Vibrational relaxation of hexafluoroaluminate ion inmolten cryolites

摘要

Raman spectra have been measured for dilute solutions of sodium aluminum fluoride (cryolite) in alkali metal chlorides, CsCl, RbCl, KCl, NaCl and LiCl to study the effect of the cations on the relative concentration and vibrational relaxation of the hexafluoraluminate and tetrafluoraluminate ions. Since the chloride ion does not enter the coordination sphere of the aluminum the hexafluoro and tetrafluoraluminate ions are essentially isolated in the environment of the alkali metal cation. Assignment of bands at about 560 and 620 cm~(-1) to the symmetric stretching modes of the AlF_6~(3-) and AlF_4~- ions in the melts is consistent with assignment bands of similar frequency in solid cryolite type crystals known to contain the discrete AlF_6~(3-) and AlF_4~- ions. From measurements of the relative intensities of the bands at about 560 and 620 cm~(-1) in the melts an approximate AlF_6~(3-) to AlF_4~- ratio of 4:1 was determined (except for lithium). Measurement of the full widths at half height (FWHH) indicated a considerable cation effect. The relative values for the FWHH were found to decrease with increased mass and size of the cation and the effect was much more pronounced for the hexafluoraluminate ion. Since the FWHH for the isotropic bands is inversely proportion the vibrational relaxation time the results indicate that small cations are associated with short life-times. The results are best explained on the basis of inhomogeneous broadening caused by differences in dephasing due to differences in the cation environment about the anion.