Vibrational spectroscopic study of the structural phase transitions in Perovskite layer compounds (CH3NH3)2 CdCl4

J. Phys. Chew. Solids Vol. Printed in Great Britain.

50. No.

IO. pp. 1033-1040.

1989

0022s3697/89 13.00 + 0.00 % 1989 Pergamon Press plc

VIBRATIONAL SPECTROSCOPIC STUDY OF THE STRUCTURAL PHASE TRANSITIONS IN PEROVSKITE LAYER COMPOUNDS (CH,NH,), CdCl, P. S. R. PRASAD and H. D. BIST Department of Physics, Indian Institute of Technology, Kanpur-208016, India (Received 29 August 1988; accepted in revised form 31 May 1989)

Abstract-The infrared (IR) and Raman spectra of (CH,NH,), CdCl, [C,CdCl] are reported in the internal mode region in the 90-300 K temperature range. From an analysis of a few thermosensitive bands in the 80&2000 cm-’ range, it is concluded that C, CdCl undergoes a transition from a tetragonal low temperature (TLT) phase to a monoclinic low temperature (MLT) phase at 160 f 2 K. No spectroscopic mamfestatlons are observed tor a transltion from an orthorhomblc room temperature (ORT) phase to the TLT phase which has been reported to occur at 283 K. The observed dissimilar linewidth changes in the v,, (NH, rocking) and Y,*(CH, rocking) modes around 160 K have been attributed to a dynamical motion of the methylammonium ion. The spectral parameter variations for v,, and v,~have been explained using a soft-hard mode coupling model and the value of B (order parameter critical exponent) was found to be 0.26 f 0.05. A change in the hydrogen bonding scheme is clearly shown by the temperature dependence of the IR active mode (vg+ Ye)and by the Raman frequency shifts in the v, and Y, modes. From an Arrhenius type plot of the Raman peak intensity ratio variation for two v2 components, the activation energy of 845 cal mol-’ has been calculated. Keywords: Structural phase transitions, soft-hard order parameter exponent, hydrogen bonding

mode coupling, dynamical motion, line broadening,

1. INTRODUCTION

0:;

formula general with the Compounds NH,),MCl,; M = Cd, Mn, Fe and n = 1, 2, K” HI, + I 3, belong to a large family of crystals with two-dimensional perovskite-type structures. They are built up from infinite sheets consisting of corner-shared MCI, octahedra. The interlayer space is occupied by the alkylammonium ions oriented perpendicular or nearly perpendicular to the octahedral plane. The NH, polar ends of the alkylammonium ions are attached to the chlorine atoms of MCI, by hydrogen bonds. The interlayer interaction occurs through both van der Waals and Coulomb forces between the CHx ends of the organic ions. Previous phase transition studies using different experimental techniques, such as calorimetry, DTA, DSC, X-ray, NMR, NQR and light scattering, reveal that these types of compounds undergo many first and second order structural transitions [ 1,2]. In short chain compounds, i.e. n $3 all the phase changes are essentially due to the reorientational motion of the organic [C, H,, + , NH,] ions. The n = 1 compounds with M = Cd, Mn and Fe show at least three structural phase transitions (SPT) in the temperature range 490-90 K. The SPT sequence for these compounds is: (a) tetragonal high temperature (THT); (b) orthorhombic room temperature (ORT); (c) tetragonal low temperature (TLT); and (d) monoclinic low temperature (MLT). The space groups and transition temperatures for (CH,NH3)* CdCl, (abbreviated hereafter as C, CdCl) have been reported as follows [l]:

THT

Tc, 424 K

0;; ORT

Tc,

0;;

Tc,

C;,,

283 K

TLT

163 K

MLT’

It is known that the THT to ORT transition is second order and the other two are first order in nature. The crystallographic data [3] show that the TLT-MLT transition occurs at 173 K; however, calorimetric, Raman and FIR spectroscopic studies [4-71 indicate it to be at 163 K. Infrared studies [8,9] on polycrystalline samples of C,CdCl in nujol mull have shown this transition at 173 K with a dominant factor group splitting in the MLT phase. With this discrepancy in mind, and to get additional information concerning the order-disorder transition, we have investigated the IR and the Raman spectra of C, CdCl. A platelet of C, CdCl was used to record the spectra in the temperature range 90-300 K as part of our ongoing program of phase transition studies using spectroscopic methods. We have not used any matrices (like KBr) or mulls (like nujol) to avoid additional interactions [lo] and the overlap of mull bands with the bands of the compound. Based on the temperature dependence of a few thermosensitive bands, we conclude that the TLT-MLT transition occurs at 160 f 2 K, which corroborates the previous spectroscopic studies [5-71.

