Structure and stability of solid and molten complexes in the MgCl2-AlCl3 system

Polyhedron Vol. 6, No. 5. pp. 975-986, Printed in Great Britain

0277-5387/87 Pergamon

1987

s3.00+ .oo Journals Ltd

STRUCTURE AND STABILITY OF SOLID AND MOLTEN COMPLEXES IN THE MgCl,-AlCl, SYSTEM M.-A. EINARSRUD,

H. JUSTNES,

E. RYTTER*

and H. A. 0YE

Institute of Inorganic Chemistry, The Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim-NTH, Norway (Received 10 April 1986 ; accepted 20 September 1986) Abstract-A

visual determination of the phase diagram of the MgC12-AlC13 system reveals the formation of two intermediate compounds. One of these compounds is MgAl,Q crystallizing with a monoclinic unit cell, space group 12/c, with dimensions a = 12.873(l) A, b = 7.8959(7) A, c = 11.617(l) A, B = 92.348(8)” and z = 4. The two crystal modifications of anhydrous magnesium chloride and the lattice parameters of aluminium chloride have been reexamined. Lattice energies for a- and /3-MgC12 have been measured, respectively, as 661(l) and 646(4) kcal mol- ‘. The liquidus curve on the acidic side has been used to estimate the activity of aluminium chloride. Fourier-transform IR spectra of melts with compositions ranging from 0 to 30 mol% MgCl, have been interpreted in terms of A1&16, strongly perturbed Al,Cl; and AlCl; entities. In particular, the tetrachloroaluminate ion acts as both a tri- and a bidentate ligand towards Mg 2+. Neutral species, e.g. Mg(A1C1J2 and Mg(A12C1,)(AlCl,)C1, dominate in the melt. Ab initio molecular-orbital calculations have been performed to obtain a better understanding of the Mg2+ * * . AlCl; interaction.

The MgCl*-AlCl, system is of considerable fundamental and industrial importance. Anhydrous MgCl, is used as a support material in third-generation Ziegler-Natta catalysts for production of stereoregular polypropylene. More recent MgC12 supports have been modified with AlCl,, ‘s2whereas the chloroaluminate species in the molten and gaseous states are related to new aluminium processes. 3 As part of a more general spectroscopic investigation of the complex formation of AlC13 in molten chlorides,“6 the present system represents an extension to a highly polarizing counterion. A previous investigation of the phase diagram of MgC12-AlC13 (O-30 mol% MgCl,) by Kendall et aL7 indicated the formation of a 1 : 2 compound. However, the narrow range and limited number of data prompted the present redetermination of the liquidus curve. Belt and Scott8 confirmed the formation of magnesium chloroaluminate (MgAl,Cl,) by X-ray powder diffraction and pointed out a possible isomerism with COA~~C~~.~ Anhydrous MgC12 has been known to exhibit two different crystal modifications, a high-temperature

*Author to whom correspondence should be addressed.

form” (a-MgC12) made from thermal decomposition of hydrates and a low-temperature modification’ ’ (/?-MgCl,) produced by chemical dehydration. An extensively milled MgCl,, as employed for the support material of third-generation Ziegler-Natta catalysts, may be looked upon as a mixture of the two forms. Action therefore was taken to determine any difference in the closest coordination sphere of magnesium and to measure accurate lattice parameters. EXPERIMENTAL

The high-temperature cc-form of MgClz was prepared by decomposition of magnesium chloride hexahydrate (99%, E. Merck AG, F.R.G.) in a flow of HCl gas (25600°C) followed by 3 times vacuum distillation (1OOO‘C) in quartz cells. b-MgC12 was produced by chemical dehydration of MgC12 * 6H2O in refluxing thionyl chloride (75°C) for 3 days. Excess SOC12 was distilled off, and the product dried in vacua AlC13 (99%, Fluka AG, Switzerland) was purified by 3 times distillation in sealed quartz tubes. Needle-shaped crystals of MgA12ClB were synthesized by fractional crystallization. A mixture of

