Lithium tetra-chlorido-aluminate, LiAlCl 4 : a new polymorph ( oP 12, Pmn 2 1 ) with Li + in tetra-hedral inter-stices.

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 ( oP 12, Pmn 21 ) of the ternary salt besides the long known monoclinic form [LiAlCl4 ( mP 24, P 21 / 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 ( mP 24) 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 ( T peak = 152 °C). The melting point is nearly identical to the literature data for LiAlCl4 ( mP 24) [146 °C; Weppner & Huggins (1976 ▸). J. Electrochem. Soc. 124 , 35-38]. From the melts of both polymorphs, the monoclinic modification recrystallizes.