[show abstract][hide abstract] ABSTRACT: The anionic species (Al n X 3n+1) ; with X = Cl or Br and n 1 have been recognized for a number of years to form in acidic liquid 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 compounds. To this end we first extend an earlier study of the ionic interactions in the Al 2 Cl 6 molecule [Z. Akdeniz and M. P. Tosi, Z. Naturforsch. 54a, 180 (1999)] to determine microscopic ionic models for Ga 2 Cl 6 , Al 2 Br 6 , and Ga 2 Br 6 . The models are then used (i) to evaluate the polymeric clusters for n 4 in the two trivalent-metal chlorides, and (ii) to explore the potential-energy hypersurface of alkali counterions in the case n = 2. We present tests of the results against available data and an evaluation of the convergence of the energy of the polymeric series towards a value of about 0.5 eV per monomer.
[show abstract][hide abstract] ABSTRACT: We determine a model of the ionic interactions in RX 3 compounds (where R is a metal in the rare-earth series from La to Lu and X = Cl, Br or I) by an analysis of data on the static and dynamic structure of their molecular monomers. The potential energy function that we adopt is patterned after earlier work on Aluminium trichloride [Z. Akdeniz and M. P. Tosi, Z. Naturforsch. 54a, 180 (1999)], but includes as an essential element the electric polarizability of the trivalent metal ion to account for a pyramidal shape of RX 3 molecules. From data referring mostly to trihalides of elements at the ends and in the middle of the rare-earth series (i. e. LaX 3 , GdX 3 and LuX 3), we propose systematic variations for the effective valence, ionic radius and electric polarizability of the metal ions across the series. As a first application of our results we predict the structure of the Dy 2 Cl 6 and Dy 2 Br 6 molecular dimers and demonstrate by comparison with electron diffraction data that lanthanide-ion polarizability plays a quantitative role also in this state of tetrahedral-like coordination.