975

976

M.-A. EINARSRUD

2.5 mol% a-MgCl, in AlCl, was kept at 190°C for 12 h and then melted at 250°C in a tilting furnace to ensure a homogeneous mixture. The quartz tube with the melt was quenched in ice-water. The mixture was again melted in a zone refinement furnace, and MgAl& was solidified as the cell was lowered at a rate of 0.6 mm h- ’ through the furnace. Alternatively, single crystals were made from a 1: 3 mixture of MgCl, and A1C13 by gas-phase transportation from 230 to 170°C. Crystallization times were 12-80 h. Attempts to prepare the compound MgAICIS by these methods failed. The liquidus in the MgC12-AlCl, phase diagram was determined by visual inspection. The melting points in the acidic region of the diagram were observed while the quartz tubes with the mixtures were shaken in a nitrate bath. The temperature in the bath was increased by 0.2-0.5”C and allowed to stabilize for 5-6 h when approaching the melting point. On the basic side of the diagram, however, the melting points were observed visually in a shaking quartz furnace, giving a considerably larger uncertainty, particularly in the 50-70 mol% A1C13 region. Neutron powder diffractograms were obtained by the OPUS III diffractometer accommodated at a radial channel of the JEEP II reactor (2 MW, central thermal flux 3 x 10’ ‘) at the Institute of Energy Technology, Kjeller. The monochromatic beam is produced by reflection from the (111) planes of a squashed Ge crystal, and has a wavelength of 1.877( 1) A. The instrument utilizes a multidetector system consisting of five 3He detectors spaced 10” apart, and each of these covers a range of 80” for the diffraction angle (28). The detectors are cylindrical with diameter 5 cm and an active length of 20 cm. They are filled with 3He gas to a pressure of 4 atm. Each detector has a Soller collimator of 10’ opening angle with plates made of gadolinium oxide coated Mylar foils. The diffractometer is operated through a Mycron microcomputer which allows programming of both the temperature variation of the sample and the scanning procedure. A step length of 0.05” was used, and a range of 5-80” for the diffraction angle was recorded. The sample holder was an aluminium cylinder with inner diameter 12 mm, wall thickness 1 mm and length 40 mm. The lid was sealed with Araldite glue. IR transmission spectra of the solid samples were recorded in the frequency range 300-800 cm- ’ with a Perkin-Elmer 580B ratio recording IR spectrophotometer. The samples were Nujol suspensions between CsI windows. The resolution of the spectra is about 5 cm-‘. Far-IR spectra to 50 cm- ’ were recorded with a Bruker IFS 113~ Fourier-transform IR spectrometer. The radiation

et al.

source was an Hg lamp, the beam splitter a 6.0 pm Mylar film, and the detector a He-cooled Ge bolometer. For the low-frequency spectra, the samples were in a Nujol suspension between polyethylene windows. The spectra were recorded with 96 scans, mirror velocity 0.235 cm s- ‘, resolution 1 cn- I, and a triangular apodization function. The specular reflection spectra of the molten mixtures were obtained with the Bruker IFS 113~ Fourier-transform instrument. The spectra were recorded of thin films (cu 1 pm) pressed against a diamond window (type IIA, D. Drukker & Zn., Amsterdam, Netherlands) by a gold-plated nickel piston. Details of the cell and procedure have been described previously. 6*1 ’ Thick samples (2-3 mm) were used as reference to avoid false band splitting. 13*14 The averaged spectra consisted of 500 single scans obtained with a DTGS detector at a resolution of 8 cn- ‘. A 3.5~pm Mylar beam splitter was used in the frequency range 150-700 cm-‘, whereas a 12-pm Mylar was employed between 90 and 210 cm-‘.

RESULTS The phase diagram of MgCl*-A1C13 is drawn in Fig. 1 together with detail of the acidic region (&,,I, > 0.67). The diagram indicates the formation of two intermediate compounds. The presence of MgAlzCls was verified by X-ray and neutron powder diffraction, chemical analysis, morphology, IR and Raman spectroscopy and X-ray investigations

4

100

9

0

’ ’ ’ ’ ’ ’ ’ ’ ’ 10 20 30 LO 50 60 70 80 90 100 Mot% AU,

Fig. 1. The MgCl,-AIC13 phase diagram.

Structure and stability of solid and molten complexes

971

of single crystals. Attempts to prepare MgAlCl, failed. Neither could the possible existence of both a high- and a low-temperature modification of MgAl& be verified. The melting point of A1C13 under its own equilibrium pressure was determined to be 193.8”C. The refinement of the crystal structures is based on a Rietveld analysis of the individual neutron powder diffraction patterns shown in Fig. 2. The calculations were performed by a computer program developed by Wiles and Young. ’ ’ In all analyses, the 2” ranges 46-48 and 54-56” were excluded to avoid contribution from the sample holder. Listings of diffraction angles and calculated and observed integral intensities of the reflections can be obtained from the authors. The complete profiles also are available. 20 Solid MgC12

111111

30 I,

50

60

ANGLE, 2 EI(deg)

I

Fig. 2. Neutron powder diffraction patterns of the two crystal modifications of MgCl, and MgAlzCls.

The intensity profile of /3--MgC12in Fig. 2 shows that this compound is contaminated by a small amount of the a-phase. The refinement was performed by using separate scale factors for the two modifications and the already obtained structural parameters for a-MgCl,. The two scale factors refined to C(B) = 0.114(3) and C(a) = 0.0040(2). Because of the structural similarities, it was possible to calculate the amount of a-MgCl, as 9.5 mol%. The refinement of /?--MgCl, is based on 17 independent reflections in the 28 range l&80”, and the R-factors refined to RB = 0.061 and RpW= 0.337. The unit cell parameters have been included in Table 1. In the structure of jI-MgC12, the magnesium atom is placed at the origin, while the chlorine atom is in the special position (l/3, 2/3, z). The final atomic position z = 0.235(3) was obtained.

cc-MgCl,

a (4 c CA) V(A3) $& (gcn-3) Space group “Recalculated,

80

I

Table 1. Unit cell parameters for the two modifications of MgCl,

Parameter

70

2 3 LATTICE SPACING. d(A)

