785

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 5, pp. 785–793. © Pleiades Publishing, Ltd., 2008.

1

INTRODUCTION

The phase transitions in tetramethylammonium tet-rahalogenometallic compounds

[N(CH)

4

]

2

MX

4

, whereM represents divalent metals (M = Zn, Co, Cu, Mn…)and X halogens (X = Cl, Br, or I), have been well inves-tigated. Their highest-temperature phase (the normalphase) commonly belongs to the

K

2

SO

4

-II type struc-ture. As temperature decreases, they undergo variousseries of phase transitions. Phases with incommensu-rate structures, and ferroelectricity of lock-in commen-surate phases have been found in a number of crystals[1–8]. It has been reported that

[N(CH

3

)

4

]

2

CuCl

4

and

[N(CH

3

)

4

]

2

ZnCl

4

single crystals with high optical qual-ity undergo phase transitions at 265, 293, and 299 K,and at 161, 181, 276, 280, and 298 K, respectively [9–11]. In the case of

[N(CH

3

)

4

]

2

CuCl

4

, the room tempera-ture phase crystallizes in the orthorhombic system withthe space group

Pmcn

and the following unit celldimensions:

a

= 8.946,

b

= 15.515, and

c

= 12.268 Å[12]. The crystal structure of

[N(CH

3

)

4

]

2

ZnCl

4

is iso-morphous to

[N(CH

3

)

4

]

2

CuCl

4

with cell parameters:

a

=9.039,

b

= 15.515 and

c

= 12.268 Å [13].

1

The text was submitted by the authors in English.

In order to diversify material properties, in our lab-oratory, we are interested in the partially metal transi-tion substitution in

[N(CH

3

)

4

]

2

MX

4

. Indeed, in thiswork, we report the chemical preparation, single struc-ture study and infra-red spectroscopy at room tempera-ture of a new mixed crystal of composition

[N(CH

3

)

4

]

2

Cu

0.5

Zn

0.5

Cl

4

.

EXPERIMENTS

The synthesis of the title compound,

[(CH

3

)

4

N]

2

Zn

0.5

Cu

0.5

Cl

4

, was performed by a mixtureof

(C

2

H

5

)

4

NCl, CuCl

2

and

ZnCl

2

(molar ratio 4 : 1 : 1)in distilled water. The mixture was stored at room tem-perature. After a few days, square yellow crystalsappeared.

Schematically, the reaction can be written:

4(CH

3

)

4

NC

l

+ CuCl

2

+ ZnCl

2

2[(CH

3

)

4

N]

2

Zn

0.5

Cu

0.5

Cl

4

.

The chemical analysis of single crystal was moni-tored using a scanning electron microscope (SEM)Philips XL 30 equipped with an energy dispersiveX-ray analyzer (EDS).

H2O

Phase Transitions in the A

2

BX

4

-Compound: Tetramethylammonium Tetrachlorozincate Tetrachlorocuprate,

[(CH

3

)

4

N]

2

Zn

0.5

Cu

0.5

Cl

4

, and Room Temperature Crystal Structure Determination

1

F. Hlel, A. Ben Rhaeim, and K. Guidara

Laboratoire de l'état solide, Faculté des Sciences de Sfax, Tunisiee-mail: faouzihlel@yahoo.fr

Received January 30, 2007

Abstract

—Preparation, crystal structure, and infra-red absorption spectra are reported for the first material inan

A

2

BX

4

compound with metal transition substitution in tetrahedral anion, tetramethylammonium tetrachlo-rozincate tetrachlorocuprate:

[(CH

3

)

4

N]

2

Zn

0.5

Cu

0.5

Cl

4

. The calorimetric study shows five endothermic peaks at248.75, 271.75, 278.6, 286.7, and 293.7 K. The determination of unit cell in the 240–298 K temperature rangeconfirms those observed by the DSC technique. At room temperature, the compound crystallizes in an orthorhombicsystem (

P

2

1

c

n

space group) with

Z

= 4 and the following unit cell dimensions:

a

= 8.988 (3),

b

= 15.527 (2) and

c

=12.269 (4) Å. The structure was solved by using 1986 independent reflections down to an R value of 0.048. The

crystal structure consists of alternating organic–inorganic [

(TMA)

+

/(Cu)Zn

] layers and organic sheets

(TMA)

2+

. All Organic groups and

(Cu)Zn ar

e not disordered. Their main geometrical features are thosecommonly observed in the atomic arrangements of

(TMA)

2

ZnCl

4

and

(TMA)

2

CuCl

4

.

