Lithium tetra­chlorido­aluminate, LiAlCl4: a new polymorph (oP12, Pmn21) with Li+ in tetra­hedral inter­stices

Prömper, S.W.; Frank, W.

doi:10.1107/S205698901701235X

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Volume 73| Part 10| October 2017| Pages 1426-1429

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Lithium tetrachloridoaluminate, LiAlCl₄: a new polymorph (oP12, Pmn2₁) with Li⁺ in tetrahedral interstices

Stephan W. Prömper ^a and Walter Frank ^a ^*

^aInstitut für Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
^*Correspondence e-mail: wfrank@hhu.de

Edited by M. Weil, Vienna University of Technology, Austria (Received 27 July 2017; accepted 25 August 2017; online 8 September 2017)

Dissolving lithium chloride and aluminium chloride in boiling para- or meta-xylene and keeping the colourless solution at room temperature led to crystal growth of a new modification of lithium tetrachloridoaluminate, LiAlCl₄, which represents a second modification (oP12, Pmn2₁) of the ternary salt besides the long known monoclinic form [LiAlCl₄(mP24, P2₁/c); Mairesse et al. (1977 ). Cryst. Struct. Commun. 6, 15–18]. The crystal structures of both modifications can be described as slightly distorted hexagonal closest packings of chloride anions. While the lithium cations in LiAlCl₄(mP24) are in octahedral coordination and the aluminium and lithium ions in the solid of orthorhombic LiAlCl₄ occupy tetrahedral interstices with site symmetries m and 1, respectively, the lithium cation site being half-occupied (defect wurtz-stannite-type structure). From differential scanning calorimetry (DSC) measurements, no evidence for a phase transition of the orthorhombic modification is found until the material melts at 148 °C (T_peak = 152 °C). The melting point is nearly identical to the literature data for LiAlCl₄(mP24) [146 °C; Weppner & Huggins (1976 ). J. Electrochem. Soc. 124, 35–38]. From the melts of both polymorphs, the monoclinic modification recrystallizes.

Keywords: crystal structure; polymorphism; lithium tetrachloridoaluminate.

CCDC reference: 1570829

1. Chemical context

The series of known crystal structures of alkali metal tetrachloridoaluminates MAlCl₄, with M = Li (Mairesse et al., 1977), Na (Baenziger, 1951 ), K (Mairesse et al., 1978a ), Rb (Mairesse et al., 1979 ) and Cs (Gearhart et al., 1975 ; Mairesse et al., 1979) was completed about 40 years ago and comparative structural studies were made (Mairesse et al., 1979; Meyer & Schwan, 1980 ). With respect to ionic conductivity, both solid lithium tetrachloridoaluminate [LiAlCl₄(mP24, P2₁/c); Mairesse et al., 1977] and melts of the salt were investigated (Weppner & Huggins, 1976, 1977 ). Besides the importance of common commercial lithium–thionyl chloride battery systems (Winter & Brodd, 2004 ), recently published studies on the conductivity of LiAlCl₄ in dimethyl carbonate or mixtures with ethylene carbonate (Scholz et al., 2015 ) indicate that the substance is of continous interest. In the course of our on-going studies on arene complexation of main group metals (Frank, 1990 ; Frank et al., 1987 , 1996 ; Frank & Wittmer, 1997 ; Kugel, 2004 ; Bredenhagen, 2014 ), we isolated a new polymorph of LiAlCl₄(oP12, Pmn2₁) from mixtures of lithium chloride and aluminium chloride in boiling para- or meta-xylene, determined its crystal structure by single-crystal X-ray diffraction and unequivocally proved polymorphism of this ternary compound.

2. Structural commentary

LiAlCl₄(oP12, Pmn2₁) crystallizes in a defect wurtz-stannite-type structure, an orthorhombic superstructure of the wurtzite-type structure, known from quaternary compounds of the type Cu₂M^IIM^IVM₄^VI (M^II = Mn, Fe, Co, Zn, Cd, Hg; M^IV = Si, Ge, Sn; M^VI = S, Se; except selenides of cobalt; Schäfer & Nitsche, 1977 ). The unit cell of the title compound contains four chloride anions and two aluminium cations, located in special positions (Wyckoff position 2a), as well as two lithium cations and another four chloride anions in general positions (4b), with the lithium site being half occupied, i.e. the asymmetric unit of the crystal structure is defined by half a tetrachloridoaluminate anion and one half-occupied lithium ion (Fig. 1a).

