Melting of aluminium and iron trichloride crystals is accompanied by a stru
ctural transition from octahedral to tetrahedral coordination of the metal
ions, and a widely accepted interpretation of their liquid structure is tha
t it mainly consists of strongly correlated dimeric units. Such Al2Cl6 and
Fe2Cl6 molecules are stable in the vapour phase and coexist in gaseous mixt
ures together with AlFeCl6 molecules. In this work we extend to Fe2Cl6 and
AlFeCl6 our earlier study of the ionic interactions in Al2Cl6 [Z. Akdeniz a
nd M. P Tosi, Z. Naturforsch. 54a, 180 (1999)], using a model which account
s for ionic deformability through (i) effective valences and (ii) electrost
atic and overlap polarizabilities. The main disposable parameters of the mo
del 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 molecul
e. 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 mix
ed vapours. Extensive comparisons with data on molecular vibrational freque
ncies are also presented for Fe2Cl6 and AlFeCl6.