2. CRYSTAL STRUCTURE AND ROOM TEMPERATURE POLARIZATIONS The group theoretical analysis made by Petzelt [ 1 l] has considered only the lattice vibrations of the methylammonium ion in the THT and ORT phases.

1033

P. S. R. PRASADand H. D. BET

1034

Table I. Factor group analysis for (CH,NH,)+ in (CH,NH,),CdCl, in the orthorhombic room temperature (ORT) and monoclinic low temperature (MLT) phases

n
I 2

2

2

1

1

A, 18 3 3

1 2

2 1

I

1

2

2

A, 18 3 3

B* 18 3 3

2 1 B” 18 3 3

int: internal, V: librational, T: translatory.

Further analysis was carried out in a similar fashion by Couzi et al. [5] for the TLT and the MLT phases. The factor group analysis results considering the free ion symmetry of CH,NH: as C,, are summarized in Table 1. In the ORT and MLT phases C, CdCl contains two molecules in the unit cell. The site group symmetries are C, and C,, respectively. A good crystal with typical dimensions (3 x 2 x 0.5 mm3) was used to record room temperature polarized Raman spectra. The sample was excited with the 514.5 nm line (~80 mW) of an argon ion laser. Other experimental details have been published elsewhere [12]. The internal modes of the methylammonium group do not show any significant changes among the A,, I+,, B,, and B3, species for the following polarization geometries XX; XZ; YZ and YX (see Fig. 1); however, noticeable changes in the external modes were in fair agreement with the existing data [6]. Since there are no observable variations in the internal mode region, we have taken unpolarized spectra for the temperature dependence study in order to avoid the scattering intensity losses due to additional optical elements.

3. EXPERIMENTAL The compound C,CdCl was prepared by slow evaporation of an aqueous solution containing stoichiometric amounts of CH,NH,Cl (AR grade Koch-

3200

3000

2800 ” RAMAN

Fig. 1. The polarization

Raman

spectra

Light Laboratories, England) and CdCl, (AR grade BDH). After initial preparation, it was purified by repeated recrystallization. For the IR study a thin plate of C,CdCI (3 x 2 x 0.02 mm3) was fixed on a metal disc with an appropriate hole at the centre. This disc was sandwiched between two nickel-plated metallic gaskets and kept inside the cold finger. A copper-constantan thermocouple was mounted close to the sample plate to measure the temperature. The sample temperature was varied at the rate of 1” min’, during both the cooling and heating cycles to ensure minimum temperature gradients. Enough time was allowed to attain the equilibrium temperature before recording the spectra. The IR spectra were recorded on a PE-580 spectrometer. The frequencies were calibrated against standard polystyrene bands and are accurate to k 2 cm-’ for strong and sharp bands; to f 5 cm-’ for broad bands; and + lOcm-’ for broad and weak bands. The temperature accuracy was better than 2 K. Overlapping bands were resolved graphically, assuming Gaussian band shapes [13]. 4. RESULTS 4.1. Infrared

spectroscopic

results

C,CdCl undergoes two SPTs below ambient temperature. In Fig. 2(a), the IR spectra of this compound in the ORT and MLT phases are shown in the 200-4000 cm-’ range. The spectrum of the intermediate phase (TLT) shows no marked changes. All fundamental modes of the methylammonium ion according to C,,. symmetry have been identified. In Table 2 all observed frequencies of the ORT and MLT phases are given. Many modes show marked changes during the TLT-MLT transition. The temperature dependence of C, CdCl in different regions is discussed in the following. The spectral parameter variations are clearly studied for the three most thermosensitive regions, comprising the rocking modes and a combination mode involving the vg (torsional) mode.

l! SHIFT

(cd)

of (CH,NH,),CdCl,

in the ORT phase.