Keywords

: Copper halide, Structural phase transition, Crystal structure, Infrared spectroscopy.

DOI:

10.1134/S0036023608050197

Cl42–

Cl42–

PHYSICAL METHODS OF INVESTIGATION

786

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

Vol. 53

No. 5 2008

HLEL et al.

The quantitative analysis of copper and zinc ele-ments was performed using a Hitachi Z–6100 atomicabsorption spectrophotometer.

The infrared spectrum was recorded in the370−4000 cm–1 range with a Perkin-Elmer FT-IR 1000spectrometer using samples pressed in spectroscopi-cally pure KBr pellets. Differential scanning calorime-try measurements were performed on heating the sam-ple from 150 to 420 K using a Netzsch-DSC 204 calo-rimeter.

Unit cell parameter variations were studied usingsingle crystal (0.19 × 0.23 × 0.16 mm) gone up on Non-ius Kappa CCD. A nitrogen flow thermostat was used;

the temperature was controlled using a thermocoupleand stabilized to an accuracy of about 0.3 K.

The examination of samples dissolved in distilledwater by atomic absorption spectroscopy shows amolar ratio of Zn/Cu elements at about 0.98. Electronicmicroscope (SEM) analysis shows the same amount.Those results permit us to confirm the proposed for-mula.

A single crystal (0.28 × 0.22 × 0.18 mm) wasselected by optical examination. Data were collected onan Enraf-Nonius CAD4 four-circles diffractometer.Unit-cell parameters were derived from least squareanalysis of 25 reflections in the 2θ range 13°–15°. 2264intensities were collected. A Ψ-scan absorption correc-tion was applied. The structure was solved by using thecomputer program SIR92 [14] in the space groupP21cn. One metal transition atom (Zn or Cu, in refine-ment we take Zn) and four chlorine atoms were firstlocated using the direct method analysis. All the refine-ment calculations were performed with the SHELXL–93program [15]. This program permits us to localize allnitrogen and carbon atoms. Anisotropic displacementfactors were determined for all atoms down to a reli-ability factor R equal to 0.055. In the Fourier series andfrom the beginning of refinement, a weighted peak(13.8 e/Å3) is observed near Zn atoms. An attempt tosupply Zn atoms is not significant. However, weattribute the more disordered tetrahedral to the copperatoms and the other to Zn. Indeed, bibliographic inves-tigation shows that CuCl4 tetrahedra are more disor-dered due to the Jahn–Teller effect. Referring toreported analysis, the occupancy factor of Zn and Cuatoms is fixed at 0.5. The distance between Zn and Cusites, which are not simultaneously occupied, is equalto 0.20(1) Å. In order to eliminate this distance, we playEXYZ option but the refinement diverges. For thesereasons, we maintain the virtual distance calculatedbetween Zn and Cu sites. The hydrogen atoms werelocked from successive least-squares refinement anddifference Fourier maps. After refinement, the final reli-ability factors dropped to R = 0.044 and wR = 0.11. TheFlack absolute structure parameter x [16, 17] is refinedto x = –0.08 (8) ≈ 0.

The experimental parameters used during these mea-surements, the strategy followed for the structure deter-mination, and its final results are gathered in Table 1.

RESULTS

Infrared Absorption Spectroscopy

According to the literature, we present an attempt toassign the important bonds observed in the infraredspectrum of the [(CH3)4N]2Zn0.5Cu0.5Cl4 compound(Fig. 1).

A detailed assignment of all the bands is difficult,but we can attribute some of them by comparison withsimilar compounds [18, 21]. Table 2 summarizes a ten-tative assignment of more significant bands. The prin-

Table 1. Summary of crystal data, intensity measurements,and refinement parameters for [(CH3)4N]2Zn0.5Cu0.5Cl

Formula: [(CH3)4N]2Zn0.5Cu0.5Cl4 Fw = 354.57 g mol–1

Color/shape Yellow/polyhedra

Crystal dimensions 0.28 × 0.22 × 0.18 mm

Crystal system Orthorhombic

Space group P21cn (N° 33)

a = 8.988 (3) Å, b = 15.527 (2) Å, V = 1712.3 (7) Å3

c = 12.269 (4) Å Z = 4, µ = 1.957 mm–1

Temperature (K) 293(2)

