H. Z ~ N G A R c t al. : Structura,l Phase Transition in (C,H,NH,),PbCl, 107

phys. stat. sol. (a) 115, 107 (1989)

Subject classification: 64.70; 61.14; 77.20; S11

Ddpartement de Physique, Uniriersitd de T u n i s (a), Lnhoratoire de Physique Exphimentale et des Xicrosondes (h) , Luhoratoire de Cristullographie et de Physique Cristulline, U.A. 144, associP'au C.N. R.S., Univsrsiti de Bordeaux I , Talencel) ( c ) , Laboratoire de Spectroscopie Moldculuire Pt C'ristuJline, U.A. 12-2, ussocie' nu C.N.R.S. Universite' de Bordeuux I , Talencel), (d), and Ecole Xutionale d'lngenieurs E N I S , Sfiix (e )

Structural Phase Transition in the Perovskite-Type Layer Compound (C3H7XH3)ePbCla

BY H. ZANGAE (a), J. L. MIANE (b), C. COURSEILLE (c), N. B. CIIANH (c) , M. COUZI (d), and Y. MLIK ( e )

(C3,H,NH,),PbCl, is a new member of the family of perovskite-type layer compounds. Calorimetric., X-ray diffraction, and dielectric memurements show the occurrence of a structural phase transi- tion at T , = 339 K, from an ort'horhombic room temperature phase (ORT) with space groiip Pnma (2 = 4) to a monoclinic high temperature phase (MHl') with space group P2Jc (2 = 2) . This phase transition is analyzed according to group theoretical methods; in many respects the structural change of (C,H,NH,),PbCl, a t 339 K is similar to that observed previously in (CH,PU'H,), .MCI, compounds (M = Mn, Cd) at high pressure and low temperature.

(C,H,NH,),PbCI, est un nouveau derive faisant psrtie de la skrie bien connue des composhs de structure bidimensionnelle de type pkrovskite. Une ktude par calorimktrie, diffraction RS c t par mesures diklectriques montre que ce dkrivk prksente une transition de phase structurale & Tc = 2 349 K faisant passer d'une structure orthorhornbique basse tempkrature Pnma (2 = 4) B unc structure monoclinique haute temperature P2Jc (Z = 2). La transition a pu &re a,nalysi.e cn terme de thkorie de groupes. Le comportement transitionnel de ce derive prbsente de nombreuses similitudes avcc celui affectant les composks de la skrie homologne (CH,NH3)2111Cl, (M = Cd, Mn) sous haute pression et a basse temperature.

1. Introduction The perovskit'e-type layer compounds with t'he general formula (C,H~,+~NH,),MCI, where M = Cd, Mn ... have at'tracted a great deal of at't'ention because of the unique propert>ies of these materials due to the two-dimensional charact.er of the structure. This struct,ure consists of two-dimensional layers built up from corner-sharing chlorine octahedra with the divalent met'al cat>ion M2+ in the cent.res; the cavit,ies between oct,ahcdra are occupied by t'he NH, polar heads of t>he alkylammonium groups which form NH _.. C1 hydrogen bonds with the chlorine atoms. The existence of numerous structural phase transitions is one attract'ion of these systems [l t>o 71. They arc clue mainly to the reorientational motions of the rigid alkyl chains when n 5 5 and to orient,ational and conformational disorder of tjhe chains when n, 6. The t>hernio- dynaniical aspects of the phase transitions have been studied by heat capacity and enthalpy measurements: the enthalpy values may be relat'ed to the different) types of motions involved a t each transition [S to 101.

l) F-33405 Talence Cedex, France.

108 H. Z.IXG-AR, J. L. M I I ~ , C. C‘OURYEILT~E, N. B. CHANII, M. Conzr, and Y. MLIK

In the series of compounds with n = 3, previous investigations have been made for the manganese [ll to 131, cadmium [5, 14,151, and mercury [I61 derivat’ives: when M = Mn, C‘d, they exhibit complex phase sequences wibh the presence of incommen- surate phaess [I] , 12, 151.

