Lithium tetra­chlorido­aluminate, LiAlCl4: a new polymorph (oP12, Pmn21) 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 tetra-chlorido-aluminate, 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 hexa-gonal closest packings of chloride anions. While the lithium cations in LiAlCl4(mP24) are in octa-hedral coordination and the aluminium and lithium ions in the solid of ortho-rhom-bic LiAlCl4 occupy tetra-hedral inter-stices 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 ortho-rhom-bic modification is found until the material melts at 148 °C (Tpeak = 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.