7654

Fourteen reflections with diffraction angles between 16 and 77” were the basis for the refinement of a-MgClz. The final R-factors were RB = 0.038 (the error of fit for the integral intensities of all the considered reflections) and RpW= 0.121 (the error of fit for each measured point in the intensity profile weighted by the inverse square of their standard deviations). The obtained unit-cell parameters are given in Table 1. In this structure, the magnesium atom is placed at the origin, while the chlorine atom is in the special position (0.0.~). The final atomic position and isotropic temperature factors refined to z = 0.2551(2), B(Mg) = 1.9(2) and R(C1) = 1.4(l) A’. These values correspond to the scale factor C = 0.11 S(2), half-width parameters u = 3.2(l), u = - 1.8(l) and w = 0.34(2), preferred orientation parameter G = 0.27(l) and an asymmetry parameter applied for the first reflection A = 2.0(2). The preferred-orientation parameter is necessary in the structure determination of this compound because of the pronounced flake shape of the crystallites.

03

DIFFRACTION

B-MgCl,

Neutron

X-ray ’ 6

Neutron

X-ray ’ ’

36435(l) 17.689(2) 203.4 2.332 3 R3m

3.640(4) 17.673(H) 202.8” 2.339 3 R3m

3.6376(5) 5.897(2) 67.58 2.339 1 P3m

3.641(3) 5.927(6) 68.05 2.323 1 P3m

different from the value given in Ref. 16.

978

M.-A. EINARSRUD

The isotropic temperature factors were transferred from a-MgClz as attempts to refine them gave preposterous values because of the diffuse reflections in the diffractogram due to the small crystallites in the sample. The positional parameter and the temperature factors match the half-width parameters u = 14(2), v = - 16(2) and w = 5.3(4), and the preferred-orientation parameter G = 0.31(2). No significant improvement or change in structural parameters and RpWwere obtained by using a Lorentzian profile function instead of the standard Gaussian one, although the half-width parameters were reduced to ca 4. The lattice energies for CI- and b-MgC12 were measured to 661(l) and 646(4) kcal mol- ‘, respectively. The low-temperature /I modification was found by differential scanning calorimetry to be metastable with a transition temperature of ca 520°C at a heating rate of 20°C min- ‘. The IR spectra of the two modifications of anhydrous magnesium chloride (Fig. 3) are virtually identical. The frequencies, relative intensities and assignments in terms of the D3,, factor group are 407 m (v,+v~, E,), 372 m (v2+v3, E,), 329 sh (v,+vz-~3, &), 276 sh (v1+vq-v3, A,,+A&,), 248 vs (vz, A,,,), 185 m (vq, E,), 140 w (vz+v3-v,, EJ and 126 w (v2-v3, EJ. It cannot be excluded, however, that some of the shoulders and weak bands appear due to traces of oxychlorides. Solid MgAlzCls and MgClr-A1C13 melts The suggested structure of MgAlzCls is based on c-axis oscillation and zero-level Weissenberg X-ray

0

200

400

600

800

WAVENUMBER(cm-'? Fig. 3. IR spectra of the two crystal modifications of MgClz and MgAl,Cl,.

et al.

photographs of a single crystal. This crystal was not suitable for a complete single-crystal analysis because of extended mosaic structure. The unitcell axes obtained from the X-ray photographs are comparable with the values for CoAlzCls obtained by Ibers.g Furthermore, the systematic extinctions (ho0 and OM) for h, k = odd and hM) for h-k = odd) can be explained by the space group for the cobalt compound, and the symmetry elements of the space group are reflected in the observed Laue symmetry. MgAl#& thus was assumed to be isomorphous with cobalt(I1) tetrachloroaluminate (CoAl,Cl,), and the trial model was given atomic coordinates from this compound. The analysis of the neutron diffraction pattern for MgAlzCls (Fig. 2) was complicated by a rather large contribution from AlC13. The combined analysis of the total diffraction pattern for the two phases was initially based on the structural data for AlC13 taken from Ketelaar et al. I7 Since, however, their unit-cell dimensions gave calculated peaks displaced from the observed ones, it was necessary to refine the axes of A1C13as well. In addition, the preferred-orientation parameter of AlCl, had to be varied independently. The refinement took into account 194 reflections for MgAlzCls and 73 reflections for A1C13in the 28 range 15-80”. The final Rfactors were Rs = 0.018 for MgAlzCls, Rs = 0.011 for A1C13and RpW = 0.163. The parameters for the monoclinic unit cell of MgAl,C& with space group Z2/c and four formula units are given in Table 2 together with results obtained for several isomorphous compounds. ‘, Is3lg Monoclinic unit-cell dimensions of AlC13 with space group n/m and four formula units are a = 5.933(2) A, b = 10.255(3) A, c = 6.180(4) A and /I = 108.24(3)“. The derived atomic positions for MgAl#& are given in Table 3. The y-coordinates of magnesium and aluminium were fixed in their ideal positions (0.00 and 0.25, respectively) during the refinement. Otherwise they would arrive at values (0.04 and 0.22) leading to unlikely bonding distances without a significant better fit (Rp”= 0.159). The applied set of atomic coordinates corresponds to scale factors C(MgAl,ClJ = 0.058(l) and C(A1C13)= 0.073(4), common half-width parameters u = 3.3(2), u = - 1.8(2) and w = 0.36(4), common overall isotropic temperature factor B = 4.0(l) A’ and preferred-orientation parameters G(MgAl,Cl,) = 0.19(l) and G(AlC13) = 0.34(3). Because of the ill-conditioned y-coordinates of magnesium and aluminium compared with CoA12Cls [y(Co) = 0.0017(12) and y(A1) = 0.2516(18)], another model, with the chain in the middle of the ab plane of the unit cell (Fig. 6)

