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

Abstract

Dissolving lithium chloride and aluminium chloride in boiling para- or metaxylene 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-stannitetype 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 (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.