Radiation, λ (Å), monochromator MoKα, 0.71069, graphite

Scan angle (°) θ–2θ2θ Range (°) 2–27

Range of h, k, l 0 11, 0 15, –2 19

Reflections collected/unique 2264/1986

Req, int 0.056, 0.038

Observed reflections [Fo > 2σ(Fo)]

749

Absorption correction Ψ-Scans

Tmin, Tmax 0.712, 0.998

Structure resolution: Direct methods: Sir92 [14]

Structure refinement with SHELXL-93 [15]

Refinement F2 full matrix

Refined parameters 183

Goodness of fit 0.884

Final R and Rw 0.048, 0.118

Final R and Rw (for all data) 0.084, 0.17

Flack absolute parameter (x) –0.08(8)

Largest feature diff. map 0.087, –0.317e–/Å3

w = 1/[σ2(Fo)2 + (0.0758P)2 + + 0.064P]

P = [Fo2 + 2Fc2]/3

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

PHASE TRANSITIONS IN THE A2BX4 COMPOUND 787

cipal bands are assigned to the internal modes of methylgroup: the anti-symmetric stretching νa(CH3) at3135 cm–1, symmetric stretching νs(CH3) at 3030 cm–1,anti-symmetric bending δa(CH3) at 1463 cm–1, degener-ate bending δd(CH3) at 1446 cm–1, symmetric bendingδs(CH3) at 1407 cm–1, and rocking vibration ρr(CH3) at1042 cm–1. The bands observed at 460 and 954 cm–1 areassigned to ν3 and ν4 modes of the [N(CH3)4] tetrahe-dron.

Structural Phase Transitions Characterization

Calorimetric study shows five endothermic peaks at248.75, 271.75, 278.6, 286.7, and 293.7 K, indicatingthe availability of six phases.

The derivative compounds [N(CH3)4]2CuCl4 and[N(CH3)4]2ZnCl4 single crystals undergo phase transi-tions at 265, 293, and 298 K, and at 161, 181, 276.3,278, and 293 K, respectively. In Table 3, we recapitu-late space group and cell parameters of all phases of[N(CH3)4]2CuCl4 [22] and [N(CH3)4]2ZnCl4 [23] andcell parameters of the mixed compound observed ver-sus temperature.

The phase transitions observed in the title com-pound were studied by X-ray diffraction. We shouldunderline that the temperature unit cell parameter mea-surements are determined with decreasing temperaturefrom 298 K down to 240 K. The measurements a and c,b and volume, cell angles and the volumetric thermalexpansion coefficient against temperature are repre-sented in Figs. 2a, 2b, 2c, and 2d, respectively. The alattice parameter exhibits anomalies in the vicinity of252 and 274 K coinciding with DSC peaks observed at248 and 271.7 K. The c cell parameter presents a jumpbetween 282 and 284 K. Volume and b lattice parame-ters show the same temperature dependency; they presentthree distinct temperatures ranges: the first for tempera-ture lower then 278 K, a jump from 278 to 280 K; the sec-ond in the range 280–282 K, a jump from 282 to 284 K;the third range is observed for temperature greater than284 K. Examination of the cell angles temperaturedependency shows, at lower temperature, a dispersion

Table 2. IR frequencies of [N(CH3)4]2Cu0.5Zn0.5Cl4

Frequency (cm–1) Assignment

460 ν4[N(CH3)4]

810 ν3[N(CH3)4]

976 ν3[N(CH3)4]

1043 ρr(CH3)

1251

1375 δs(CH3)

1408 δd(CH3)

1464 δa(CH3)

3030 νs(CH3)

3135 νs(CH3)

Table 3. Phase's relatives to the mixed compound are missing

[N(CH3)4]2ZnCl4

T(K) 161 181 276 280 298

Space group P212121 P21/c P21/n P21cn IC PmcnSymmetry orthorhombic monoclinic monoclinic orthorhombic orthorhombic orthorhombicParameters(Å) Z = 12, a = a0,

b = b0, c = 3c0 Z = 4, a = a0,b = b0, c = c0

Z = 12, a = a0,b = b0, c = 3c0

Z = 20, a = a0,b = b0, c = 5c0

Z = 20, a = a0,b = b0, c = 5c0

Z = 4, a = 8.946 b = 15.515, c = 12.276.

