X-ray diffraction, differential scanning calorimetric and spectroscopic studies of phase transitions in the bidimensional compound (C12H25NH3)2CdCl4

OQ22-3697185 $3.00 + .OO Q 1985 F%rmmon Plaa Ltd.

1. Phys. Chem. Solids Vol. 46, No. 12, pp. 1413-1420. 1985 Rintcd in orort Britain.

X-RAY DIFFRACTION, DIFFERENTIAL SCANNING CALORIMETRIC AND SPECTROSCOPIC STUDIES OF PHASE TRANSITIONS IN THE BIDIMENSIONAL COMPOUND (C,2H25NH3)2CdC14 NGUYEN BA CHANH, CHRISTIAN HAUW and ALAIN MERESSE Laboratoire de Cristallographie-UA144au CNRS-35 1,Coursde la Liberation, 33405 Talence Cedex, France

and MADELEINE REY-LAFON and LAURE RICARD Laboratoire de SpectroscopicInfra-rouge-UA 124 au CNRS-35 1, Cours de la Lit&ration, 33405 Talence Cedex, France (Received 29 January 1985; accepted 2 May 1985) Abstract-The existence of three main crystalbe phases (called III, II and I) in (C12H&lH&CdCl, has been revealed by differential scanning calorimetry, X-my dilhction and spectroscopic studies. The crystallographic evolution with increasing temperature appears to be monoclinic (III) - orthorhombic (11) tetmgonal (I). The low temperature phase III is the only ordered structure. The phase transition (III-II), which is of first order type, corresponds to an order-disorder mechanism involving the organic part of the structure (alkylammonium chains) whereas the phase transition (II-I), which is of second-order type, is

related to the arrange.mentof the mineral matrix (octahedra of perovskite layers).An intermediate disordered form II’, stable in a very narrow temperature range and structurally similar to the form II, has also been observed so that the tran~ormation (III-II) proceeds,in fact in two steps(III-II-II). The variation enthalpies observed at the transitions (III-III-II) and analyzed through an order-disorder mechanism demonstrate the high disorder of the alkylammonium chains in form II, in agreement with spectroscopicresults. No thermal anomaly or spectroscopic modification is observed for the high temperature transition (II-I). Keywords:bidimensional comcound, structural phase transition, order-disorder,X-ray di5action, calorimetry,

Infrared spectroscopy and &man spectroscopy. 1. INTRODUCTION

compounds of general formula (CnHln+,X NH3)&fX4 (with M = Cd’+, MI?+ Cu’+, = Cl, Br) exhibit a bidimensional str&ture. R’&&ly their polymorphic behaviour with temperature or pressure has drawn the interest of physicists and chemists [l-9]. The crystalline structure of these derivatives can be-described as a sequence of alternating layers of comer-sharing ikf& octahedra, and of aligned alkylammonium chains situated between these layers. The NH, polar heads of the chains are linked to the chlorine matrix by three hydrogen bonds with two axial and one equatorial chlorine atoms. This configuration is called monoclinic and is the only one allowed for steric reasons. The CH, ends of the chains are directed towards the interlayer space and are bonded to each other through Van der Walls forces. These compounds undergo some magnetic transitions 2d-3d [ lo] related to the nature of the metallic atom M. They have also been extensively studied through their phase transitions related to the dynamics of the alkylammonium chains. Up to now, two main types of transitions have been demonstrated. The first type is related to a flipping of NH3 polar heads between several potential wells in the The

cavities and the second one involves conformational disorder of the chains when n > 3. On the other hand, disorder of the octahedra of the perovskite layer has been shown and the planar disorder in the sequence of these layers has been demonstrated by the existence of diffuse scattering streaks [ 111on the X-ray ditfmction patterns. The thermodynamical aspects of the phase transitions have been studied by heat capacity and enthalpy measurements. The values obtained may be related to the different types of motions involved at each transition [ 12- 15, 251. In the series of cadmium compounds with long alkylammonium chains (n 2 8) the published results concern essentially the compounds with an even number of carbons: the derivatives with n = 8 and 10 are the only ones which have been the subject of a diversified study [7, 16, 17, 81, but the derivatives with n = 12 and 16 have been investigated only by spectroscopic methods [ 17, 191. The aim of the present work is to study the polymorphic behaviour of (C,2H25NH3)&dCl, from the results of X-ray diffiction and calorimetric analysis. These data have been supplemented with Raman and infrared results which are expected to give information on the geometry of the chains in the different phases and on the mechanism of the transitions.