The present work concerns the structure and the phase sequence of t.he lead com- pound (C,H,KH,),PbCI,. The phase transition occurring in t’his material is studied by means of differential scanning caloiimetric (DSC), X-ray diffract.ion, and dielectric measurements. The results show t’he existence of a phase t’ransition a t 339 K, from an orthorhornbic room t’eniperature phase (ORT) to a nionoclinic high temperature phase (MHT) : t,liis behariour is analyzed by group t’heoretica,l methods, in view of ageneral “family tjree” ofperovskite-type layer compounds. The structural determination of t’he ORT phase has been published elsewhere [17].

2 . Experiiiiontal 2.1 S p t h e s i s

The synthesis of the compound is realized according to the following procedure. First, lead chloride PbCI, is prepared by reaction of HCI on PbO,. Then, a reaction between propylammoniiim chloride C,H,XH,Cl and PbCI, in HCI solution with the stoichio- metric ratio 2 : 1 gives directly (C,H,NH,),PbCI,. Platelet-shaped single crystals are obtained by slow evaporation of the solution. The chemical composition of the crys- tals has been controlled by elementary analysis.

2.2 Differential scaiinirig ralo%+net?vy

Calorimetric measurements are performed on a Dupont de Nemours DSC model 910/990. The powder sample (about 5 to 10mg) is put in platinum capsules. The recording conditions are : temperature increasing and decreasing speed of 5 K/min, sensitivity of 5000 pV mW-l, and temperature range 393 to 400 I(.

2.3 S - r a g diffractioti

X-ray diffraction studies are carried out on single crystals and powder samples. For single crystals, precession and Weissenberg camera with an air heated system are used to characterize the crystallographic parameters and space groups of the ORT and MHT phases. Single crystal data in the ORT phase were collected on a CAD-4 diffrac- tometer [lli] : the temperature variation of the powder diffraction patterns through the ORT - MHT transition are measured with a Guinier Simon camera; the recording conditions are: C’uB, radiation, quartz monochromator, window width of 1 mm, and film speed of 1 mm/h; the ternperatute range 293 to 423 K is explored during a heating time of 16 h.

2.4 Dielectric measiiw~?ietit .s

Dielectric measurements are made in the microwave range (9.5 GHz) by a cavity perturbation method. The crystal under test is glued on a quartz rod and is placed in the centre of the cavity (TE,,, modc) a t a maximum of the electric field. The tempera- ture is measured by an optical thermometer which does not interact with the micro- wave field. At a given temperature T, the resonant frequency and quality coefficient of the cavity are measured by means of a scalar network analyser. From these values, the complex permittivity E ‘ - j ~ ‘ ’ of the crystal can be obtained.

For a better determination of the transition temperature, the same microwave device is used, but only the power transmitted a t resonance by the cavity P ( T ) is

Structural Phase Transition in the Layer Compound (C',H,?L'H,),PbC'l, 109

measured, allonring dynamic measurements ; the temperature T of the crystal is simul- taneously recorded.

For platelet crystals with the microwave field in the (ac) plane the transmitted power is

where Po and KO are t'wo constants characteristic of the resonant carit,x

3. Itesults 3.1 l'he i'oom tentperature phuse

The structure of the O R F phase [17] is shown in Fig. 1, where the two-dimensional perovskite arrangement is clearly evidenced. l'he space group is Pnrna (standard notation) and the lattice parameters are CL = O.7815( 1) nm, b = 2.50:34(:3) nm, c == 0.7954(1) nm, and 2 = 4. In this description, the perovskite layer planes contain the a and c directions. The propylammonium groups are ordered [17], which is in contrast with the ORT phase of manganese and cadmium derivatives (1, 4, 5, 11, 14). Another characteristic of this structure is the strong distortion of the PhCI, octa- hedra [17]. Indeed, the in-plane Cleq-Cleq distances vary from 0.386 to 0.432 nm and, in addition, the Pb-C1 axial bonds are bent with an angle Cl,,-Pb-Cl,, = 165.2". A s a result, the Pb2+ sites no longer correspond to inversion symmetry, and the PbCl,

octahedra conserve the rsv mirror plane as a unique symmetry element. Again, this i s in contrast with most of the MII and Cd derivatives, where the NC1, octahedra hare nearly tetraponal sgm- metry.