[N(CH3)4]2Zn0.5Cu0.5Cl4

T(K) 248 271.7 278 284 293.7

Space group P21nb

Symmetry orthorhombic

Parameters(Å) Z = 4, a = a0, b = b0, c = 3c0

Z = 4, a = a0,b = b0, c = 5c0

Z = 4, a0 = 8.988,b0 = 15.527, c0 = 12.269

Z = 4, a = a0,b = b0, c = c0,

[N(CH3)4]2CuCl4

T(K) 247 293 299

Space group P21/n P21/c1 IC Pmcn Symmetry monoclinic monoclinic incommensurate orthorhombicParameters a = a0, b = b0,

c = 3c0; Z = 4a = a0, b = b0, c = 3c0; Z = 12

a = a0, b = b0, c = 3c0; Z = 12

a0 = 9.039, b0 = 15.59,c0 = 12.30; Z = 4

788

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

HLEL et al.

of angles values around 90°. When temperatureincreases, angles became more close to 90°. We shouldnote that the volume expansion coefficient α =(1/V)(dV/dT) displays a large perturbation between 278and 286 K and becomes constant elsewhere. The exam-ination of temperature cell parameters dependency per-mits us to identify three structural phases: phase I aboveT = 284 K, phase II in the temperature range 284–278 Kand phase III below T = 278 K.

Room Temperature Structure Description

Relevant final atomic coordinates for all non-hydro-gen atoms, selected interatomic distances and anglesare summarized in Tables 4 and 5, respectively.

The atomic arrangement in [(CH3)4N]2Zn0.5Cu0.5Cl4consists of alternate organic-inorganic layers

50

3500 3000 2500 2000 1500 1000 500Wave number, cm–1

4000

60

70

80

90

100

Fig. 1. Infrared spectrum of [(CH3)4N]2Zn0.5Cu0.5Cl4.

9.00

9.02

9.04

9.06

9.08

9.10

15.4

15.5

15.6

b(Å)a(Å)

274ä

252ä

a

c

240 250 260 270 280 290 300

80

20

30

40

60

70

10

40

50(V/100)

c(Å)

50 30

20

60α = 1/V(dV/dT)

–4

250 260 270 280 290 300T(ä)

240

–3

–2

–1

0

1

alpha

89.8

89.9

90.0

90.1

90.2

α, β, γ(°)

betagamma

b

(‡)

(b)V/100 (Å3)90

(c)

(d)

T(ä)

Fig. 2. Temperature dependence of a and c lattice parameters of [(CH3)4N]2Zn0.5Cu0.5Cl4 (a). Temperature dependence of cell vol-ume and b lattice parameter of [(CH3)4N]2Zn0.5Cu0.5Cl4 (b). Temperature dependence of α, β, and γ angles of triclinic cells for[(CH3)4N]2Zn0.5Cu0.5Cl4 (c). Temperature dependence of the volume expansion coefficient α = (1/V)(dV/dT) of[(CH3)4N]2Zn0.5Cu0.5Cl4 (d).

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

PHASE TRANSITIONS IN THE A2BX4 COMPOUND 789

(N(1)CH3)4/Zn(Cu)Cl4) and organic sheets (N(2)CH3)4both parallel to (001) plane (Fig. 3).

The organic–inorganic layers are centered at z ≈ 1/4but the organic ones are observed at z = 0 and z = 1/2(Fig. 3). The first kind of planes are built up in zigzagchains along [110] direction, as shown in Fig. 4. Chainscan be regarded by repeating the sequence (Cu)ZnCl4and N(1)(CH3)4, where the sense of tetrahedron orienta-tion changes in the same chain alternatively, unlike thoseobserved in [(CH3)4N]2MnCl4 [22], [(CH3)4N]2ZnBr4[23], and [(CH3)4N]2MnBr4 [24], which change chain bychain. N(2)(CH3)4 tetrahedra are disposed betweenchains and in both sides of the organic-inorganic planesas to build the organic layers.

N(1)(CH3)4 and N(2)(CH3)4 present a C1 point sym-metry. For the first cation, C–N–C angles and N–C dis-tances vary from 86(3)° to 119(3)° and from 1.41(6) to1.50(4) Å, respectively. For the second cation, C–N–Cangles vary between 86(4)° and 128(3)° and N–C dis-tances vary between 1.37(3) and 1.48(3) Å. The anglesand distances show an important distortion of TMA(1)and TMA(2) cations, as observed elsewhere [25]. Wenote that each carbon atom of the second cation may besplit into two positions. The refinement of duplicatedpositions and their occupancy factors is unreasonable.