Figure 1
(a) The unit cell of the crystal structure of the title compound, with displacement ellipsoids drawn at the 50% probability level; (b) a view of the crystal structure in polyhedral representation perpendicular to a stacking direction ([010]) of the slightly distorted hexagonal closest packing of chloride anions; (c) a view of the crystal structure along [00 $[\overline{1}]$ ].

The crystal structures of the title compound, as well as of the monoclinic modification of lithium tetrachloridoaluminate, can be described as slightly distorted hexagonal closest packings of chloride anions. While the lithium cations in LiAlCl₄(mP24) are in octahedral coordination (Mairesse et al., 1977), the aluminium and lithium ions in the solid of orthorhombic LiAlCl₄ occupy tetrahedral interstices with site symmetries m and 1, respectively, the lithium cation site being half-occupied (Figs. 1b and 1c). Hence, the solid state of the title compound represents a three-dimensional network of corner-sharing tetrahedra, while in LiAlCl₄(mP24), the octahedral and tetrahedral polyhedra are connected via corners as well as edges. LiAlCl₄(oP12) exhibits, as expected, shorter Li—Cl bonds (coordination number 4) as compared to corresponding bonds in monoclinic LiAlCl₄ (coordination number 6). Using the Brown formalism (Brown & Altermatt, 1985 ), in both cases, bond orders which differ significantly from the expected value in view of the monovalent cation are computed (Table 1). In the case of orthorhombic LiAlCl₄, the strong deviation is based on the statistical disorder mentioned above and corresponding averaged geometric parameters obtained for occupied and non-occupied tetrahedral interstices, leading to higher Li—Cl bond orders in view of the exponential relationship between bond length and bond order.

Table 1
Selected bond lengths (Å) in LiAlCl₄(oP12) and LiAlCl₄(mP24) (Perenthaler et al., 1982 ) in the left and right column, respectively, and corresponding sums of bond orders, calculated using the Brown formalism (r₀ = 1.91, B = 0.37; Brese & O'Keeffe, 1991 )

Li1—Cl1	2.322 (17)	Li—Cl1	2.475 (7)
Li1—Cl2	2.381 (21)	Li1—Cl2^x	2.729 (7)
Li1—Cl2ⁱ	2.356 (14)	Li1—Cl2^xi	2.841 (7)
Li1—Cl3	2.413 (17)	Li1—Cl3	2.594 (7)
		Li1—Cl3^xii	2.769 (7)
		Li1—Cl4^xiii	2.493 (7)
Σs(Li—Cl)	1.17	Σs(Li—Cl)	0.87
Symmetry codes: (i) −x + $[{1\over 2}]$ , −y, z − $[{1\over 2}]$ ; (x) −x, y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (xi) x, −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (xii) −x, y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (xiii) 1 − x, y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ .

3. Raman spectra

Raman bands in the vibrational spectrum of the title compound (Fig. 2) can be assigned to the four normal modes of vibration of a five atomic tetrahedral moiety of composition AX₄ (Nakamoto, 1986 ) ν_s(A₁: 350 cm⁻¹), δ_d(E: 136 and 126 cm⁻¹), ν_d(F₂: 523, 502, 487 and 478 cm⁻¹) and δ_d(F₂: 180 and 170 cm⁻¹). As in the Raman spectra of other alkali metal tetrachloridoaluminates (Rytter & Øye, 1973 ; Rubbens et al., 1978 ) or NH₄AlCl₄ (Mairesse et al., 1978b ), splitting of the bands is observed corresponding to the site effect and perturbation of the ideal tetrahedral symmetry of free AlCl₄⁻ anions caused by cation interactions.

Figure 2
Raman spectrum of the title compound.