X-ray diffuse scattering experi- ments performed on crystals of (CH,-NH,),C'dCI, [ 18 and (C,H,SH,), . .Cd@l, [19] have shown the presence of planar disorder in the perowkite layers. Similar experiments realized with a fised crystal photographic method (monochromatized Mo radia- tion) on (C3H7NH,),PhCl, do not re- veal the presence of any tliffuse scat- tering, which is again in accordance with an ordered structure.

Fig. 1. The room temperature phase of (C,H,NH,)2PbC1, [17]

110 H. ZANGAR, J. L. MIANE, C. COURSEILLE, N. B. CHANR, M. Couzr, and Y. MLIK

b

Fig. 2. Calorimetric results corresponding to the ORT-AIHT transition. a ) Heating curve, b) cooliiig curve

3.2 Calorimetric inues tiga t ion Fig. 2 represents the DSC heating curve of (C,H,NH,),PbCI,. A phase transition is evidenced by the presence of an intense endothermic signal situated a t the “onset” temperature To = 339.5 K. The enthalpy of the transition determined from peak area measurements is

AH = (3350 & 300) J/mol.

3.3 Characferization of the O R T e M H T transition

3.3.1 Powder diffract ion investigation The Guinier Simon pattern, obtained in the temperature range 395 to 423 K (Fig. 3) confirms the existence of a crystallographic phase transition a t approximatively 335 K. The characteristic changes, described in the orthorhombic reference are :

(i) The interlayer distances doxo (080, 040 ...) exhibit an important and abrupt variation a t the transition temperature. For instance, one has d,, = 0.635 nm in the ORT phase and do40 = 0.650 nm in the high temperature phase, which represents an increase by = 4%.

(ii) A number of reflections, such as (111) corresponding to = 0.543 nm, are apparently not affected by the phase transition.

(iii) Some other reflections, such as (131) or (151), disappear in the high temperature phase.

On the other hand, the phase transition is found reversible when decreasing the tem- perature.

Precession photographs a t 350 K show a monoclinic symmetry for the high tempera- ture phase (MHT). The presence of several domains is detected in the crystal when going into the M H T phase, as expected for such a ferroelastic transition.

By combining the single crystal and powder diffraction results, the cell parameters and space group of the MHT phase are determined. Approximate values of the par- ameters obtained from precession photographs are then refined through the 30 reflec- tions observed with the Guinier Simon camera, and the following values (360 K) are retained: (1 = 1.351(3) nm, b = 0.786(5) nm, c = 0,781(2) nm, and f i = 71.9(8)’. The space group is P&/c (standard notation) and Z = 2 . The crystallographic changes

3.3.2 Binglc crystal inwstiqation

Structural Phase Transition in the Layer Compound (C,H,NH,)?PbCI,

293K -

335 K -

423 K - 293 K -

111

orthorhombic phasc

monoclinic phase

orthorhombic phase

Fig. 3. Guinier Simon diffraction pattern showing the phase transitions “room temperature phase -+ high temperature phase” in (C,H,PU’H,),PbCl,

a t the ORT - MHT transition are shown in Fig. 4. Table 1 shows the comparison of experimental with ca1culat)ed reticular dist,ances.

3.4 Dielectric m*eszilts

Fig. 5 represents the transmission z = P(T) / Po of the microwave cavity in the range 310 to 360 K. The temperature of the sample is given within an accuracy of k0 .5 K. The phase transition is characterized by an abrupt change in the transmission be- tween 335 and 341 K,

At room temperature, the anisotropy of the permittivity in the (ac) plane of a monocrystal, is measured according to the method described in [20],

F; = 4.20 & 0.20,

.& = 4.60 & 0.20,

gi = 0.09 f 0.01 , E: = 0.10 & 0.01 .

