Structure and vibrational spectra of the vapour molecules Fe2Cb and AlFeCl6

Melting of aluminium and iron trichloride crystals is accompanied by a structural transition from octahedral to tetrahedral coordination of the metal ions, and a widely accepted interpretation of their liquid structure is that it mainly consists of strongly correlated dimeric units. Such Al2Cl6 and Fe2Cl6 molecules are stable in the vapour phase and coexist in gaseous mixtures together with AlFeCl6 molecules. In this work we extend to Fe2Cl6 and AlFeCl6 our earlier study of the ionic interactions in Al2Cl6 [Z. Akdeniz and M. P Tosi, Z. Naturforsch. 54a, 180 (1999)], using a model which accounts for ionic deformability through (i) effective valences and (ii) electrostatic and overlap polarizabilities. The main disposable parameters of the model are adjusted to the Fe-Cl bond length in FeCl3 monomer molecule and to the Fe-Fe bond length and a bond-stretching frequency in the Fe2Cl6 molecule. The results are used to evaluate the structure of the AlFeCl6 molecule, which has so far only been inferred from the observed Raman spectrum in mixed vapours. Extensive comparisons with data on molecular vibrational frequencies are also presented for Fe2Cl6 and AlFeCl6.