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

The new polymorph of lithium tetrachloridoaluminate, LiAlCl4, adopts a defect wurtz-stannite-type of structure and is metastable.


Chemical context
The series of known crystal structures of alkali metal tetrachloridoaluminates MAlCl 4 , 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 [LiAlCl 4 (mP24, P2 1 /c); Mairesse et al., 1977] and melts of the salt were investigated (Weppner & Huggins, 1976. Besides the importance of common commercial lithium-thionyl chloride battery systems (Winter & Brodd, 2004), recently published studies on the conductivity of LiAlCl 4 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 ongoing studies on arene complexation of main group metals (Frank, 1990;Frank et al., 1987Frank et al., , 1996Frank & Wittmer, 1997;Kugel, 2004;Bredenhagen, 2014), we isolated a new polymorph of LiAlCl 4 (oP12, Pmn2 1 ) from mixtures of lithium chloride and aluminium chloride in boiling para-or metaxylene, determined its crystal structure by single-crystal X-ray diffraction and unequivocally proved polymorphism of this ternary compound.   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).
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 LiAlCl 4 (mP24) are in octahedral coordination (Mairesse et al., 1977), the aluminium and lithium ions in the solid of orthorhombic LiAlCl 4 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 LiAlCl 4 (mP24), the octahedral and tetrahedral polyhedra are connected via corners as well as edges. LiAlCl 4 (oP12) exhibits, as expected, shorter Li-Cl bonds (coordination number 4) as compared to corresponding bonds in monoclinic LiAlCl 4 (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 LiAlCl 4 , 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.

Figure 2
Raman spectrum of the title compound.

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 LiAlCl 4 (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 LiAlCl 4 (oP12) to represent a metastable phase of lithium tetrachloridoaluminate whose melting point probably is nearly identical to that of monoclinic LiAlCl 4 because it is very unlikely that a phase transition would not have been observed with the chosen DSC methods. The lower density of orthorhombic LiAlCl 4 (1.89 g cm À3 ) compared to monoclinic LiAlCl 4 (1.98 g cm À3 ; Mairesse et al., 1979) supports the assumption of its metastability.

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 AlCl 3 (powder, 99.99%) were purchased from Sigma-Aldrich and while LiCl was used as received, AlCl 3 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 pxylene 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. LiAlCl 4 (oP12, Pmn2 1 ) was isolated in 60%    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 À1 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.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2016); software used to prepare material for publication: publCIF (Westrip, 2010). Special details 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.

Crystal data
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )