The structure of molten mixtures of iron(III) chloride with caesium chloride

Raman spectroscopy has been used to elucidate the structure of molten iron(III) chloride and its binary mixtures with caesium chloride. In order to overcome difficulties arising from the dark coloration of these melts and to ensure the spectral features, both conventional resonance Raman and micro-Raman spectra have been obtained. The spectral changes upon melting the solids Cs2NaFeCl6, CsFeCl4 and FeCl3 have also been measured. The FeCl4- tetrahedra were found to be present in both the solid and molten CsFeCl4, while a change of coordination from six-fold (FeCl63-) to four-fold (FeCl4-) occurs upon melting Cs2NaFeCl6. The FeCl4- tetrahedra are the predominant species in CsCl–FeCl3 mixtures containing up to 50 mol% FeCl3. The systematics and the temperature dependence of the spectra of the molten mixtures with mole fractions from 50 to 100 mol% FeCl3 indicate the presence of Fe2Cl7-, FeCl2+ and Fe2Cl5+ ionic species. Resonance Raman spectra of molten FeCl3 were measured and compared with the spectra of the corresponding solid and vapor. The data are interpreted to indicate a self-ionization of the molecular Fe2Cl6 melt: 2Fe2Cl6Fe2Cl5+ +Fe2Cl7-. The proposed model accounts for both the near-ionic conductivity of the melt and the structural data obtained by neutron diffraction.