1413

1414

N. B. mANI 2. ExpERlMEh’TAL

2.1. Preparation This compound has been prepamd following the method proposed by Kind and Roes [20]. By mixing alcoholic solutions of n-alkylammonium chloride (prepared by HCl on the corresponding amine) and cadmium chloride according to the reaction 2(C,rHs~NHsCl) + CdClz --) (C,rHZSNH&CdC14 very small platelets of product were obtained, which were recrystallized several times from methanol. Attempts to obtain good single crystals at room temperature were not entirely successll, the crystals were twinned as revealed by further X-ray inve&igation. This phenomenon appears to be frequent for this type of structure. Nevertheless, complementary data have been obtained from single crystal diffraction patterns and corroborate the powder difh-action results for the characterization of the room temperature phase. 2.2. X-ray dlflraction Powder analysis has been performed by diffiactometer and Guinier-Len& camera, using CuKa! radiation. In the first case, the reflections were recorded with a very low counter speed; 10.05” in 26 per min, and accurate positions obtained atIer Karrcvs decotivolution and internal stamkd (quartz) corrections. Because of the fine platelet form of the microcrystals, a pellet of the product was prepared and cut across. A double analysis on the face of the pellet gives the main (001) reflections and, on the cross-section, the main (I&O) reflections, so that a correct and complete record of the diffraction diagram was realized. The GuinierLen& camera experiments were carried out using the following conditions: heating or freezing speeds from O.O3”/min to 0.004°/min, quartz monochromator, window width of 1 mm and film speed of 1 mm/hr with a X-ray generator power of 1 KW. The powder was sealed between two thin aluminium sheets (0.0 12 mm), allowing good thermal equilibrium throughout the diffraction sample and still reasonably intense observed reflections (transmission method). The uncertainty in the temperature was estimated to be equal to +2”. Single crystal investigations were carried out at room temperature by classical photographic methods (We&se&erg camera). 2.3. D@erential scanning calorimetry (DSC) Calorimetric measurements were performed on a Dupont deNemours DSC model 9 10/900. The powder sample (about 2 mg) was put into hermetically sealed capsules. The weight of the sample was controlled before and after each experiment. The thermal recording conditions were as follows: temperature speeds (heating or freezing) of 0.5 to 1 degree/mm, temperature range 220 to 360 K, sensitivity of 5000 rV/mW. The ternperatures are defined according to ICTA (International Confederation for Thermal Analysis) recommenda-

ef al.

tions: the transition temperatures are given through the “onset” temperature To and ‘peak” temperature Ts. The transition enthalpies were calculated from the areas of the DSC signals from the method described by Wilburn [21].

2.4. Infrared and Raman spectroscopies For the infrared study, the powdered sample was dispersed in nujol or fluorolube mulls, then squeezed between two CSI windows; the spectra were recorded using a Perkin-Elmer 180 spectrometer with a resolution of l-2 cm-’ or 2-4 cm-‘, depending on the frequency range. Spectra from 100 to 3 10 K were performed with the help of a cell built in the laboratory. Raman spectra of the powder were obtained using a Coderg T 800 spectrometer and accumulated in a Mint 1 l-03 computer. Spectral slit widths were 1 to 3 cm-‘. The 5 14.5 nm line of a Spectra Physics argon ion laser model 17 1 was used as the exciting beam with a power of 80 to 400 mW.