Fig. G represents the thermal behaviour of another crystal oriented in such a way that the microwave electric field is in the (ar) plane: the transition begins near 330 K with a sharp increase a t 340 to 341 K. The value of E“ a t room temperature is 0.20 for this crystal, which is slightly larger than for the previous crystal: the difference may be due to the quality of the crystal.

The large increase in F“ observed a t the phase transition suggests that, in the high temperature phase, dipoles are released in the ( a c ) plane of the structure with a relaxa- tion time in the order of the microwave period s). Probably, the propylammo- nium groups are responsible for this relaxation phenomenon.

-1. Group Theoretical Analysis and Discussion

In order to get a better insight into the mechanism of the observed phase transition, it is meaningful t o consider the group theoretical implications related to the occurrence of the different structural modifications, in view of the “family tree” of perovskite-

h a b r----’ I I I I I

Fig. 4. Crystallographic relations for the ORTctMHT transition. a) Orthorhombir phase (room temperature), b) monoclinic phasc (high temperature)

113 H. ZAWJAR, J. 1,. MIME, C'. COURSEILLE, N. B. CHANH, M. Cor~zr, and P. MLIK

Table 1 Crystal data of high temperature form of (C,H,NH,)2PbCI,

No. dobs (nm) &iC (nm) 11 x-1 I ~~

1 ? 3 4 5 6

8 9

10 11 12 13 14 16 18

17

n

18

19

20

21

22

23

24

25

1.30 0.65 0.542 0.468 0.428 0.391 0.376 0.321 0.307 0.292 0.286 0.278 0.372 0.257 0.254 0.247

0.239

0.234

0.224

0.216

0.211

0.207

0.196

0.294

0.188

1.2850 0.6425 0.5498 0.4688 0.4283 0.3906 0.3i59 0.3213 0.3083 0.2917 0.3858 0.2773 0.2719 0.2570 0.2532 0.2475

0.2394 0.2389 0.2388

0.2344 0.2344

0.0238

0.2165 0.2361

0.2114 0.2115

0.2071 0.2070

0.1969

0.1937 0.1937

0.1882 0.1581 0.1879

100 200 111 21 1 300 102 2002 400 411 402 202 122 222 500 12" 03 1

23 1 512 313

512 422

330

123 611

323 522

332 513

020

621 304

620 314 404

type layer compounds. First, it must be recalled that all phases observed up to now in the series of compounds (CnH2;11+1NH3)2MX4 have been connected by group- subgroup relations to a parent undistorted phase with space group IS/mmm [l, 3 to ti]. The primitive I4/mmm unit cell contains Z = 1 formula unit. Let us call a, and ct the lattice constants of the conventional body-centred unit-cell of multiplicit,y 3, referred to a right-handed orthogonal set of axes Oxyz [?I]; then, the basic vectors of the pri- mitive unit cell correspond to [23]

t , = $ ( - a t , at, c t ) , tz = T ( a t , --at , CJ , t -1 - (a t , a t , - ~ t ) .

(1)

Structural Pliasr Transition in the Layer Compound (C,H,NH,)IPbCl, 113

I I / 1 300 320 340 360

T I K ) - Fig. 5 320 340 360

TCKI - Fig. 6

Fig. 5. Tmnsmission t of the resonant cavity (9.5 GHz) containing three crystds of (C,H,?U'H,), . . PbCI,. The microwavc electric field is in t,he (ac) plane

Fig. 6. Microwave (9.5 GHz) permittivity of (C,H,NH,),PbCI, measured in the (ac) plane; -i-, o citvity perturbation method; solid curve with black circles transmission measurement (rf. Fig. 5)

Big. 7 shows the first Brillouin zone associated with t,he Oetragonal body-centred Bravais lattice, with basic vectors defined as [ZO]