4. Thermal analysis and X-ray powder diffraction

From DSC measurements of the title compound (Fig. 3), no evidence for a phase transition is found until the material melts at 148 °C (T_peak = 152 °C). The melting point is nearly identical to literature data for LiAlCl₄(mP24) (146 °C; Weppner & Huggins, 1976), which seems to be the only modification that recrystallizes from the melts of both modifications. This is demonstrated by high-quality X-ray powder diffraction patterns of the title compound, crystallized from para-xylene solution, and of the crystalline solid obtained by recrystallization from the melt (Fig. 4). In view of the current data, we suppose LiAlCl₄(oP12) to represent a metastable phase of lithium tetrachloridoaluminate whose melting point probably is nearly identical to that of monoclinic LiAlCl₄ because it is very unlikely that a phase transition would not have been observed with the chosen DSC methods. The lower density of orthorhombic LiAlCl₄ (1.89 g cm⁻³) compared to monoclinic LiAlCl₄ (1.98 g cm⁻³; Mairesse et al., 1979) supports the assumption of its metastability.

Figure 3
DSC curves of multiple runs of the title compound.

Figure 4
X-ray powder diffraction pattern of the title compound before (top) and after (bottom) melting and corresponding simulations. [a] Single-crystal data for LiAlCl₄(mP24) are taken from the literature (Perenthaler et al., 1982

5. Synthesis and crystallization

All sample preparations and manipulations were carried out in an atmosphere of dry argon (argon 5.0) using either Schlenk techniques or an MBraun LABstar glove-box. LiCl (beads, 99.9+%, anhydrous) and AlCl₃ (powder, 99.99%) were purchased from Sigma–Aldrich and while LiCl was used as received, AlCl₃ was first overlayed with elemental aluminium (grit, ≥97%, Sigma–Aldrich) and sublimed in a sealed ampoule in vacuo at 190 °C. p-Xylene (99%, Sigma–Aldrich) and m-xylene (99%, TCI) were refluxed with aluminium chloride, washed with 0.2 M NaOH, as well as distilled water, and distilled on molecular sieve 4 Å afterwards. In a typical reaction, 0.112 g (2.64 mmol) lithium chloride and 0.268 g (2.01 mmol) aluminium chloride were treated with 5 ml p-xylene and the mixture was refluxed for 30 min. Seperation of the warm colourless solution from residual LiCl and removal of 4 ml of the solvent under reduced pressure at room temperature led to the formation of colourless crystals of the title compound. LiAlCl₄(oP12, Pmn2₁) was isolated in 60% yield after washing the crystalline material with p-xylene and drying the solid in vacuo at room temperature.

The FT–Raman spectrum was recorded using a Bruker MultiRam spectrometer (OPUS; Bruker, 2006 ) equipped with an RT-InGaAs-detector and an Nd:YAG-Laser at 1064 nm (Stokes: 3500–70 cm⁻¹; resolution: 2 cm⁻¹): ν_d(F₂, AlCl₄⁻): 523 (w), 502 (w), 487 (m), 478 (w); ν_s(A₁, AlCl₄⁻): 350 (vs); δ_d(F₂, AlCl₄⁻): 180 (s), 170 (s); δ_d(E, AlCl₄⁻): 136 (m), 126 (s), 104 (m).

Thermal analysis (differential scanning calorimetry) was carried out with a Mettler Toledo DSC 1 calorimeter (STARe; Mettler-Toledo, 2008 ) equipped with an FRS 5 sensor using medium pressure steel crucibles without sealing rings. Measurements were carried out in an atmosphere of dry nitrogen at a heating/cooling rate of 5 °C min⁻¹ between 0 and 170 °C. First measurement heating: T_onset = 148 °C (T_peak = 152 °C), endothermic, melting; first measurement cooling: T_onset = 132 °C (T_peak = 132 °C), exothermic, crystallization; second measurement heating: T_onset = 149 °C (T_peak = 152 °C), endothermic, melting; second measurement cooling: T_onset = 139 °C (T_peak = 138 °C), exothermic, crystallization; third measurement heating: T_onset = 148 °C (T_peak = 152 °C), endothermic, melting; third measurement cooling: T_onset = 139 °C (T_peak = 139 °C), exothermic, crystallization. An alternative melting-point determination was carried out with a Mettler Toledo MP 90 Melting Point System: T_mp = 149 °C.

