Lithium tetra­chlorido­aluminate, LiAlCl(4): a new polymorph (oP12, Pmn2(1)) with Li(+) in tetra­hedral inter­stices

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, LiAlCl(4), which represents a second modification (oP12, Pmn2(1)) of the ternary salt besides the long known monoclinic form [LiAlCl(4)(mP24, P2(1)/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 LiAlCl(4)(mP24) are in octa­hedral coordination and 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 (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 LiAlCl(4)(mP24) [146 °C; Weppner & Huggins (1976 ▸). J. Electrochem. Soc. 124, 35–38]. From the melts of both polymorphs, the monoclinic modification recrystallizes.