VIBRATIONAL-MODES AND STRUCTURE OF LIQUID AND GASEOUS ZIRCONIUM TETRACHLORIDE AND OF MOLTEN ZRCL4-CSCL MIXTURES

Raman spectra of liquid and gaseous zirconium tetrachloride and of mol ten ZrCl4-CsCl mixtures have been measured. Changes of the relative Ra man intensities in the vapor phase, near and above the critical point, have established the dimerization reaction 2ZrCl(4)(g) reversible arr ow Zr2Clg(g), Delta H-R = -56.6 +/- 1.1 kJ mol(-1). Spectra of solid a nd molten zirconium chloride show that in the liquid phase ZrCl4, mono mers are present in equilibrium with polymerlike (ZrCl4)(n) species. A systematic investigation of the Raman spectra of the solid mixtures f ormed upon cooling the ZrCl4-CsCl melts at different compositions has shown that apart from the known Cs2ZrCl6 a new compound with stoichiom etry CsZr2Cl9 is also formed. The spectra of solid and molten Cs2ZrCl6 and CsZr2Cl9 show that ZrCl62- and the Zr2Cl9- are present in both ph ases. In molten ZrCl4-CsCl mixtures and at compositions x(ZrCl4) < 0.3 3 the ZrCl62- octahedra are predominant. In the range 0.33 < x(ZrCl4) < 0.66 the spectral changes with temperature and composition reflect a n equilibrium involving three ionic species: ZrCl62-, Zr2Cl9- and Zr2C l102- (or ZrCl5-). At mole fractions rich in ZrCl4 (x(ZrCl4) > 0.66) t he spectra indicate an equilibrium between the ionic Zr2Cl9-, the ZrCl 4 monomer and the (ZrCl4)(n) polymer-like species. All data suggest th at the value of n is small and most probably hexamers and/or dimers ar e the predominant 'polymer' species in these melts.