ZnCl4 and CuCl4 tetrahedra present a C1 point sym-metry with main geometrical features which are similarto those commonly observed [12, 13, 26, 27]. The Cu–Cldistances vary from 2.12(4) to 2.39(4) Å with a meanequal to 2.253(4) Å. The Cl–Cu–Cl angles are in therange 104(1)°–112(2)° with an average equal to109.38°. The Zn–Cl distances vary between 2.24(2) and2.28(4) Å with a mean equal to 2.252(4) Å. The Cl–Zn–Cl

angles are observed between 105(1)° and 112.1(5)°with an average equal to 109.25°. Taking into accountthese parameters and considering the calculated aver-age values of the Baur distortion indices [28] for CuCl4

Table 4. Final atomic coordinates for [(CH3)4N]2Zn0.5Cu0.5Cl4(Esd are given in parentheses)

Atoms x y z Ueq, Å2

Cu –0.557(5) 0.4064(10) –0.2545(11) 0.060(6)

Zn –0.537(5) 0.4071(7) –0.2534(8) 0.045(4)

Cl1 –0.5445(16) 0.4063(3) –0.4369(3) 0.1035(15)

Cl2 –0.3387(11) 0.3357(9) –0.1898(9) 0.116(4)

Cl3 –0.542(2) 0.5410(2) –0.1856(4) 0.138(2)

Cl4 –0.7507(8) 0.3415(9) –0.1992(9) 0.117(4)

N1 –0.553(4) 0.4038(7) 0.1468(9) 0.065(4)

Cl1 –0.681(3) 0.348(4) 0.111(4) 0.130(18)

Cl2 –0.579(7) 0.485(2) 0.090(4) 0.22(2)

Cl3 –0.463(6) 0.345(3) 0.088(4) 0.063(13)

Cl4 –0.515(7) 0.4064(19) 0.264(2) 0.25(3)

N2 –1.038(3) 0.1745(7) 0.0096(9) 0.069(3)

C21 –1.193(3) 0.1480(19) –0.007(3) 0.149(15)

C22 –1.047(7) 0.2568(13) 0.063(2) 0.179(12)

C23 –1.034(9) 0.110(2) 0.098(2) 0.26(2)

C24 –0.954(6) 0.177(3) –0.083(3) 0.35(5)

N2

N1

c

a

Fig. 3. [010] view of the structure of [(CH3)4N]2Zn0.5Cu0.5Cl4. Cu/Zn anions are represented by grey tetrahedrons. N(Ccations are represented by empty tetrahedrons.

Cl42–

H3 )4+

790

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

HLEL et al.

and ZnCl4 tetrahedron (ID Cu–Cl = 0.032, ID Cl–Cu–Cl =0.024(2), ID Zn–Cl = 0.006, and ID Cl–Zn–Cl =0.020), we deduce that, due to the Jahn–Teller effect,the CuCl4 tetrahedron is more distorted than the ZnCl4one. For CuCl4 and ZnCl4 tetrahedra, Cl–Cl distancesvary between 3.60(1) and 3.699(3) Å with ID equal to0.01. The Cl–Cl distances are almost twice r(Cl–).

DISCUSSION

Both isomorphic compounds (TMA)2ZnCl4 (Z) and(TMA)2CuCl4 (C) and their derivative compounds[(CH3)4N]2Zn0.5Cu0.5Cl4 (ZC) crystallize at room tem-perature in the orthorhombic system with space groupsPmcn and P21cn, respectively. At first, we can deducethat metal substitution doesn’t allow an isomorphicstructure. The cell volume of the substituted com-pounds is almost the average of (TMA)2ZnCl4 and(TMA)2CuCl4 ones, which are 1702.76(3), 1720.46(4),and 1712.3(7) Å3, respectively. For all structures, MCl4tetrahedra are analogous to those commonly observed.All structures are of layered organization, built up byorganic–inorganic planes alternating with organicsheets.

The organic–inorganic layer of those structures isbuilt up zigzagging, formed by alternating TMA andMCl4 tetrahedra so as to build infinite chains. In twoadjacent ones, the metal atom of three MCl4 tetrahedrabuild a triangle with M at the corner, and another trian-gle between three TMA tetrahedra with nitrogen at thecorner. Organic and inorganic triangles are shifted, as inFig. 5.