For convenience, we shall adopt the same setting of crystallographical directions in all phases, i.e. with the Ox and Oy directions in the octahedra layer planes, as in the parent phase. Under this condition, the space group of the orthorhombic room tem-

Fig. 7. The Brillouin zone for the I4/mmm parent phase

8 physica (a) 1 1 5 / 1

114 H. ZANGAR, J. L. MIANE, C. COURSEILLE, N. B. CHANH, M. COWZI, and P. MLIK

perature phase (ORT) should be noted Pnam or equivalently Pbnm (non-standard notations), and that of the monoclinic high temperature phase (MHT) P12,/al, or equivalently P2Jbll. Furthermore, let us take the origin of symmetry operations on the sites occupied by the metallic (Pn2+) cation in the parent phase (sites of Dph sym- metry). The latt,ice constants ao, bo, co of the ORT phase (Pnam or Pbnm setting) are related to those of I4/mmm by a. = at i, 6, = C I ~ I/, co = c t ; the basic vectors are such as [22]

which yields

ti = t , + t2 - 2, ,

t 2 ' = - tl + t z >

ti = t , + t, . (4)

Thus, according to the structural determination [ 171, the symmetry elements of the ORT unit cell, with translational parts expressed in ti, ti , f i units must be written as

( 5 )

(6)

E I 0, 0, 0, Cz, 1 0 1/2 1/2, (22, 1 1/2 0 1/2, Cz, I 1/2 1/3 0 , Pnam setting .

1 I 0 1/3 1/3, az 1 0 0 0, a, l/3 1/2 0, bz I 1/2 0 1/3 , E I 0 0 0, Czz I 1/3 0 1/2, C;, 1 1/2 1/3 0, Cz, I 0 1/3 1/3 ,

{ { 1 I 1/2 0 1/3, az I 0 0 0, 5 y 1 0 1/2 1/2, oz I 1/2 1/3 0 .

Pbnm setting .

Knowing that the Cz, and C2, directions in the ORT phase correspond respectively to the C2, and Cza diagonal directions in the parent phase, it can be easily shown that all symmetry elements found in the ORT phase are contained in those of the I$/mmm space group. This means that Pnam (or Pbnm) is a subgroup of I4/mmm. However, the structural arrangement found in the ORT phase [17], with strongly distorted octahedra, is a unique case in the series of (CnH2n,1NH,),MX, compounds with 5 3. Indeed, it implies for the position of the metallic (Pb") cation (origin of

symmetry operations) the following constraints : (i) it is brought out of a centrosyminetric position, (ii) it conserves the oz mirror plane as a unique symmetry element. It follows that a new mechanism has probably to be found in order to account for

the occurrence of such a structure. I n a first step, we shall search for a possibility of direct transformation from I4/mnim

to Pnam, i.e. connected with only one order parameter. From (I) to (4), it can be estab- lished that such a phase change will replace simultaneously the points X (0 0 1/3), x (1/2 1/3 O), and Z (1/2 1/3 i/3) of the I4/mmm Brillouin zone (Fig. 7) a t the zone centre in the O R T phase (the different components of the wave vectors kx, k,, and kZ are expressed in gl, g,, g, units). Since the star of the wave vector a t point X con- tains two arms such as lix+ k., = k, + (01 I) =_ k, an instability occurring at point X can generate a Bi-avsis lattice consistent with (4). On the other hand, only the Xuzu and Xzau little representations of the wave vector group (D;t,) comply with the con- straints (i) and (ii). I n Table 3, we have represented the character tables of the full

Structural Phase Transition in the Layer Compound (C3H,NH.J2PbCI, 115 __ 0 3

I 3 0 I - __ 0 3

I 3 0 I - 3 0 I

O H I __ - 3 0 I 0 3

I __ ~

0 3

1 0 ~

__ 3 0

O H - ~

1 0

0 3 ~

__ 0 3

H O __ ~

01 I

3 0 I - - 0 1

I 3 0 I

~

~

1 0 I 0 -

I - __ 3 0 I 0 3

I -

__ 0 3

3 0 __

I_

n o

0 3 ~

~

3 0

0 1 __ tn s

y.