Structural phase transitions in (CH,NH,),

(4

CdCI,

1035

72 .Z 5 d

b 8 f

F f

z

a

E 3000

4000

2000

1400

WAVENUMBER

3200

3000

1550

2800 RAMAN

Fig. 2. (a) Infrared

and (b) Raman

spectra

2. The IR and Raman

Present 293 K

studyt 91 K

1489 s

1462m

1419 sh 992 sh

1422 s 1002 m 332~

From 298 K

of (CH3NH,)2CdCl,

1500 1488 1420 994

1500 1490 1422 1000 380 3120 3080 2955 1605 1585 1590 1475 1465 1460 1288 1265 941 937 1925

1496

s m s m

2955 m 1585s

1464 s

1512 sh 1502 s

1460 w

1654 sh

1932 m 1900 w 1737 m

ORT 3105 2968

2960 sh 1602 s

1881 m

Ref. 9 120K 3040 s 2900 sh

$ 1584s

934 s

in

the ORT and MLT phases.

Temperature evolution of these bands is shown in Fig. 3. The most dramatic changes are noticed in the v,~ band position [Fig. 4(a)] and linewidth [Fig. 4(b)] around 160 K. There is no appreciable change in the

3040 m 2900 sh

$

1482 s 1290s 1269 s 941 s

100

(cd)

band positions of (CH,NH,),CdCl, in the orthorhombic (ORT) and monoclinic low temperature phases

$

1266 s

400

300

950

SHIFT

At room temperature two fundamental bands, viz (CH, rocking: 934cm-‘) and v5 (C-N stretching: 992cm-‘); and a combination mode (v12+ lattice: 1040 cm-‘) are seen in the 75GllSOcm~ range. Table

800

(cni’)

1263 s 935 s -

s m s m w s m m m s sh m m m m s s s

1420 990 3175 3028 1594 1574 1462

Raman freq. MLT 3070 2968 2954 1496

995 330 3112 3042 1609 1593 1578 1466 1450

932

943 934

“,(A,): v,(A,):

NH, Sym. stretch CH, Sym. stretch

“,(A,):

NH,

Sym. deform.

vq(A,): v,(A,): vs(A,): v,(E):

CH, Sym. deform. CN Stretch Torsional NH, Asym. stretch

v,(E): v,(E):

CH, Asym. stretch NH, Asym. deform.

v,,(E):

CH, Asym.

v,,(E):

NH,

Rock

v,,(E):

CH,

Rock

Vb+ VI0 “6 + “II peaks,

temperature

Assignment and approx. description

“6 + vg

1897 1880 1725

t s = strong, m = medium, w = weak and sh = shoulder-type SIndicates part of the broad band at 3100cm-‘.

room

respectively.

deform.

P. S. R. PRASAD and H. D. BIS~

1036

1350

1270

WAVENUMBER

WAVENUMBER

Fig. 5. Spectral

(cd)

Fig. 3. Typical IR spectral evolution of the CH, rocking mode (v,~) in the temperature range 91-295 K. Numbers above each curve represent the corresponding temperatures.

peak intensity for this mode during this transition. There are no observable changes for the v5 band. Sharpening of the combination mode similar to that of v,~ is also observed, but measurements of the band parameters are difficult because of its weak and broad nature. The predominant changes with temperature occurring in the NH, rocking mode (v,, ) in the frequency range 1230-1350cm-’ are shown in Fig. 5 with the temperature axis displaced diagonally. At room temperature, v,, has only one component at 1266 cm- ‘; however, a slight asymmetry developed within a 2’ temperature variation (160-158 K). The peak frequency of 1269 cm-’ (at 93 K) remains unchanged, while the 1290cm-’ component of v,, exhibits a softening and merges into the lower frequency com-

(Cd)

thermograph of C,CdCI region.