3. IWSULTS

3.1. Evidence of phase transitions Two phase tmnsition zones situated around 326 and 345 K appear clearly on the Guinier-Len& pattern, recorded while increasing the temperature at a speed of O.O3”/min pig. l(b)]. Three crystalline phases can be defined initially:

(b)

(X)

(xl

Fig. 1.Guinier-Len& Miaction results. (a) Room temperatire phase of(Cl&12,NH&GlCl,, mono&nic space group FWn. (b) Evidence of phase transitionsin (C,~H&JHMdCL with k&easing ten$erature. (c) Phase transit6ns in (CI~H2~NH&CdCl, with decreasing temperature. (d) Phase transitions observed with very low rate of increase of temperature. (The lines marked X are those from the sample holder.)

1415

Phase transitions in the bidimensional compound (CltH~NH&CdC&

-a low temperature phase, called III, stable below 326 K, -a middle temperature phase, called II, stable between 326 and 345 K, -a bigb temperature phase, called I, stable above 345 K. These transitions are reversible as can be seen on a Guinier-Lenni diagram obtained with decreasing temperature [Fii. l(c)]. With a very low rate of increase of temperature (O.O04”/min) it may be noted that the lirst transition at 326 K looks like a sequence of several tm~o~ations extended over a short range of temperature. A weak modification of the spectrum is detected at 323 K and affects only the first two (001) intense reflections (very weak splitting) whereas the main diffraction pattern is nearly the same as that of form III; the whole discontinuity of the d&-action diagram is really observed only at 327 K. Such phenomena are not observed for the second transition zone at 345 K, between forms II and I. Thermal analysis gives new information. In Fig. 2 are the reported DSC signals corresponding to the transition phenomena around 326 K. On the heating curve, a deviation of the baseline is detected at 320 K (Td) before an intense endothermic signal characterized by an onset temperature To = 327.7 K and a peak temperature T,, = 328.7 IL A hump? with a peak temperature T, N 330.2 K is observed, which seems to reveal the existence of a new crystalline form, called II’, in the narrow temperature range between 328.7 and 330.2 & Thus, the combination of X-my diffraction and thermal results allows us to conclude that the transition from form III * form II proceeds through several steps: pretransitional effects observed at 320 K before a first-order phase transition from form III + form II’ at 327.7 K, and finally, a phase transition from form II’ - form II at 330.2 K with a weak enthalpy variation. Diffraction results show the very similar structural characteristics of forms II’ and II. The phase transition between forms II and I, which is detected very clearly by X-my diffraction at 345 K, corresponds to a quasi-athermal transformation. 3.2. ~~~~rirnetr~c analysis Ten independent experiments have been performed and allow us to determine the transition temperatures and enthalpies with good accuracy. Concerning the phase transition from form III - form II, we have chosen to give the total enthalpy, including the two tmnsfo~ations from form III - form II’ - form II which present overlapping thermal signals. It has been found that AZ&r, = 40,000 + 2,000 j - mol-‘. For the second unction we can assume that Al&i

from form II --3 form I, 2~ 0 as no heat capacity

t On some records, this hump appears as a little separated signal.

I

Ends

Fig. 2. DSC curves of (C,~H~~NH~k~. (B) cooling curve.

(A) heating curve;

anomaly is detected. This transition is certainly of second-order type. Our results are slightly different from those in a recent work of Needham et al. [26].

We will give the crystallographic results for the three main forms III, II and I of (C,2H25NH3)2CdCL,. The crystalline data for form II’ are very similar to those of the form II. The three forms have been studied through their Guinier-Len& diagram, supplemented by powder ~~~orne~c and single crystal results for the room temperature phase. The relative intensities of the reflections were measured with a Joyce-MK III C densit0meter.t (a) Form III. We have taken into account the strong crystallographic analogy between this form III and the room temperature phase III of the lower homologous compound (C&-12,NHs)$ZdCl,+, which is known to be monoclinic [7] (PZ,/n, 2 = 4). This analogy appears clearly in Figs. l(a) and (b). The isomorphism criteria allow us to index the whole powder diagram (35 observed mtlections) and the results of the final refinement are reported in Table 1. The differences between observed and calculated B (di&action angle) values are less than I/ 100 degree. We obtained:

