Infrared-Reflection Spectra of Molten Fluoride Solutions: Tantalum (V) in Alkali Fluorides

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Abstract
Infrared-absorption spectra derived from reflectivity measurements over the range 4000 to 200 cm−1 have been obtained for molten fluoride solutions of electrochemical interest. The results for alkali fluoride melts containing tantalum (V) over the composition range 21.3 KF:21.3 LiF:TaF5 to 2 KF:2 LiF:TaF5 show two strong bands which increase in intensity with increasing tantalum concentration. One band is found at 535 cm−1 and the other is a split (17-cm−1) band centered at 300 cm−1. The band positions are independent of concentration and temperature from the melting point to 800°C. Comparison of these spectra with those for well-characterized solid compounds such as K2TaF7 and CsTaF6 demonstrate that the TaF72— complex ion is the predominant species in these melts. A study of the influence of cations on the spectra of these melts show that when KF is present TaF72— is the only species observed. In the NaF:LiF solvent, TaF6− also appears. The spectra of vapors condensed from the various melts studied show interesting contrasts which are related to the vapor—liquid equilibria.