Structure and stability of solid and molten complexes

979

Table 2. Unit cell parameters for isomorphous MAl,Cl, compounds (M = Mg, Co, Ni or Ti) Mg

Co”

Nib

12.873(l) 7.8959(7) 11.617(l) 92.348(8) 1179.8 2.037

12.81(2) 7.75(l) 1lSO(2) 92.2(2) 1141 2.308

12.72(1) 7.672(7) 11.47(2) 92.2(1) 1119 2.353

Parameter a (A) b (A) c (A) B (“) v (A’) pcalc.(g cm- ‘)

Ti 13.000(3) 7.71l(2) 11.772(3) 92.2d 1179(l) 2.171

LIReference 9. bReference 18. ’ Reference 19. dNot reported, but calculated from the given volume. translated one-half of the repetition length along the c-axis, was tried out. It turns out that only the y-coordinate of aluminium is altered. The new model has the space group C2/c, which explains equally well the observed systematic extinctions and Laue symmetry. However, the fit for the unit-cell axes was less satisfactory and the R-factors, standard deviations and overall temperature factor increased. Hence, the first model was assumed to be correct, although some mixing of the two or lack of stoichiometry is possible. The IR spectra of solid MgAlzCls and MgClz are compared in Fig. 3. The IR reflectances of the different molten compositions are shown in Fig. 4, whereas the wavenumbers of melts and solid MgAl& are listed in Table 4.

c-axis (and volume) found by Bassi ef al. ” seems too large. In this context it is pertinent to point out that a comparison between the Debye-Scherrer results of these authors with the lattice spacings calculated from the present unit cell axes, gives as good a fit as for the unit cell parameters from the

DISCUSSION Solid MgCl 2

The unit-cell dimensions for ol-MgClz obtained by neutron diffraction in this work compare excellently with the recent reinvestigation by X-ray diffraction (Table 1). This agreement gives us confidence in the somewhat less well defined wavelength of the neutrons compared to X-rays. For /?MgC12, the discrepancy is larger. In particular, the Table 3. Atomic coordinates for MgAlzCls Atom Mg

Al Cl(l) Cl(2) Cl(3) Cl(4)

x

Y

0 0.0874(20) 0.1597(9) 0.1579(8) 0.0956(7) -0.0625(g)

O.OOOO(-) 0.2500(-) 0.4818(29) 0.0187(27) 0.2369(27) 0.2263(22)

Z

l/4 0.5494(24) 0.6127(10) 0.6317(11) 0.3672(g) 0.6145(g)

0

200

400

600

WAVENUMBER (CM-')

Fig. 4. IR reflectance spectra of molten mixtures in the MgCl,-AlC13 system at 260-200°C. (A) AlCl; (C,,), (0) AlCl; (C,,), and (Cl) AW;.

980

M.-A. EINARSRUD

er al.

Table 4. Observed IR frequencies (cm- ‘) for solid MgAl&

Solid MgALCls

70% AlCl 3 260°C

75% AlCl 3 250°C

80% AlC13 250°C

85% AlCl, 250°C

90% AICl, 200°C

617 sh 599 sh

617 sh 595 sh

615 vs 596 sh

573 vs 497 sll 470 s

571 vs 497 sh 475 vs

453 s 410 sh 370 sh 341 m 316m 291 m

447s 407 sh 374 sh 340 m 320 m 280 m

AlC13 200°C

vs sh sh vs vs

347 m 318 w 271 s 245 s 211 w 176~ 165 w 157s 140m 134 sh 122vw 108 w 103 vw 94 w

Interpretation

of melt spectra*

614 vs 579 vw

552 502 476 468 457

and molten magnesium chloroaluminates”

573 vs 498 sh

575 vs 500sh

500 sh 472 vs

473 vs

Al&l, 64 “wx (VII) AWMv,z+v,,) M&W&). . . , Al% (~3; Czu,Cd &Cl; (v, A Al’X (~3; Cd Al#& (VIA

465 s 452 s 404sh 341 m

409 sh 374 sh 340m

288 m

294m

172~

442 sh 410 s 376 sh 339 m 315 m

410 s 383 sh 316 m

Mg(AI,Cl,). . . , AlCl; (vj ; C,,) AlzCls (v,g), AlCl; (~3; C,,) Al& (v,+v,), A&l; (~5) Al&l; (v,), AlCAlCl; (v,) ALCl&17) Mg2+ . . . AlCl;, Cl-, Al&l,

265 sh

251 vw

A12C45

(V~+VIO)

172~

174w

173 w

170w

178~

A12Cls

Cd,

146 w

148 w

147 sh

149w

145w

-412%

(v,dr

A12Cl;

A12Cl;

b),

AW

(v.,)

6~121

“vs = very strong, s = strong, m = medium, w = weak, vw = very weak, sh = shoulder. *The numbering of fundamentals refers to the idealized Tr and D,bsymmetries of AlClh and Al,Cl;, tively (see Ref. 6). For Al*&, see Ref. 25. Ionic entities are parts of neutral species.