Form III: monoclinic at T= 293K

a = 7.463 (1) A, b = 7.523 (1) A,

c= 59.152(8)& /.I = 96.54” (2), z = 4. The indexing is compatible

with the space group

P2,fn. This result is confirmed by single crystal data,

t We wish to thank Dr. J. Lwhet, Lahoratoirede Physique expirimentale University de Bordeaux I, for these mcasuremerits.

N. B. CHANH

1416

Tabie 1. Crystal data of the form III of (C,2H25NH3)&KI, at T= 293K %,

8ak.

4 5 6 7 8 9 10 11 12 13 14 15 16 17

3.010 4.515 6.025 7.535 8.430 8.520 9.048 9.485 9.660 9.857 10.170 10.273 10.570 11.840 12.097 12.106 12.420

18 19 20 21

12.710 13.651 15.203 16.766

22

17.070

23

17.221

24

17.376

3.0054 4.5 107 6.0191 7.5317 8.4277 8.5134 9.0496 9.4798 9.6618 9.8604 10.1859 10.2837 10.5739 11.8417 12.0895 12.1059 12.4168 12.4228 12.7100 13.6467 15.1976 16.7601 17.0787 17.0842 17.2133 17.2224 17.2270 17.3761 17.3770 17.5327 17.5340 17.7635 18.0395 18.0406 18.3353 18.3435 18.9668 19.0426 19.0437

No. 1

2 3

25

17.536

26

17.762

27

18.040

28

18.333

29

18.966

30

19.050

31

19.326

32

19.655

33

19.728

34

19.930

35

20.108

19.3219 19.3266 19.6576 19.7208 19.7318 19.7398 19.9252 19.9376 20.1063

h

k

I

0 0 0 0

4 6 8 IO 8 9 12 5

:, 0

A 1 1 ; ; 0 2 1 0 : 0

1 2 2 0 1 2 : 1 x 3 8 3

11 6 11 7 14 1 13 16 5 13 4 18 20 22 21 5 6 18 16 I 19 14 13 21 3 3 24 7

x 3

43 33 11 3; 39 2; 3”; 40 33 11

(b) Form II. Guinier-Len& data showed a strong simiiarity in the polymorphic behaviour of (Clcr H&lHs)&dCI, and (C1$12~NH&CdCl, dative to the III-II phase transition. For the first compound, it was found that phase II has orthorhombic symmetry (space group Amaa). On the basis of these isomorphic c&e& the ~o~es~n~ng form II of (C,*H~~NH~~~d~ was indexed through the 17 observed reflections. Refinement of the lattice parameters leads to the results given in Table 2. The differences between observed and calculated %values are less than 2/100 degree, which can still be considered as reasonable. We obtained:

Form II: orthorhombic at T = 334 K:

63: 11

u = 7.470 (7) A, b = 7.553 (7) A,

3 3

c = 63.50 (4) A,

: 9

z = 4.

100 25 11

The indexing is compatible with the space group Amua In this hypothesis, the chain probably would be symmetry equivalent and statisticaUy distributed on both sides of the bc mirror plane, as in the high temperature phase of C&d [7].

11 9 3 6

(a’

10 :

: 3 1 1 1 2

II&X

et al.

; 7 9 17 18 13 26 9 11

II 16 6

reciprocal

lattice (strate k=O)

3 4 3

although the crystal appears to be twinned. Effectively, the Weissenberg patterns revealed an apparent orthorhombic symmetry but no correct space group was found. On Fig. 3 are given schematic reproductions of reciprocal (MI/) and (kll) planes and the relative orientation of the two twinned reciprocal lattices &P and a*%” [Fig. 3(a)] and of the corresponding direct lattices UCand a% [Fig. 3(b)]. The direction of b* and b*’ (monoclinic axis) is the same. This interpretation allows us to confirm entirely the lattice parameters obtained from the powder diffraction investigation. Twinning phenomena appear very frequently in these types ofderivatives [22] and must be taken into account as a real difficulty in the interpretation of singIe crystal data.

a+

(a’

reciprocal

lattice (rttate

k=l

I

lb) a

(a*) .

b-fb’) direct

Ir

lattice

Fig. 3. Schematic representation of twinned-crystal phenomenon in form III of (C12HZ,NH&CdCl~.

