observed in seriesof other compounds (with the exceptions of M A C I [13] and M A N 0 3 [16] ), in which the mechanism of the phase transition is also related to a freezing of the reorientation of M A cations. The higher position of the ~ band compared with M A X (X = Cl, Br, I ) salts [10] is caused by relativelyweaker hydrogen bonding in M A C A and M A C B but comparable with that observed in (CH3NH3)2PtCI6 [14] and (CH3NH3)2SnCIe [15]. For the last two crystals it is well known that M A cations have a similar freedom of reorientationalmotion in the high temperature phase, as in the studied crystals.W e could not observe the shiftingof this band with temperature towards lower values due to its large halfwidth.

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