An ionic model for FeCl3-based melts from a study of the molecular dimer and other molecular structures

A model of ionic interactions accounting for ionic polarizability through electrostatic and shell deformation dipoles is developed for applications to molten iron trichloride and its mixtures with alkali chlorides in the acidic range of composition. The main focus is on (i) bond lengths and bond angles in molecular clusters as possible precursors of local structures in melts, and (ii) stability of local structures against fluctuations into ionized stales. The determination of the ionic interactions is based on structural properties of the neutral monomer and dimer in the vapour phase, in parallel with, and using some input from, our earlier study of aluminium trichloride. The model is used to study the structure of charge-carrying clusters obtained from the monomer ((FeCl4)(-) and (FeCl2)(+)) and from the dimer ((Fe2Cl7)(-) and (Fe2Cl7)(-)); it is also tested through calculations of the vibrational frequencies of the various clusters. The role of the metal ion polarizability is discussed with regard to FeCl3 and Fe2Cl6. Comparisons with data from experiment and from molecular orbital calculations are presented.