Vibrational modes and structure of liquid and gaseous zirconium tetrachloride and of molten ZrCl4–CsCl mixtures‡

Abstract

Raman spectra of liquid and gaseous zirconium tetrachloride and of molten ZrCl₄–CsCl mixtures have been measured. Changes of the relative Raman intensities in the vapor phase, near and above the critical point, have established the dimerization reaction 2ZrCl₄(g) ⇄ Zr₂Cl₈(g), ΔH_R = –56.6 ± 1.1 kJ mol^–1. Spectra of solid and molten zirconium chloride show that in the liquid phase ZrCl₄ monomers are present in equilibrium with polymer-like (ZrCl₄)_n species. A systematic investigation of the Raman spectra of the solid mixtures formed upon cooling the ZrCl₄–CsCl melts at different compositions has shown that apart from the known Cs₂ZrCl₆ a new compound with stoichiometry CsZr₂Cl₉ is also formed. The spectra of solid and molten Cs₂ZrCl₆ and CsZr₂Cl₉ show that ZrCl₆^2– and the Zr₂Cl₉^– are present in both phases. In molten ZrCl₄–CsCl mixtures and at compositions x_ZrCl4 < 0.33 the ZrCl₆^2– octahedra are predominant. In the range 0.33 < x_ZrCl4 < 0.66 the spectral changes with temperature and composition reflect an equilibrium involving three ionic species: ZrCl₆^2–, Zr₂Cl₉^– and Zr₂Cl₁₀^2– (or ZrCl₅^–). At mole fractions rich in ZrCl₄ (x_ZrCl4 > 0.66) the spectra indicate an equilibrium between the ionic Zr₂Cl₉^–, the ZrCl₄ monomer and the (ZrCl₄)_n polymer-like species. All data suggest that the value of n is small and most probably hexamers and/or dimers are the predominant ‘polymer’ species in these melts.