__ 0 3 3 0 I __

__ 0 3

I 3 0

~

_.

H O

0 3 __ ~

3 0 I 0 3

I

~

0 3

1 0 __ __ 3 0

0 3 I I

- 3 0 I 0 3 __ __ 0 3

I 3 0 I

~

__ 0 3

I 3 0 __ __ 0 3

3 0 I

~

__ 3 0 I 0 1

I __

-_ 1 0

0 4 ~

_ _ 0 1

I 4 0 I __

__ 3 0

0 3 I

~

__ 3 0 I 0 3 __

c.3 c ici

0 3 I

3 0 I __ - 3 0

0 3

0 3

3 0

~

0 1

- 0 I __ __ 3 0 I

O H I

3 0 I 0 3

I

__ 0 3

I 3 0

~

3 0

0 3 __ 0 3

3 0 0 3

3 0 0 3

1 0 3 0 0 3

I _.

3 0 I 0 3 -

0 3 I

3 0 I

II c 4- Y - w Ill u" 3- r"

w -

.^

- 3 0 I 0 3 c

a d x,

3 0

0 1 I 0 3

I 3 0 I - __ 0 3

- 0 I

a Ld

- 0 I 0 -

I 0 1

I 3 0

d o

0 3

__ - 0

0 3

0 3

1 0 I

3 0 I 0 3

I __ ~

o r ( 1

1 0 I __

0 3 I

- 0

0 3 I

1 0

__ 0 3

I 3 0 I

3 0

3 Q I

3 0 0 -

I 1 0 I 0d

1 0 I 0 3

0 3

3 0

0 -

3 0 -

3 'A x

II 0 -

n*

116 H. ZANGAR, J. L. MIANE, C. COURSEILLE, S. B. CHANFI, 31. Coczr, and Y. MLIK

representations of the space group I4/mmm (XB2" 1 I4/mmm) and (XB3u 1 I4/mmm), generated from X B ~ ~ and X B ~ ~ [B]. These representations are t,wo-dimensional and so, are associat,ed with a two-component order paramter ( q , ~ ) . Hence, i t can be easily determined that an order parameter of the type ( T I , 0) or (0, T I ) subduces the space group Arnam or Bbnm (SB,,) and Abnm or Bmam (X,,,), while solutioiis of the type ( 7 1 , ~ ) or (q , 7 ) conserve the tetragonal symmetry; finally, t,he solutions of the type (q , p ) subduce the space groups Pmam and Pbmm. Thus t>he Pnam or Pbnm space ~ r o u p , as definecl by ( 5 ) or ( t i ) , is not a maximal subgroup of I4/nimm.

Then, a more complex mechanism involving a t least two symmetry independent order paramet>ers has to be found. By following the group t)heoret>ical procedure de- scribed above, several possibilities are determined involving the combination of two order parameters among those belonging to the Xlllg, XnaU, and Zun representations. Indeed, it has been shown already that X133u subduces the space group Amam (7, 0) or Bbmm (0 ,q ) . Similarly, it can be established that XI,,, (Table 3) subduces the space group Acam (v, 0) or Bbcm (0, v) and that ZJ3, (Table 3) subduces the space group Ccmm (p , ,u) or Cmcm (p, F). Now the relevant, Pnam or Pbnm space group (see (5) or (6)) is t'he maximal subgroup common to all these space groups and so can be gener- ated from the combined ZE,, and X B ~ , or Z E , and Xe,, or XBl, and Sn,, order par- ameters. From the atomic positions determined in the ORT phase [17], this structure is related to the 14/mmm parent phase by antiphase translational motions of the octa- hedron layers perpendicular t.o ct , combined to octahedra rotations around ct. The former mobion is the transverse acoustic mode a t point Z of the Brillouin zone and belongs to the Zg, representat>ion, and the latter one belongs to X B ~ ~ . Furthermore, the XBl, and XI<,, represent>ations contain also pseudo-spin coordinates attached t'o the orientation of the C,H,NH$ cat'ion [83], that, make possible an orientationally ordered configuration of these groups in the ORT phase, in agreement with the experi- mental data [17]. Thus, it can be safely stat,ed t'hat the observed Pnam (Pbnm) phase corresponds to the freezing of two independent order parameters, with ZE, and X,,, symmetry, respectively.

