Structure and stability of solid and molten complexes in the MgCl2-AlCl3 system

A visual determination of the phase diagram of the MgCl2-AlCl3 system reveals the formation of two intermediate compounds. One of these compounds is MgAl2Cl8 crystallizing with a monoclinic unit cell, space group I2/c, with dimensions a = 12.873(1)Å, b = 7.8959(7)Å, c = 11.617(1)Å, β = 92.348(8)° and z = 4. The two crystal modifications of anhydrous magnesium chloride and the lattice parameters of aluminium chloride have been reexamined. Lattice energies for α- and β-MgCl2 have been measured, respectively, as 661(1) and 646(4) kcal mol−1. The liquidus curve on the acidic side has been used to estimate the activity of aluminium chloride. Fourier-transform IR spectra of melts with compositions ranging from 0 to 30 mol% MgCl2 have been interpreted in terms of Al2Cl6, strongly perturbed Al2Cl7− and AlCl4− entities. In particular, the tetrachloroaluminate ion acts as both a tri- and a bidentate ligand towards Mg2+. Neutral species, e.g. Mg(AlCl4)2 and Mg(Al2Cl6)(AlCl4)Cl, dominate in the melt. Ab initio molecular-orbital calculations have been performed to obtain a better understanding of the Mg2+ —d AlCl4− interaction.