The layer structures (CdC12 and CdIz) of the two modifications of anhydrous MgClz both contain a two-dimensional net as a common structural unit. This unit, a Cl-Mg-Cl sandwich, has D,bsymmetry around magnesium. Distances and angles for aMgClz are Mg-Cl = 2.518(2), Cl-Cl = 3.643(l) within a closed-packed layer, Cl-Cl = 3.477(2) between layers in a sandwich, Cl-Cl = 3.771(3) A between sandwiches and Cl-Mg-Cl = 87.31(7)0 in a sandwich. The values for the /&compound are within 2a(a) with the exception Cl-Cl(within layer) = 3.638(l) A. Thus, the difference between the two modifications is mainly due to the stacking of the Cl-Mg-Cl sandwiches along the c-axes ; aMgC12 consists of a cubic close packing of anions, while /I-MgCl* is based on a hexagonal close packing. Both modifications have the magnesium atoms placed in all octahedral holes between every second chlorine layer. The CdClz stacking arrangement found in the high-temperature a-modification gives a more ionic character because of closer secondary Mg2+-Cl-

respec-

interactions. However, the ionic part of the lattice energy has been calculated by Hoppe” to be 589.7 (a-MgC12) and 589.8 kcal mol-’ (B-MgC12). These values correspond to Madelung constants of 4.486 and 4.481, respectively. In order to determine the influence of the packing sequence on the Madelung constant, a calculation was made on fi-MgC12 where the shape of the sandwiches, and the distance between them, were as determined for a-MgCl,. The resulting Madelung constant is 4.472, which means that the cubic close packing has about 0.3% higher ionic character than the hexagonal arrangement. Even though the standard deviations of the structural parameters for /I-MgC12 are rather large, it may look as if the structure of this modification is adjusted to achieve a higher ionic lattice energy. The considerably larger difference21 in the Madelung constants for CdC12 (4.489) and CdIz (4.382) using idealized structures is probably due to a different z-parameter. The interpretation of the IR spectra in Fig. 2 of MgC12 can be understood from the correlation

Structure and stability of solid and molten complexes

981

Table 5. Correlation diagram for a- and fl-MgClT Factor Acoustic group mode

Basis

Site

Sandwich

Mg

&i

&i

&d

T,

A zu

A __‘a

T. 2;

E

A __kzE _a -E

Optical mode

_a

23,

2T;

A,

2T;,

-RE -

Azu-

AZ,,

2 -

-2 E

T,

T; (248)

Txy

T;$ (185)

“Selected representations. IR activity is indicated by underlined symmetry species and Raman activity by underdashed species.

diagram in Table 5. The same diagram is applicable for a- and fl-MgC12 as both have site symmetry DJd for magnesium, site symmetry Caofor chlorine, Dsdsymmetry for the structural unit (sandwich) and factor group D3+ There is only one formula unit in the Bravais cell, and thus a total of six optical and

three acoustic modes are expected. An examination of the correlation diagram shows that two IR-active and two Raman-active vibrations are predicted. The observed far-IR frequencies for CI-and /I-MgC12 are equal within the uncertainty limits, as expected from the structure determinations. The assignments in terms of IR-active fundamentals, v,(A,,) = 248 and v@,,) = 185 cm-‘, and combination bands Raman-active fundamentals lead to the v ,(A ,& = 223 and v&E& = 125 cm- i. These values may be compared with the results of Anderson et al ” 243 (A lg), 225 (A,“) and 176 cm-’ (EJ. Note thl apparent interchange of the A Igand AZuvalues. The Raman-active EB mode was not observed for MgCl*, but other isomorphous chlorides were reported to have the Eg frequency in the range 130170 crnl.** Solid MgA12Cls

The structure of MgA12Cls can be described within a hexagonal close packing of the chlorine atoms. Magnesium and ahuninium atoms are placed, respectively, in one-eighth of the octahedral and tetrahedral holes between alternating layers. The atoms are arranged in such a way that infinite chains of composition (MgAI,Cl& are formed with propagation direction along the c-axis of the unit cell. The site symmetry of the magnesium is C2, which is the symmetry of the infinite chain as well. The two-fold rotation axis is perpendicular to the chain, and intersects it through the magnesium atom. A drawing of the chain is shown in Fig. 5

Fig. 5. The repeating part, C, of the in&& chain in MgAl,Cl,. The right drawing is turned 90” compared to the left and lacks the three upper and the three lower chlorine atoms for clarity.

and interatomic distances are given in Table 6. The bonding distances comply well with the corresponding values for CoA12Cls : ’ Co-Cl 3.45(l)-

3.47(l) and Al-Cl 2.1 l(2)-2.19(2) A. The standard deviations do not allow elaborate discussions of the bonding within the MgC& and AlC& units. It is noteworthy, however, that the rather short chlorine-chlorine contact distance of 3.28 A is across the double chlorine bridge between Al and Mg (3.30 8, for CoA12ClB).