1417

Phase transitionsin the bidimensionalcompound (C1~H~SNH#ZdC~ Table 2. Crystaldata of the form II of (C12H2~NH&CdCh at T= 334K

: 3 4

2.77 4.18 5.56 6.97

5

8.36

6

8.58

7

9.06

8

9.66

9 10 11 12

10.44 11.76 11.98 16.86

13

17.06

14 15 16 17

17.40 17.80 18.85 18.98

2.781 4.174 5.569 6.967

8.368 8.370 8.592 8.599 9.045 9.668 9.675 10.459 11.769 11.985 16.861 17.045 17.057 17.385 17.810 18.849 18.980

0 8

0 0

0 1 0

0 1 0 1 1 1 1 1 1

0

1 1 0 1 1 0

4 :

40 32

10 1 12 9 3 5 11 7 9

: 30 30 100 35 2 20 20 70

0

2 2 I 2

; 2 I

I

2 0 21 15

x 1 1

: :

18 8 :

25 10 8 4 3

(c) Form I. The Guinier-Lenne pattern shows that the transition from form II - form I does not affect the interplanar distances &I, and modifications are observed only for the reflections involving h and k indices. It is known that for this type of structure, the high temperature form can show a tetragonal symmetry. The relation between the orthorhombic low temperature phase and the tetragonal high temperature one is that the direction of the c axis is common to the two lattices but the a axis (=b) of the tetragonal phase is oriented at 45 o with regard to the orthorhombic ones and nearly equal to h(&2) [or b-(&Z)]. On the basis of this assumption, the Guinier-Len& data of form I were indexed with 16 observed reflections (Table 3). The differences between observed and calculated 8 values are less than 2/ 100 degree. We obtained: Form I: tetragonal at T = 360 K:

u = b = 5.310 (1)

A,

c = 64.31 (4)

A,

z=

2.

The indexing shows that the space group is not a bodycentered I group as usually found for the prototype phase in this series. As the transition II-I is a secondorder one, there is a group-s&group relation between the space groups of phase I and phase II. X-Ray diffraction has shown that the unit volume is about the same in the two phases, and them are four space groups defining a primitive tetragonal cell which are supergroups of Amaa. They are PWmbc, PWmmc, P4/mnc, Wlmcc. As the unit cell contains two molecular entities, the group WJmbc must be excluded since its highest symmetry site implies at least four molecules.

3.4. Spectroscopy study Infrared spectra have been obtained from 200 cm-’ to 3500 cm-‘. In the Raman spectra, an important Rayleigh wing overlaps the frequency range below 150 cm-‘. Thus, only bands due to internal vibrations of the akylammonium

chains and Cd%

octahedra are

by spectroscopy. Infrared and Raman spectra have been recorded at different temperatures (300,3 15, 321 to 329 K by steps of 2,335 and 350 K). Spectral changes are observed only at 328 and 330 K. (a) Low temperaturephase. As shown in Fii. 4 and ObStZNd