ponent at about Tc, (160 K). The linewidth of both components increases linearly [Fig. 6(b)] with temperature until Tq, after which it remains almost constant. The ratio of peak intensity of the 1290cm-’ band (I’) to that of the 1269 cm-’ band (I) shows a slight decrease with increase in temperature up to 160 K [Fig. 6(c)]. There are five clear bands in the 1300-1800 cm-’ range at room temperature of which four have been assigned as fundamentals: the CH, symmetric deformation mode (vq at 1419 cm-‘); the CH, asymmetric

TEMPERATURE TEMPERATURE

(K)

Fig. 4. Temperature dependence of (a) peak frequency (b) linewidth for the v,> band of C,CdCl.

and

in the Y,, mode

( K)

Fig. 6. (a) Peak frequency, (b) linewidth and(c) relative peak intensity of VI:* (with respect to VI?“) of C,CdCl in the temperature range 90-298 K.

Structural phase transitions in (CH,NH,),

CdCI,

1037

deformation mode (vIO at 1464 cm--‘); the NH3 symmetric deformation mode (v) at 1489cm-‘) and the

asymmetric deformation mode (vg at NH, lSX4cm-r). The shoulder at 1654 cm-’ could be assigned as vg + v,, 191.On lowering the temperature, at about 160 K, i.e. at the TLT-MLT transition, the vg and vrO modes split into two components, 1572-1602 cm-’ and 1482-1502 cm-‘, respectiveiy, as shown in Table 2. These observations broadly corroborate previous reports [8,9]. It is interesting to note that almost all the prominent band positions at room temperature agree with earlier observations [8,9]; however, several split components in our platelet spectra at low temperature differ beyond experimental error. For example one component of the vg mode at 1572 cm-’ is observed in the present study compared with a 1590-1595 cm-’ doublet [9]. These variations could be attributed to orientational effects. Significantly, the strengthening 280 of the hydrogen bonding, which has been deduced TE~PERATURECK) from a change of 20 cm--’ in the mean frequency of Fig. 8. Thermal variations of (a) peak frequency and (b) the vgmode at low temperature [S], is corroborated by linewidth of the (v6+ vg) band. the present study. Additional evidence for strengthening of H-bonding is clearly seen from the temperature more rapidly towards the higher frequency side and dependence of the combination mode (v,+ vg) the 1889 cm- ’ band shows a marginal upward shift in appearing at 1881 cm-’ at room temperature. its peak position. In Fig. 7 some typical IR spectra in the Similar effects, i.e. peak frequency shift towards the 1800-2000 cm- * region for C, CdCl in the temperahigh frequency and decrease in the linewidth, for the ture range 95-298 K are shown. The band at (vg + vii) mode (1654 cm-’ at RT) were also observed 1881 cm-’ in the ORT phase could be assigned to the on approaching the transition temperature. vg+ vg combination mode [ 14, 151. Thermal variaThe other combination modes at 2230, 2460, 2550 tions for (a) the peak frequency and (b) the linewidth and 2775 cm” also show sharpening near Tc,. The are depicted in Fig. 8 for this band. At room temperwhole spectrum becomes better resolved at low ature and up to about 160 K only one mode at temperature (93 K); however, the CH, and NH, 1881 cm-” is observed: however, on lowering the stretching modes overlap strongly and form a temperature further to 158 K two components are broad unresolved band near 3lOOcm-‘. This broad seen at I889 and 1925 cm-‘. On approaching liquid structure shows a definite sharpening near 160 K. nitrogen temperature, the 1925 cm-’ band shifts A weak band at 332 cm-’ is observed in the MLT phase. This could be assigned as the vg (torsional) mode [9] corresponding to the Raman value of 341 cm-’ (at 20 K) 161. In Fig. 9(a) ln(cu - oo) (the peak frequency difference: Ao) of the v,* mode is plotted against ln(Tc, - T) while Fig. 9(b) shows ln(r r,) (the linewidth difference: Ar) vs In{T/(Tc, - T}) for this band. In Fig. 9(c) a In-ln plot of Au.rvs (Tc, - T) for I WI J/L-\\ the v,, mode is shown. The values of w0 and To are taken far from the critical region. The error bars on each experimental point indicate the uncertainties in the measurements. There is no detectable shift for the TLT-MLT transition temperature between the cooling and heating cycles. This corroborates the observation that there is no hysteresis during this transition [3]. The sudden changes in the spectrum could be due to the first order nature of this transition (see Discussion).

