Polymeric structures in aluminium and gallium halides

The anionic species (AlnX3n+1)(-) with X = Cl or Pr and n greater than or e qual to 1 have been recognized for a number of years to form in acidic liqu id mixtures of aluminium chloride or bromide with the corresponding halides of alkali or organic cations, in relative proportions which vary with the composition of the mixture. In this work we evaluate the structure and the energetics of such polymeric series in a comparative study of Al and Ga com pounds. To this end we first extend an earlier study of the ionic interacti ons in the Al2Cl6 molecule [Z. Akdeniz and M. P Tosi, 2. Naturforsch. 54a, 180 (1999)] to determine microscopic ionic models for Ga2Cl6, Al2Br6, and G a2Br6. The models are then used (i) to evaluate the polymeric clusters for n less than or equal to 4 in the two trivalent-metal chlorides, and (ii) to explore the potential-energy hypersurface of alkali counterions in the cas e n = 2. We present tests of the results against available data and an eval uation of the convergence of the energy of the polymeric series towards a v alue of about 0.5 eV per monomer.