AN IONIC MODEL FOR MOLECULAR UNITS IN MOLTEN ALUMINUM TRICHLORIDE ANDALKALI CHLOROALUMINATES

A simple ionic model including anion polarizability is developed for t he Al2Cl6 molecular dimer and is tested against data on bond lengths, bond angles and vibrational frequencies from experiment and from molec ular orbital calculations. The transferability of the model is tested through calculations of structure and vibrational frequencies for the (AlCl4)(-) and (Al2Cl7)(-) molecular ions as well as for related ionic clusters, the results being compared with available data. Further cal culations are reported showing that the Al2Cl6 dimer is strongly stabl e against fluctuations into ionized states, the fluctuations examined being chlorine exchange between neighbouring dimers and the appearance of ionized products al intermediate stages in the dissociation of the dimer into neutral monomers. The relevance of the results to theoreti cal understanding of molten chloroaluminates in the acidic range up to pure molten AlCl3 is discussed.