Table 6. Interatomic distances (A) in MgAl&l, Bonding distances Mg-Cl(2) Mg-Cl(3) Mg-Cl(4)

2.50(l) 2.59(3) 2.49(3)

Non-bonding

Between groups 3.66(3) 3.52(2) 3.50(2) 3.58(2) 3.28(2) 3.59(2)

Non-bonding WlHJ(2) Cl(l)-Cl(2) Cl(lWJ(3)

2.17(4) 2.24(4) 2.13(3) 2.11(3)

distances within the chain

AlCl, group Cl(WCl(2) Cl(WCl(3) Cl(lWJ(4) Cl(2)--cl(3) Cl(2)--cl(4) Cl(3)--cl(4)

Al--cl( 1) Al-Cl(2) Al-Cl(3) Al-Cl(4)

Cl(2+~1(3) Cl(2>--cl(3) Cl(2>-cl(4) Cl(2>-cl(4) Cl(3)--cl(3) Cl(3)--cl(4) Cl(4>-cl(4)

3.52(2) 3.84(2) 3.63(2) 3.63(2) 3.60(l) 3.69(3) 3.48(l)

distances between chains

3.71(2) 3.77(2) 3.74(2)

Cl(l>--cl(3) Cl(l)--cl(4) Cl(2>-cl(3)

3.82(2) 3.68(2) 3.88(2)

M.-A. EINARSRUD

982

The chains are kept together by interactions between electron-deficient bridging chlorines and electron-rich terminal chlorines from neighbouring chains. It is seen from the stereographic drawing in Fig. 6 that one might expect van der Waals’ forces between bridging chlorines from different chains as well. However, the non-bonding distances shorter than 3.90 %, within and between chains given in Table 6, show that only one bridge-bridge distance is below this limit and that terminal-bridge distances are significantly shorter. The vibrational correlation digram for the infinite chain in MgAl& is given in Table 7. The basis is taken in the 15 degrees of freedom of the unperturbed ligand AlC14 with Tbsymmetry. Fundamental frequencies of the A1C14unit as foundz3 in NH4AlC14 have been included in Table 7. The v,-mode is the symmetric stretch, v2 is a Cl-Al-Cl deformation, v3 is the antisymmetric stretch and v4

Fig. 6. A stereographic plot of the unit cell of MgAl#&. The viewer looks along the b-axis, and the u-axis points upwards in the paper plane.

et al.

Structure and stability of solid and molten complexes

is a deformation. However, since the AlCl; group in the chain has three chlorines bridged to magnesium and one terminal chlorine, the Thsymmetry is strongly perturbed to a C3,-symmetry as shown in the first part of the diagram. This “chemical correlation” is in turn correlated to the site symmetry (C,) of the AU4 group and then to the symmetry of the chain (C,). Even though both the A- and B-representations of the factor group are IR-active, the main effect probably is the “chemical perturbation” of the AlCl, ligand. Five bands with their origin in the antisymmetric stretch (vJ are observed at 552, 502, 476, 468 and 457 cm- ‘. The characteristics split into two groups by the chemical perturbation of the AlCl, entity, as previously found for TiAlzCls,24 is clearly seen in Fig. 3. The band at 347 cn- ’ (and possibly 318 cn- ‘) is the symmetric stretch, IR-inactive for the free. ligand (TJ, but activated by the “chemical perturbation” (C,,). Bending modes (v4) are observed at 211, 176, 165 and 157 cm- ‘, whereas v2 occur at 146, 134 and 122 cm- ‘. Two of the Mg-Cl stretching fundamentals appear at 271 and 245 cm-‘, and further skeletal modes (librations) are found at 108,103 and 94 cm-‘. Because of the fine splitting of the absorption band at 468 cm-’ into three or four components, we conclude that this frequency corresponds to the E-representation and 552 cm-’ to the A ‘-representation (C,,). These assignments are confirmed by force field calculations. The calculations also show that the frequencies of 211 and 157 cm-’ belong to the Al- and E-representations (C,,), respectively, of the v,-mode.

Molten

MgC12-AlC13

Vibrational spectra of molten alkali chlorideA1C13 mixtures show the presence of the anions AlCl; and A12C1;, with some evidence for cationanion interactions, particularly for Li+ .4*6Accordingly, it is expected that the IR reflectance spectra of the acidic MgA12Cls melts can be interpreted in terms of perturbed anionic entities. If the polarizing power of magnesium is high enough to cause directional Mg-Cl bonds, ion pairs, clusters etc. may be formed. The spectra are interpreted in terms of both the perturbed anion and cluster models. The magnesium cation can distort the AlCI; group from Tfl to C30-or C2,-symmetry depending on its action as a tri- (mono-) or bidentate ligand. Figure 4 shows that both configurations seem to be present. First it may be noted that the appearance of the totally symmetric stretch, v,(A ,), at 341 cm- ’ verifies that distortions do take place. The fre-