5, the observed spectra of form III are typical of ordered, almost extended chains. In the Raman spec-

trum, the most intense bands correspond to the Raman active vibrations of an infinite extended polyethylene chain. Indeed, characteristic bands of trans planar chains are observed for instance at 1063 cm-’ and 1139 cm-‘, which correspond to elongations of the C-C bonds. In the same way, the spectral domains corresponding to the C-H s&etchings (2850-3000 cm-‘) and CHr bendings ( 1420-1480 cm-‘) look like those of solid paraffins [23]. However, the Frequency of the longitudinal acoustic mode (LAM) is slightly higher than expected for an all-trans chain [24] (195 cm-‘), which suggests the existence of a gauche (G’) configuration in the vicinity of one end of the chain. As the NH3 polar head linking can only have a monoclinic configuration, the all-trans cation would be approximatively perpendicular to the chlorine layers. This is not compatible with the low value of the interlayer distances, and the chains must therefore be tilted with respect to the c’ axis (perpendicular to the perovskite layer), which implies the existence of a gauche form near the NH, group. Assignment of the infrared speo trum with the help of a normal mode calculation [ 171 shows that, as for analogous compounds, two conformers are present, one with a single G defect between the first two carbon atoms and the other with a G form between the second and the third carbon atoms. (b) Disordered phases. At 328 K, the Raman spectrum loses all the characteristic features of long ex-

Table 3. Crystal data of the form I (C,2H2SNHShCdCl,at T= 360K No. 1 : 4 5 6 7 !z 10 11 12 13 14 15 16

Bm

&&.

2.75 4.12 5.50 6.88 8.34 8.45 8.79 9.31 10.01 11.79 11.92 16.87 17.10 17.38 18.89 18.97

2.745 4.118 5.498 6.878 8.341 8.455 8.788 9.317 10.012 11.783 11.920 16.866 17.101 17.391 18.892 18.982

h

k

8 0 0 1 1 1

8 0

1 I

I 1 2 2 2 2 1

8 0 0

8

0

I

WUM

2 4 6 10 0 2 4 6 8 12

43 48 6

I 8

f 4

: 2

1: 2

3: ; 65 :

40

100 20 18 10 10

1418

N. B. CHANH

et al.

phase transition does not seem to be related with a further conformational disorder of the alkylammonium chains. This is consistent with its very low and undetected enthalpy. 4. DISCUSSION

X-Ray diffraction, calorimetric and spectroscopic studies show that (Ci2HzsNH&CdCh presents the following phase transition sequence: 328K

330K

Form III ---+ Form II’ +

Fig. 4. Temperature dependence of the Raman spectrum of

(C,~H~,NH&CdCl,+*) Bonds assignedto octahedra modes.

tended chains. Modes at 1063 and 1139 cm-’ collapse. Between 328 and 330 K, the LAM is split into two components which appear as shoulders of the CdCl stretching band at 195 and 230 cm-‘. The former frequency indicates that rather extended chains still remain. The frequency of the latter is characteristic of an important conformational disorder. Above 330 K, the wing at 195 cm-’ completely disappears (Fig. 4). In the CH1 bending domain, the splitting into two components at 1445 and 1420 cm-’ has disappeared, showing a weakening of the intermolecular forces. The peak intensity near 1465 cm-’ decreases with respect to that at 1445 cm-‘; this is consistent with an increase of G defects. The analysis of the CH stretching region leads to the same conclusion. The collapsing of the 2885 cm-’ mode and the upward shift of the frequency of the band near 2850 cm-’ show a diminution of the extended sequences in the chains. It is difficult to determine whether there are more G forms in phase II than in phase II’. However, all the Raman spectrum changes indicate that conformers are more distorted in phase II. Infrared spectra complete this description (Fig. 5). The large absorption near 1306 cm-’ which appears at 328 K is characteristic of the existence of kinks of the form GT2,,+,G’. No other type of defect could be detected within the accuracy of our measurements. In conclusion, the spectroscopic study of the disordered phases of (CL2H&H3hCdCl, shows that the dodecylammonium cations exhibit conformational changes which consist of motions of kinks along the chains. In the II’ phase, it seems that some rather extended configurations remain; they disappear in phase II. No spectral change is observed at 345 K, thus this

345K

Form II ---f Form I.