In the (Z) arrangement, the inorganic triangle isisosceles with Zn–Zn distances equal to 8.980(3) Å and8.998(1) Å at the bases. This triangle is pseudo-equilat-eral. The organic triangle is equilateral with N1–N1distance equal to 7.803(1) Å. Those triangles areshifted, with distances between adjacent corners,Zn−N1, equal to 5.514(6) and 4.984(3) Å. The shortestdistance, Zn–Zn, between successive inorganic–organic layers is equal to 7.838(1) Å. In the (C) struc-ture the inorganic triangle made up of CuCl4 is equilat-eral, with Cu–Cu distances equal to 8.998(1) A, but theorganic one is isosceles with N1–N1 distances equal to9.446(2) A and 8.998(2) A in bases. The distancesbetween the adjacent corners of organic and inorganictriangles (Cu–N1) are 4.976(3) and 5.654(2) Å. Theshortest distance, Cu–Cu, between successive inor-ganic–organic layers is equal to 7.787(6) Å. One notesthat in organic triangle, nitrogen atoms are more spacedin the (C) than those observed in the (Z) arrangement.

In the (ZC) structure, there are two types of inor-ganic triangles. The first, made up of ZnCl4, is equilat-eral with Zn–Zn distance equal to 8.971(3) Å, the sec-ond, made up of CuCl4, is isosceles with Cu–Cu dis-tance equal to 8.971(2) and 8.988(3) Å in bases. Thesecond triangle is slightly rotated from the first one by

Table 5. Main inter-atomic distances (Å) and bond angles(degree) in [(CH3)4N]2Zn0.5Cu0.5Cl4 (Esd are given in paren-theses)

Distances (Å) Angles (deg.)

CuCl4

Cu–C11 2.24(1) C11-Cu-C12 107(2)

Cu–C12 2.39(4) C11-Cu-C13 111.8(6)

Cu–C13 2.26(2) C11-Cu-C14 111(1)

Cu–C14 2.12(4) C12-Cu-C13 104(1)

C12-Cu-C14 110.5(6)

C13-Cu-C14 112(2)

ZnCl4

Zn–C11 2.25(1) C11-Zn-C12 111(1)

Zn–C12 2.24(4) C11-Zn-C13 112.1(5)

Zn–C13 2.24(2) C11-Zn-C14 105(1)

Zn–C14 2.28(4) C12-Zn-C13 110(1)

C12-Zn-C14 110.4(5)

C13-Zn-C14 107(1)

N(1)(CH3)4

N1–C11 1.50(4) C11-N1-C12 103(4)

N1–C12 1.45(3) C11-N1-C13 86(3)

N1–C13 1.41(6) C11-N1-C14 119(3)

N1–C14 1.48(3) C12-N1-C13 114(3)

C12-N1-C14 119(3)

C13-N1-C14 112(3)

N(2)(CH3)4

N2–C21 1.46(3) C21-N2-C22 105(3)

N2–C22 1.44(2) C21-N2-C23 86(4)

N2–C23 1.48(3) C21-N2-C24 115(3)

N2–C24 1.37(3) C22-N2-C23 106(2)

C22-N2-C24 112(3)

C23-N2-C24 128(4)

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

PHASE TRANSITIONS IN THE A2BX4 COMPOUND 791

Cu–Cu–Zn angle equal to 1.5°. The organic triangle isisosceles with N1–N1 distances equal to 9.308(3) and8.988(2) Å in bases. The shortest distance, Cu–Cu andZn–Zn, between successive inorganic–organic layersare 7.838(4) and 7.843(2) Å, respectively. The exami-nation of the crystallographic characteristics oforganic–inorganic layers of those structures shows thatthe (ZC) structure can be described by superposition ofthose observed in the (Z) and (C) ones. Moreover, theorganic triangle keeps the same N1–N1 distancesobserved in (C) structure, but the presence of ZnCl4groups causes distortion of those triangles.

a

N2

N1

CuZn

b

Fig. 4. [010] view of the structure of

[(CH3)4N]2Zn0.5Cu0.5Cl4. Cu/Zn anions are repre-

sented by grey tetrahedrons. N(C cations are repre-sented by empty tetrahedrons. Dashed boundary shows thedevelopment of zigzag chain in organic–inorganic layeralong the [110] direction.