983

quencies of 573,498 and 404 cm-’ may reflect the splitting of the antisymmetric stretch, v3(F2), of tetrahedral AlCl;, upon perturbation from Td to C20,and the strong bands at 573 and 452 cm- ’ have their origin in the descent in symmetry from Td to c3,. The latter two frequencies may be compared with the solid values 552 and 468 cm-‘. Evidently, the interactions are stronger in the melt. A partial explanation of this fact is a lower coordination number of Mg2+ in the molten state. Prevalence of smaller entities like (A1C14)Mg(A1C14),where all bridging chlorine atoms of a given tetrachloroaluminate ion ate bonded to the same magnesium atom, also may contribute to the increased splitting. The accidental degeneracy at 573 cm-’ deserves a comment. A parallel situation is found for solid TiA12Cls and TiA12Cls *C6H6 where C3”symmetry gives rise to bands at 552 and 466 cm-‘, whereas C% gives 554,504 and 442 cm-1.24 In analogy with Li+-A12C1; me1ts,6 it is proposed that the strong polarizing power of the magnesium cation stabilizes a bent (CJ rather than a linear (D3J structure of A12C1;. This interaction is manifested by the observed splitting of v1,(I$,) from 525 cm-’ for molten6 KA12Cl, to 596 and 500 cm-’ upon perturbation from D36 to C,-symmetry. The corresponding values for LiA12Cl, are 570 and 514 cm- ‘. Thus, the counterions with high charge/ radius ratios strongly perturb both the symmetric Cl-A1C13 tops of A12C1;. Other absorptions in the magnesium system at 376 and 339 cm-’ are Al-Cl stretching fundamentals of the A12Cl; group, whereas the band at 170 cm- ’ is a bending mode. The broad band at 280-290 cm-’ probably is a direct observation of Mg2+ . . . AlCl; interaction, cf. the Mg-Cl frequency situated at 271 cm- ’ for solid MgA12Cls. At higher A1C13contents, strong bands of Al#& at 615, 472, 410 and 315 cm-’ appear. Although the above interpretation in terms of Al&l6 and the distorted anions A12Cl; and AlCl; is perfectly valid, auxiliary information discussed below together with intensity considerations give further insight into the melt structure. It is seen that the spectra in Fig. 4 essentially are superpositions of Mg(AIC14)2 and Al#& with a Small contribution from A12Cl;. Simply, Mg2+ destabilizes any Al,Cl$,+, (n 2 2) polymer. At intermediate compositions, 80 and 85 mol% A1C13, the seemingly abrupt disappearance of Al&l6 and the frequency variations in the terminal Al-Cl stretching region around 450 cm- ‘, point to distortions of Al&l6 as the Mg2+ concentration is increased. The interaction between a magnesium ion and a chloroaluminate species may be formulated in terms of

984

M.-A. EINARSRUD

pressures of 1 atm or less. This observation is also contrary to the mixture with alkali chloride (and calcium chloride) and show that MgC12 does not donate Cl- as easily to AlC13, forming an ionic melt, as alkali chloride and calcium chloride.

molecular complexes as 50 67 80

mol% AlC13 i

Mg’+, AlCl;, ClMg(AlC1J2 Mg(A12C16)(AlC14)2, Mg(AlzCl&Clz.

Similar complexes obtained by substitution of A12Cl; for AlCl; exist in low concentrations. Furthermore, it is possible that more lattice like structures to some extent are formed by polymerization, cf. the Mg(AlC1J2 chain in the solid. Some dissociation into ionic species is expected. This melt model is supported by the following additional information : (a) Due to a strong, polarized band characteristic for each of the species AlCl;, A12C1; and A12Cls, Raman spectroscopy is better suited than IR spectroscopy to estimate concentrations.26~27 Comparison of relative concentrations with stoichiometric restrictions leads to the conclusion for the similar acidic MgCl,-GaCl, system, that some chlorine atoms must be bonded to magnesium only.” The same results may be inferred from a preliminary Raman study of the present system.‘* (b) Raman spectra show that the amount of A12Cl; (Ga,Cl;) relative to AlCl; (GaCl;) and Al& (Ga,Cl,) is reduced when the polarizing power of the counterion increases.4~14,27*28 The following series of diminishing A12Cl~ concentration may be constructed : Cs > K > Li > Mg > Zn. (c) The terminal IR-active Al-Cl stretching vibrations of Czv perturbed AlCl; follow a smooth trend which may be correlated with polarizing power. Giving the antisymmetric and the symmetric mode, the frequencies (cm- ‘) are :

Asymmetric Symmetric

et al.