188;w

J

-

WAVENUMBER(cni’)

Fig. 7. Several representative curves of C,CdCI in the frequency range 1800-2000 cm-’ (v6 -I- vg). Temperatures

(K) are indicated above each curve.

4.2, Raman spectroscopic results Previous Raman spectroscopic studies are restricted to frequencies below 350 cn-’ [5,6]. In view

P. S. R. PRASAD and H. D.

1038

BIST

In (T/T~~-T) 0

1

2

3

57

k

J

CC)p=o.zs /

3.0 I

4

_/-’

/’

3.0

Jpq&3.0.5?

4 t / p,,;

3p 2.0

I

Fig. 9. (a) and (c) are ln-ln plots of the peak frequency difference (w - o,,) (left-hand scale) vs (Tc, - T) (bottom scale) for the v,~ and v,, modes, respectively and (b) is a graph of ln(T - r,) (right-hand scale) vs In{T/(Tc, - T)} for the v,~ mode.

I

100

,

I

I

180

I

Temperature of the above IR study, we have investigated the Raman spectra up to 3300 cm- ‘. The Raman spectra in the ORT and MLT phases are shown in Fig. 2(b) and band positions are noted in Table 2. The main temperature variation changes are for the v, , v,, v2 and v8 modes. The v, and v, modes show a 30 and 50 cm-’ downward frequency shift, respectively, indicating stronger hydrogen bonding in the MLT phase. The vu mode also shows splitting in the MLT phase. Spectral changes with temperature in the 2900-3075 cm-’ region are depicted in Fig. 10. As the temperature approaches the TLT-MLT transition, a sharp increase in the peak intensity for the v8 mode is observed (see Fig. 1 I), Simultaneously, a clear band

RAMAN Fig.

10.

SHIFT

Temperature-dependent 29OC3075 cm-’

(cm-‘)

Raman region.

spectra

in the

I

260

I

340 f K)

Fig. 11.Thermal variation of (a) normalized peak intensity of the vQ and (b) peak intensity ratio of the v2 bands. Inset is the Arrhenius plot for the v2 components.

was observed with a small asymmetric structure at room temperature for the v2 mode (see Fig. 10) indicating an orientational change for the methylammonium ion. 5. DISCUSSION The phase transition sequence in C, CdCl has been explained by a stepwise ordering of methylammonium ions [16, 171. In the prototype phase [TLT], methylammonium ions are equally distributed among four possible orientations, making it a disordered phase. In the MLT phase these ions occupy a particular orientation, i.e. they are in an ordered state. The intermediate phases [ORT and TLT] result from the partial disorder of these ions [16, 171. It has been observed previously that vibrational lines show a large change in linewidth during order-disorder transitions [l&20]. The linewidth changes in C, CdCt during the TLT-MLT transition are discussed in the following section. 5.1. Line broadening and critical exponents It is clearly seen from the IR spectra that many vibrational lines show broadening near the MLT-TLT transition. For example, from Table 3, Fig. 6(b) and Fig. 4(b), it is clear that the change in the linewidths (Ar) for the v,, mode is smaller (24 cm-‘) than that of the vu mode (z 14 cm-‘) in the small temperature interval of 4 K on either side .. of Tc,.Additionally, for the v,, mode, one component (1282 cm -‘) shows a slight increase in linewidth near Tc,,while both components show similar be-

Structural phase transitions in (CH,NH,),

CdCI,

1039

Table 3. Typical frequency and linewidth changes for the Y,, (NH, Rock) and vu (CH, Rock) modes in the tetragonal low temperature (TLT) and monoclinic low temperature (MLT) phases close to the TLT-MLT transition temperature (Tc,) in (CH,NH&CdCI, Mode NH, Rock (VII) CH, Rock (v,,)