X-ray powder diffraction patterns were measured using a Stoe & Cie STADI P (WinXPOW; Stoe & Cie, 2003 ) Debye–Scherrer diffractometer working in transmission mode with Cu Kα₁ radiation [Ge(111) monochromator]. Simulations of powder patterns from single-crystal data were carried out using the computer program PowderCell (Kraus & Nolze, 2000 ).

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The lithium cation site (general position, Wyckoff site 4b) is half occupied.

Table 2
Experimental details

Crystal data
Chemical formula	LiAlCl₄
M_r	175.72
Crystal system, space group	Orthorhombic, Pmn2₁
Temperature (K)	173
a, b, c (Å)	7.8273 (10), 6.4466 (10), 6.1304 (8)
V (Å³)	309.34 (7)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	1.90
Crystal size (mm)	0.65 × 0.10 × 0.03

Data collection
Diffractometer	Stoe IPDS 2T
Absorption correction	Multi-scan (X-AREA; Stoe & Cie, 2009 )
T_min, T_max	0.431, 0.583
No. of measured, independent and observed [I > 2σ(I)] reflections	3388, 880, 870
R_int	0.092
(sin θ/λ)_max (Å⁻¹)	0.685

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.028, 0.075, 1.21
No. of reflections	880
No. of parameters	37
No. of restraints	1
Δρ_max, Δρ_min (e Å⁻³)	0.48, −0.46
Absolute structure	Flack x determined using 386 quotients [(I⁺) − (I⁻)]/[(I⁺) + (I⁻)] (Parsons et al., 2013 )
Absolute structure parameter	0.1 (2)
Computer programs: X-AREA (Stoe & Cie, 2009), SHELXT (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ), DIAMOND (Brandenburg, 2016 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1570829

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S205698901701235X/wm5410sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698901701235X/wm5410Isup2.hkl

checkCIF report

Computing details top

Data collection: X-AREA (Stoe & Cie, 2009); cell refinement: X-AREA (Stoe & Cie, 2009); data reduction: X-AREA (Stoe & Cie, 2009); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2016); software used to prepare material for publication: publCIF (Westrip, 2010).

Lithium tetrachloridoaluminate top

Crystal data top

LiAlCl₄	D_x = 1.887 Mg m⁻³
M_r = 175.72	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pmn2₁	Cell parameters from 6814 reflections
a = 7.8273 (10) Å	θ = 3.0–29.7°
b = 6.4466 (10) Å	µ = 1.90 mm⁻¹
c = 6.1304 (8) Å	T = 173 K
V = 309.34 (7) Å³	Needle-shaped, colorless
Z = 2	0.65 × 0.10 × 0.03 mm
F(000) = 168

Data collection top

Stoe IPDS 2T diffractometer	870 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.092
φ–scans	θ_max = 29.1°, θ_min = 3.2°
Absorption correction: multi-scan (X-AREA; Stoe & Cie, 2009)	h = −10→10
T_min = 0.431, T_max = 0.583	k = −8→8
3388 measured reflections	l = −8→8
880 independent reflections

Refinement top

Refinement on F²	1 restraint
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.022P)² + 0.166P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.028	(Δ/σ)_max < 0.001
wR(F²) = 0.075	Δρ_max = 0.48 e Å⁻³
S = 1.21	Δρ_min = −0.46 e Å⁻³
880 reflections	Absolute structure: Flack x determined using 386 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013)
37 parameters	Absolute structure parameter: 0.1 (2)

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Li1	0.246 (2)	0.1671 (19)	0.493 (3)	0.035 (3)	0.5
Cl1	0.0000	0.31452 (19)	0.34835 (19)	0.0305 (3)
Cl2	0.22483 (12)	0.18042 (11)	0.88033 (12)	0.0317 (2)
Cl3	0.5000	0.35454 (16)	0.3896 (3)	0.0295 (3)
Al1	0.0000	0.3312 (2)	−0.0004 (3)	0.0229 (3)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Li1	0.043 (8)	0.033 (6)	0.029 (8)	0.001 (6)	0.002 (6)	0.000 (4)
Cl1	0.0300 (5)	0.0405 (6)	0.0208 (7)	0.000	0.000	0.0010 (4)
Cl2	0.0313 (4)	0.0314 (4)	0.0324 (5)	0.0078 (3)	0.0067 (5)	0.0017 (5)
Cl3	0.0300 (5)	0.0226 (4)	0.0360 (6)	0.000	0.000	−0.0055 (6)
Al1	0.0244 (8)	0.0212 (6)	0.0233 (8)	0.000	0.000	0.0008 (5)