Cl42–

H3 )4+

N1

CuZn

Fig. 5. Organic and inorganic triangles representation inorganic–inorganic layers of (Z), (C) and (ZC) atomicarrangements.

Cu

C3C6

N1

N2

C3'C6'

Cu/ZnN1

N2

C3

C6

C3'

C6'

Zn

(a)

(b)

(c)

Fig. 6. From [(CH3)4N]2CuCl4 to [(CH3)4N]2Zn0.5Cu0.5Cl4structure, TMA groups rotate according to the indicatedarrow (a). From [(CH3)4N]2Zn0.5Cu0.5Cl4 to[(CH3)4N]2ZnCl4 structure, TMA groups rotate according toindicated arrow (b). Disposition of TMA groups in[(CH3)4N]2ZnCl4 structure (c).

792

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

HLEL et al.

Like the disposition of N(1)(CH3)4 groups inorganic–inorganic layers, in the organic sheetsN(2)(CH3)4 groups are observed at the corners of isos-celes triangles so as to build infinite planes. In the (Z)structure arrangement; the N2–N2 distances are equalto 8.998(3) and 11.077(1) Å in bases. In the (C) and(ZC) atomic organizations the N2–N2 distances areequal to 6.966(6) and 9.03(1) Å in bases. We remarkthat the organic layers of the (ZC) structure are similarto the (C) arrangement and are not influenced by ZnCl4groups in N(1)(CH3)4 dispositions.

In organic–inorganic layers, passing from the (C) to(ZC) structure, the N(CH3)4 tetrahedron rotates aroundaxes perpendicular to sheets containing nitrogen atomand rotates in the opposite way from the (ZC) to (Z)structure (Fig. 6a, 6b, and 6c). In organic layers, pass-ing from the (C) to (ZC) structure, the N(CH3)4 tetrahe-dron makes rotation around axes perpendicular tosheets and containing carbon atom (C6) and rotates inthe opposite way around axes parallel to the stackingdirection and containing (C6') from the (ZC) to (Z)structure. The crystallographic characteristics of thosestructures previously reported explain clearly the dyna-misms of N(CH3)4 and MCl4 tetrahedron. Moreover,the tetrahedra dynamisms are frequently observed inthis family of compounds on phase transitions [29–32].

CONCLUSIONS

Non-centrosymmetric crystals of (TMA)2Zn0.5Cu0.5Cl4have been prepared by slow evaporation, at room tem-perature, of aqueous solution obtained by a mixture ofN(CH3)4Cl, CuCl2, and ZnCl2 in the molar ratio 4 : 1 : 1.Thermal study and cell parameters determination ver-sus temperature show three structural transitions. Thestructure is characterized by infinite layers of organicanions (TMA)+ alternating with organic–inorganic

sheets made up of TMA+ and Zn both approxi-mately parallel to (001) planes. Compared to the (Z)and (C) structures, the (ZC) structure permits deduc-tion of the effect of metal substitution. The examinationof crystallographic characteristics of the organic–inor-ganic layers of those structures shows that the (ZC)structure can be described by a superposition of thoseobserved in the (Z) and (C) ones. Moreover, the organictriangle keeps the same N1–N1 distances observed inthe (C) structure, but the presence of ZnCl4 groupscauses a distortion of these triangles. The examinationof N2–N2 distances shows a similarity of organic layersof (ZC) and (C) structure which are different from (Z)ones. The metal partial substitution is accompanied bythe rotation of TMA cation. This dynamism is usuallyobserved in phase transitions progress in this family ofcompounds.

Cl42–,

ACKNOWLEDGMENTS

The authors are grateful to Professor K. Boubakeurfor the use of Kappa CCD and for his invaluable help inthe X-ray study of cell parameters temperature depen-dency at the CIM2 laboratory of Pierre et Marie CurieUniversity (Paris). The authors are also grateful toDr. T. Gurfel for the use of the Nonius CAD4 and forhis invaluable help in the data collection at the Chemis-try Solid State Laboratory at the Faculty of Sciences ofMonastir (Tunisia). The authors wish to thankMr. H. Bejaoui for his proofreading help. F. Hlelgreatly acknowledges the “Institut Francais de Cooper-ation” for the funding of his stay at the UPMC.

REFERENCES1. J. R. Wiesner, R. C. Srivastava, C. H. L. Kennard, et al.,

Acta Crystallogr. 23, 595 (1967).2. K. Gesi, Ferroelectrics 66, 269 (1986).3. H. Mashiyama, M. FuKushima, and H. Shigematsu, J.