TLT phase at I64 K w cm-’ rem-’ 1270.5 938

24 54

haviour (i.e. monotonic decreases with temperature) up to 90 K. If this broadening in the vI1 and vu modes is due only to the ordering of methylammonium ions, one would expect nearly the same amount of linewidth changes. The dissimilarities in the linewidth changes suggest that the NH, and CH, ions are under the influence of different forces apart from the C-N binding force. This fact indeed supports the NMR result that even though the C-N bond occupies a particular orientation, CH, and NH, ions undergo hindered reorientations [21]. The large linewidth change in the CH, rocking mode shows that the CH, ion is more ordered in the MLT phase. The small linewidth change for the v,! mode could be attributed to the additional disorder of NH, ions in the MLT phase. The origin of additional disorder can be understood in terms of the hydrogen bonding associated with the NH, ions. In the completely disordered (THT) state, NH, polar ends form one hydrogen bond with an axial chlorine atom and two hydrogen bonds with equatorial chlorine atoms of the MCI, octahedra on a time averaged basis [21]. In the ordered (MLT) state, the opposite hydrogen bonding scheme for NH, ions has been found, i.e. two and one hydrogen bonds with axial and equatorial chlorine atoms of the MCI, octahedra [21]. The additional disorder (hence line broadening) for the NH, ions in the MLT phase could be attributed to the time averaged hydrogen bonding scheme. The frequency changes in the v,, and Y,~modes and the drastic linewidth change in the vu mode could be understood on the basis of a soft-hard mode coupling model 1221.According to this model, the frequency of a hard mode varies like a soft mode if it is coupled to it. The temperature dependence of such a phonon frequency [o,~: soft-hard coupled mode] is given by

MLT phase at 156 K w cm-’ r cm-’ 1270 1282 943

20 26 40

Ar = ~TLT- ~MLT 4 -2 14

where fi is the order parameter exponent, then

From Fig. 9(a), it is clear that the In-In plot of the peak frequency difference (W - wO) vs (Tc, - T) is a straight line (in the temperature range 95-160 K) with a slope of 0.27 + 0.05 and with o0 = 937.5 cm-‘. The similar plot for the linewidth change [Fig. 9(b)] with r0 = 26 cm-’ in the same temperature range is also a straight line with a slope 0.52 + 0.05 (i.e. 2p = 0.52 + 0.05). Additionally, the validity of the above relation (1) is also checked from the vII mode temperature dependence. A similar In-In plot as Fig. 9(a) with o0 = 1269cm-’ [Fig. 9(c)] also gives a straight line with slope 0.26 f 0.05. All these observations are in accordance with the soft-hard mode coupling scheme, for tem~ratures over 65” below Tc, (i.e. until 95 K). The existence of a soft mode for C,CdCl was not found in previous FIR studies, but it has been observed that some low frequency modes do show drastic broadening f7] near Tc,. The absence of hysteresis and the value of the order parameter exponent (0.26 + 0.05) indicate that this could be a weak first order transition; however, the calculated value in the present study is closer to that of second order phase transitions in similar systems [23,24].

5.2. Hydrogen bonding scheme The shorter N-Cl bonds and the monoclinic distortion of the MCI, octahedra about the unique axis provide evidence for an increase in the strength of hydrogen bonding [3] in the MLT phase. The spectroscopic manifestation of this can be found from the stretching, bending and torsional modes involving N-H bonds [25]. Here, in our case, stretching modes form a broad band near 3100cm-‘. Many overlapping bands are present in the N-H bending mode o,a(Tc - 7-)“2 region. Hence, we have chosen the 18~2~O~rn-’ range, where one expects vg (torsion) coupled modes. and the linewidth rsh of this mode varies as follows From the temperature dependence of the (vg+ vp) mode (Fig. 7) it is clear that this shows splitting near ]22] 160 K. The split components of this mode show a frequency shift towards higher values. The linewidth r,,@(TI(T - Tc)i. of this mode (vg $ vg) shows sharpening near Tc,. SO, Assuming the general validity of this model, if CD,,, from the observed frequency for the vp and (vg + vg) varies like an order parameter, i.e. modes, it is concluded that hydrogen bonding is stronger in the MLT phase. Additionally, a band u&c((Tc - T)“, (1) which could be assigned to the vg mode was found

1040

P. S. R.

PRASAD

near 332cm-‘. Our Raman data indeed show a diminution in the frequency of the stretching modes, corroborating the strengthening of hydrogen bonding in the MLT phase. The v1 components show an Arrhenius-type variation in the peak intensity ratio near Tc,. The orientational barrier calculated from this, 845 cal mol-’ (see Fig. 1l), is comparable with the NMR results [26].