This mechanism for a hypothetical structural change from I4/mmm to Pnam in (C,H7NH,),PbCl, is exactly the same as that determined in (CH,NH,),MCl, (M = = Mn, Cd) compounds for the occurrence of a new high pressure phase (HP) [24, 351. In particular, i t has been speculated that the MCI, octahedra should exhibit a very strong spont,aneous strain in the HP phase [%I, a fact which is indeed directly evi- denced in the O R r phase of the lead compound [17!. However, the HP phase was found monoclinic instead of ort,horhombic : probably, this monoclinic structure cor- responds to a subgroup of Cmcm and/or Pnam space groups, resulting from a ferro- elastic transformation [ 2 5 ] .

As regards the monoclinic MHT phase of (C3H7NH,),PbCI,, with space group Pl3,/al (or P3,/bll) and Z = 3 in the primitive unit cell (Fig. 4), it, is not group- subgroup relat,ed to O R T . Furthermore, in siich a unit cell, the Pb2+ cations must be necessarily sit>uat,ed on inversion centres (sites Ci are the only ones with a multiplicity of two). I n the absence of a structural determination, a precise discussion concerning the occurrence of t,his phase is not easy to apprehend. However, in view of the "family tree" of perovskite-type layer compounds, t,his phase is probably equivalent to the morioclinic ordered structure (MLT) often observed a t low t>emperature in manganese and cadmium derivatives [ l , 6, IS]. Then, i t could correspond t,o an isotranslational subgroup of either Acam (as mentioned already, this later space group is subduced by the Xulg represent>ation) or Bmab (subduced by XBYn [ G I ) . Fig. 8 summarizes the above discussion on groups to subgroup relations in (C,H,NH,),HgCl,.

Strnctural Phase Transition in the Layer Compound (C:,H,IYH,),PbC‘l, 117

Fig. 8. Group-subgroup relations in (C,H,NH.&PbCl, (full lines). Thc dashed line corresponds to tho observed ORT - --f MHT phase transition

A cam Abma

Bbcm Bmab

(ORTI I M H U

Whatever the exact structure of the M H T phase, the ORT C- MHT transition must exhibit some “reconstructive” character, and in many respect should resemble the HI? - MLT phase change of (CH,NH,),CdCl, evidenced a t high pressure and low tern- perature [24]. Indeed, the ORT -> MHI’ transition is characterized by an abrupt and important increase of the interlayer distance (see Section 3), which is also the main characteristic of the H P - MLT transition [25 I .

From this discussion, it appears that changing the metallic cation Mn2+ or Cd2+ by Pb2+ is equivalent to a combined effect of pressure increase and temperature decrease. In many respects, the ORT .- MHT transformation is similar to the H P - MLT transition observed in manganese and cadmium compounds, which gives some general meaning to the PT phase diagram determined previously [14], in the behaviour of perovskite-type layer compounds.

AclcnotL.ledgements

The authors wish to thank Dr. C. Hauw and Dr. M. Hospital (Laboratoire de Cristallo- graphie e t de Physique Cristalline, U.A. 144 du C.N.R.S., Univ. de Bordeaux I, France) and Prof. A. Daoud (Laboratoire de Chimie MinPrale, Facult6 des Sciences et Tech- niques, Universit6 de Sfax, Tunisie) for helpful discussions and their interest in this work.

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(Received January 30, 19S9)

Chem. Solids 46, 187 (1985).

el, 795 (1984).

C 17 , 233 (1984).

Dordrecht (Holland) 1983.

Clarendon Press, Oxford 1972.

Transitions 14, 89 (1989).