Activity

of acidic melts

As discussed above, the A1C13vapour pressure and corresponding activity could not be obtained from boiling point method studies. Such information can, however, be calculated from the lowering of the A1C13 freezing point. The change in freezing point is so small that the difference in C, can be neglected. The following equation is valid for the liquidus line to the right in Fig. 1 : In

aAICI,

,

fus

[

where AT is the freezing point depression. Further aAIAI,CI,

=

pAI,CI,/p~I,CI,

=

:

dlCI,+

From

the AT values of 0.6 and 1.6”C at 0.974 and 0.936, respectively, and setting AH,,(AlCl,) = 8450 Cal, Al&l6 activities of 0.980 and 0.943 are found. .Based on these data the activity coefficient function : XAICI,

=

YAICl,

=

1 +0.6(1

-xAm,)

(XAlCI,

>

0.7)~

is proposed. This equation gives for instance = 0.74 for 75 mol% AlC13. It is hence underaAI,CI, stood that the vapour pressures above the MgCl, AlC13 liquids were higher than 1 atm.

“[Al(AlC14)]2+”

[Zn(AlCl,)]+ 5*‘4

DJgWC~d +

614 473

585 482

573 498

The hypothetical ion [A1(A1C14)]2+is part ofA12C16. The titanium values 24 for solid Ti(AlCl&(C6H6) are in the molten state expected to become close to the [Mg(AlCl,)]+ data. (d) Contrary to the alkali chloridez9 and calcium chloride3’ mixtures with A1C13,no indication of an immiscibility gap was found in the present phase diagram investigation of the MgC12 system. As Al&l6 can dissolve less than 1 mol% ionic species, there is a strong case for the prevalence of neutral entities in acidic MgC12-AlC13 melts. (e) Attempts by vapour pressure studies on acidic MgC12-AlC13 mixtures failed to establish any liquid phase in the range 0.67 < .&c[, < 1 for vapour

hq1

AJ%sWCL)&

-

=

l’WA1C14)l+@) 554 504

It is concluded that the activity of A1C13 in MgC12-AlC13 mixtures show a positive deviation from ideality, but the deviation is not so strong that liquid-liquid immiscibility occurs. When, however, the composition Mg(AlC14)2 is approached, negative deviation may prevail as the A13+ ion is saturated with respect to Cl-. Molecular-orbital

calculations

In order to gain a clearer understanding of the energetics when AlCl; and Mg2+ interact, molecular-orbital calculations on the ab initio Hartree

Fock level were performed

on [MgAlCl,]+ (C,,)

Structure and stability of solid and molten complexes

985

Table 8. Ab initio calculated properties of [MgAlCl,]+ and MgAICIS’ Parameter

MiN-.XI+ (Cd [Mtd1CL1+(Go) WWCL (‘2

Al-Cl, Al-C&, Mg-Q Mg-Cl, Mg-Al

2.028 2.213 2.280 2.734

Cl,-Al-Cl, ClrAl--Cl,, Clb-Al-Cl, E

2.046 2.289 2.181 3.203

2.062 2.207 2.086 2.274 3.231

126.4 88.4

130.2 108.0 85.8

122.1 110.2 89.1

-2254.75994

-2254.74218

- 2709.63293

uUnits : bond lengths in A, angles in degrees and energies in Hartrees. h

,clb

~~&,~~---~~,

Cl,,

“Cl;' C3”

with tridentate Mg, [MgAlCl,] (C,,) and MgAlC15

(C,,). The computations were done with the gradient program GAUSSIAN 80 employing a minimal STO-3G basis.31 The results after optimizing all geometrical parameters are given in Table 8. It may be of interest to compare the bond lengths in Table 8 with the calculated values for AlCl; (Td), 2.113 A,andMgCl, (Dmh),2.090A. The two [MgAlCl,]+ structures probably correspond to the only energy minima when Mg*+ is moved around the AlCl; ion.32 The C3v structure is favoured by 11.1 kcal mol- ’ relative to C2”.These results agree with the conclusions from the IR spectroscopic investigations ; both modifications are present in the melt, but with a higher concentration of the Crv structure. In a calculation on LiAlC14 it was verified that the alternative Ck structure with a monodentate counterion is unstable. 32 The geometrical parameters seem to be reasonable, both the absolute values and, in particular, terminal-bridge differences and the shifts from species to species. For instance, Al-Cl, < Al-C&, (cf. 2.065 and 2.252 A observed in A12C1633), Cl,Al-Cl, > Cl,-Al-Cl, (123.4 and 91 .O” in A12C1633)and one terminal bond (2.028 A in Table 8) is stronger than two adjacent terminal bonds (2.046 A). The latter effect also is seen in the Al-C& bonds. Due to the low coordination number imposed on magnesium in the models, the Mg-Q, distances are short compared to the observed bond lengths in MgAl,Cl,(s) (see Table 6). Naturally, bidentate magnesium give stronger Mg-Cls than tridentate magnesium, mainly because of the reduced MgAl repulsion. Adding another terminal chlorine to

magnesium forming MgAlC15 gives the expected reduction in the counterion influence on AlClh . Acknowle&ements-Financial support from NTNF and Norges Tekniske Hsgskoles Fond gratefully is acknowledged. The phase diagram work was sponsored by ALCOA laboratories through Dr Ed. Martin. Special thanks go to Professor J. Liitzow Holm for DSc, Professor H. J. Seifert for calorimetry, Dr A. F. Andresen for recording the neutron diffractograms, and to B. M. Faanes for phase diagram measurements.

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