Structure and Vibrational Spectra of the Vapour Molecules Fe₂Cl₆ and AlFeCl₆

54a,

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. 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.