6. CONCLUSIONS

The present study corroborates the TLT-MLT transition for C, CdCl at 160 + 2 K. At about Tc,, linewidths for the rocking modes (v,, and v,*) diverge. Frequency shifts and/or splitting of many modes were clearly seen near Tc, without any hysteresis. The observed frequency shift and linewidth change for the v12 mode were explained by the soft-hard coupling model. The order parameter exponent value (0.26 f 0.05) is close to that in the Ising model, indicating that the nature of the transition could be weakly first order. We applied this model up to 65 K below Tc,. Experiments with higher temperature accuracy are needed to verify the order of this transition. The role of hydrogen bonding is shown by the sudden frequency shift for the (vg + vg) mode and the appearance of the v6 mode itself. From the spectral variations of these bands, it is concluded that hydrogen bonding is stronger in the MLT phase.

REFERENCES 1. Kind R., Ferroelecrrics 24, 81 (1980). 2. Couzi M., Vibrational Spectra and Structure. Sixry Years on Roman Spectroscopy (Edited by H. D. Bist, J. R. Durig and J. A. Sullevan). Vol. 17. Elsevier, Amsterdam (1989).

and H. D. BIST 3. Chapuis

G., Kind R. and Arend

H., Phys. Slatus Solidi

(a) 36, 285 (1977); ibid. 31, 449 (1975). 4. Rahman A.. Clavton P. R. and Stavelev L. A. K.. J. them. Thermo&amics 13, 735 (1981). I Couzi M., Daoud A. and Perret R., Phys. Status Solidi (a) 41, 271 (1977). Mokhlisse R., Couzi M. and Lossegues J. C., J. Phys. C16, 1367 (1983). Stoelinga J. H. M. and Wyder P., J. them. Phys. 64, 4612 (1976). 8. Oxton I. A. and Knop O., J. molec. Srrucr. 37, 59 (1977). 9. Rao C. N. R., Ganguly S., Ramachandra Swamy H. and Oxton I. A., J. them. Sot. Faraday Tram 2 77, 1825 (1981). E., J. molec. Struct. 23, 449 (1974). 10. Casellucci 11. Petzelt J., J. Phys. Chem. Solids 36, 1005 (1975). 12. Pate1 M. B., Agarwal A. and Bist H. D.. J. Raman Spectrosc. 14, 406 (1976). and Processin% 13. Horak M. and Vitek A., Interwetarion of Vibrational Spectroscopy. John Wiley, New York (1978). 14. Oxton I. A., J. molec. Struct. 54, 11 (1979). 15. Depmeier W. and Oxton I. A., J. molec. Srruct. 77, 91 (1981). 16. Blinc R., Zeks B. and Kind R., Phys. Rea B17, 3409 (1978). 17. Blat D. Kh. and Zinenko V. I., Souier Phys. solid S/. 21, 588 (1979). 18. Dimtrier V. P. and Loshkarev V. V., Sooiet Phvs. solid St. 29, 669 (1987). 19. Shaak M.. Couzi M. and Huone P. V.. Ber. Bunsewes. Phys. Chem. 80, 881 (1976). 20. Prasad P. S. R. and Bist H. D., Proceedings of the XI International Conference on Raman Spectroscopy, London (1988). 21. Seliger J., Blinc R., Arend H. and Kind R., Z. Ph.rs. B25, 189 (1976). 22. Lavlicht I., J. Phys. Chem. Solids 39, 901 (1978). 23. Kind R. and Roos J., Php. Reu B13, 45 (1976). E., Laiho R., Levola T., Kleemann W. and 24. Karajamki Schafer F. J., Physica 8111, 24 (1981). 25. Turrel G., Infrared and Raman Spectra of Crystals. Academic Press, New York (1972). 26. Blinc R., Burgar M., Lozar B., Seliger J.. Rutar V., Arend H. and Kind R., /. them. Phys. 66, 278 (1976).