Geometric parameters (Å, º) top

Li1—Cl1	2.322 (17)	Cl2—Li1^iv	2.356 (14)
Li1—Cl2ⁱ	2.356 (14)	Cl3—Al1^v	2.1354 (18)
Li1—Cl2	2.38 (2)	Cl3—Li1^vi	2.413 (17)
Li1—Cl3	2.413 (17)	Al1—Cl3^vii	2.1354 (18)
Cl1—Al1	2.1404 (19)	Al1—Cl2^viii	2.1392 (11)
Cl1—Li1ⁱⁱ	2.322 (17)	Al1—Cl2^ix	2.1392 (11)
Cl2—Al1ⁱⁱⁱ	2.1392 (11)

Cl1—Li1—Cl2ⁱ	111.0 (7)	Li1^iv—Cl2—Li1	104.6 (5)
Cl1—Li1—Cl2	108.0 (7)	Al1^v—Cl3—Li1^vi	113.1 (4)
Cl2ⁱ—Li1—Cl2	109.5 (6)	Al1^v—Cl3—Li1	113.1 (4)
Cl1—Li1—Cl3	112.2 (6)	Li1^vi—Cl3—Li1	111.1 (7)
Cl2ⁱ—Li1—Cl3	108.6 (7)	Cl3^vii—Al1—Cl2^viii	108.85 (6)
Cl2—Li1—Cl3	107.5 (7)	Cl3^vii—Al1—Cl2^ix	108.85 (6)
Al1—Cl1—Li1ⁱⁱ	113.7 (5)	Cl2^viii—Al1—Cl2^ix	110.70 (8)
Al1—Cl1—Li1	113.7 (5)	Cl3^vii—Al1—Cl1	111.28 (9)
Li1ⁱⁱ—Cl1—Li1	111.9 (9)	Cl2^viii—Al1—Cl1	108.58 (6)
Al1ⁱⁱⁱ—Cl2—Li1^iv	114.3 (4)	Cl2^ix—Al1—Cl1	108.58 (6)
Al1ⁱⁱⁱ—Cl2—Li1	114.4 (4)

Symmetry codes: (i) −x+1/2, −y, z−1/2; (ii) −x, y, z; (iii) x, y, z+1; (iv) −x+1/2, −y, z+1/2; (v) −x+1/2, −y+1, z+1/2; (vi) −x+1, y, z; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y, z−1; (ix) x, y, z−1.

Selected bond lengths (Å) in LiAlCl₄(oP12) and LiAlCl₄(mP24) (Perenthaler et al., 1982) in the left and right column, respectively, and corresponding sums of bond orders, calculated using the Brown formalism (r₀ = 1.91, B = 0.37; Brese & O'Keeffe, 1991). top

Li1—Cl1	2.322 (17)	Li—Cl1	2.475 (7)
Li1—Cl2	2.381 (21)	Li1—Cl2^x	2.729 (7)
Li1—Cl2ⁱ	2.356 (14)	Li1—Cl2^xi	2.841 (7)
Li1—Cl3	2.413 (17)	Li1—Cl3	2.594 (7)
		Li1—Cl3^xii	2.769 (7)
		Li1—Cl4^xiii	2.493 (7)
Σs(Li—Cl)	1.17	Σs(Li—Cl)	0.87

Symmetry codes: (i) -x+1/2, -y, z-1/2; (x) -x, y-1/2, -z+1/2; (xi) x, -y+1/2, z+1/2; (xii) -x, y-1/2, -z+1/2; (xiii) 1-x, y-1/2, -z+1/2.

Acknowledgements

We thank E. Hammes and P. Roloff for technical support.

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