Korean Phys. Soc. 29, 478 (1996).4. Z. Tylczynski and P. Biskupski, J. Korean Phys. Soc 32,

235 (1998).5. AeR. Lim, K. W. Hyung, K. S. Hong, and S. Jeong, Phys.

Stat. Solidi (b) 219, 389 (2000).6. N. Koshiji and H. Mashiyama, J. Phys. Soc. Jpn. 69,

3853 (2000).7. I. Ruiz-Larrea, A. Lopez-Echarri, and M. J. Tello, J.

Phys. C 14, 3171 (1981).8. J. Hencoque, J. L. Sauvajaul, J. Lefebere, and G. Marion,

J. Raman Spectrosc. 14, 93 (1983).9. J. Petzelt, G. V. Kozlov, A. A. Volkov, and Y. Ishibashi, Z.

Phys. B: Condens. Matter 33, 369 (1979).10. B. Martin, J. M. Pastar, F. Rull, and J. A. De Saja, Solid

State Commun. 44, 1047 (1982).11. S. Sawada, Y. Shiroishi, A. Yamamoto, et al., J. Phys.

Soc. Jpn. 44, 687 (1978).12. B. R. Clay, J. Murray-Rust, and P. Murray-Rust, Acta

Crystallogr., Sect. B 31, 289 (1975).13. B. J. R. Wiesner, R. C. Srivastava, C. H. L. Kennard, et al.,

Acta Crystallogr. 23, 565 (1967).14. A. Altomare, G. Cascarano, C. Giacovazzo, and A.

Gualardi, J. Appl. Crystallogr. 26, 343 (1993).15. G. M. Sheldrick, SHELXL93. A Program for the Refine-

ment of Crystal Structures from Diffraction Data (Univ.of Göttingen, Göttingen, 1993).

16. H. D. Flack, Acta Crystallogr., Sect. A: 39, 876 (1983).17. G. Bernardinelli and H. D. Flack, Acta Crystallogr., Sect.

A: 41, 500 (1985).18. K. Nakamoto, Infrared and Raman Spectra of Inorganic

and Coordination Compounds. 5th Ed (Wiley, NewYork, 1992), p. 88.

19. H. G. Heddrich and C. E. Blom, J. Chem. Phys. 90, 4660(1989).

20. H. G. Heddrich and C. E. Blom, J. Mol. Spectrosc. 140,103 (1990).

21. A. D. Kirkwood, K. D. Bier, J. K. Thompson, et al., J.Phys. Chem. 95, 2644 (1991).

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 5 2008

PHASE TRANSITIONS IN THE A2BX4 COMPOUND 793

22. G. Madariaga, F. J. Zuniga, J. M. Perez-Mato, andM. J. Tello, Acta Crystallogr., Sect. B: 43, 356 (1987).

23. K. Gesi and M. Iizumi, J. Phys. Soc. Jpn. 48, 1775(1980).

24. H. Mashiyama and N. Koshui, Acta Crystallogr., Sect. B:45, 467 (1989).

25. T. Asahi, K. Hasebe, and K. Gesi, J. Phys. Soc. Jpn. 57,4219 (1988).

26. K. Hasebe, T. Asahi, and K. Gesi, Acta Crystallogr., Sect.C: Cryst. Struct. Commun. 46, 759 (1990).

27. P. Trouelan, J. Lefebvre, and P. Derollez, Acta Crystal-logr., Sect. C: Cryst. Struct. Commun. 40, 386 (1984).

28. D. R. Bloomquit and R. D. Willett, Acta Crystallogr.,Sect. B: 37, 1353 (1981).

29. Y. Nishijima and H. Mashiyama, J. Phys. Soc. Jpn. 69,3581 (2000).

30. W. Baur, Acta Crystallogr., Sect. B: 30, 1195 (1974).31. M. Amami S. van Smaalen, L. Palatinus, et al., Z. Kri-

stallogr. 217, 532 (2002).32. T. Asahi, K. Hasebe, and K. Gesi, J. Phys. Soc. Jpn. 61,

1590 (1988).33. W. Zapzrt, J. Solecki, M. B. Zapart, et al., Phys. Soc. 32,

697 (1998).34. H. Mashiyama and N. Koshiji, Acta Crystallogr., Sect.

B: 45, 467 (1989).