The evolution of the lattice symmetry with increasing temperature appears to be monoclinic (form III) orthorhombic (form II) - tetragonal (form I). The phase II’ which exists in a very narrow temperature range, probably has orthorhombic symmetry. The transformation III - II’ presents pretransitional effects detected by calorimetric and diffraction methods. The phase transitions III - (II’) - II particularly affect the interlayer distances. The value of &, which is equal to 29.38 A in form III, becomes 31.75 A in form II, which corresponds to an increase of 8%. This fact can be explained only by the variation of the tilt of the alkylammonium chains with respect to the c’ axis perpendicular to the layer. Effectively, analysis of the infrared and Raman spectra shows that, in phase III, the chains are tilted with respect to the normal to the layer due to the single gauche bond near the ammonium end. At 328 K an important conformational disorder character&d by the presence of kink defects (GTh+,G structures) is shown to occur; it becomes more important above 330 K. The second layer G’ defect causes the chain end to be approximatively parallel to the c’ axis. Thus, when the kinks are of the type GTG’, the chains are approximately perpendicular to the layer, which is allowed by the value of the a&~(interlayer distances in the disordered phases. Calorimetric results allow a thermodynamical analysis of the dynamical disorder of the chains. It is assumed that the total transition entropy can be estimated by summing the different partial transition entropies related to these orderdisorder transitions. In the case of the derivative (CloH2,NHS)2CdC4, Kind et al. [lo] have observed that the disorder of the chain can be separated into two steps: the first corresponds to the rotation of the rigid alkylammonium chains between two equivalent positions oriented at 90” to each other, and the second to the conformational disorder of the chain (attributed by the authors to a “melting” of the alkyl chains). Let us consider now the case of (ClzHzsNH&CdQ. The results show a high conformational disorder in phase II. The compatibility with space group Amaa would imply that the chains occupy at least two positions with a statistical weight of 0.5 each. The total entropy at the transition III - II’ - II thus corresponds to the occurrence of these two types of disorder. It has been found equal to ASi,, = 29.2 + 1.5 cal/K/mol = (14.6 + 0.75) R per mol. Since four consecutive C or N atoms are involved in the definition of a torsion angle, the

Phase transitions in the bidimensional compound (C12H25NH3)zCdQ

?I% 46rl2-?

1419

1420

N. B. CHANH

two (C,~H&lI-I~) chains have 10 X 2 = 20 independent possibilities to form either tram or gauche R-C-C-R” configurations, which corresponds to a minimal variation of conformational entropy AS, = 20 X R Log 2 = 13.86R per mol. On the other hand, the minimum entropy variation A& corresponding to the transition from an ordered form to a phase with two disorder configurations is R Log 2 per chain, so A& = 2R Log 2 = 1.38R per mol. The total calculated entropy relative to the disorder of the alkyl chains corresponds to a minimum value of AS$?i, = AS, + A& = 15.24R per mol. Comparison between the experimental entropy value and the calculated one shows that the disorder in phase II can be considered as high but not total. It is lower than the disorder in molten n-paraffins where the melting entropy is 0.9(n - 2)R per mol of chains [7], which would correspond to 18R per mol of compound. This is in concordance with spectroscopic results, since defects such as G forms at the methyl end or GG sequences which are important in molten n-alkanes are not observed for the phase II of (CL2H25NH3)2CdCb. The highest temperature phase transition between forms II and I would correspond to a rearrangement of the CdC& octahedra in the perovskite plane with a quasi-zero energetic expense. In agreement with this conclusion, the vibrational spectra are not modified at the transition II-I. Thus, the polymorphic behaviour of (C1rHZSNH&CdC& can be described through two specific transitions: (i) the first one around 328 K corresponds to the passage from an ordered structure to a disordered one and concerns mainly the organic part of the structure. (ii) the second one at 345 K is relative to a weak modification in the mineral matrix array in this disordered structure. The structural aspect of these transitions could be better understood if a good nontwinned single crystal could be obtained, allowing crystal structure determination.

etal.

Acknowledgment-The authors thank Mme Jany Housty for the numerous calorimetric measurements. RKFKRENCKS

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