Vibrational Spectra of Nb(V) Fluoro and Oxofluoro Complexes Formed in Alkali-Metal Fluoride Melts

Raman spectra of the molten system LiF-NaF-KF-K2NbF7 at 650 °C indicate the presence of NbF72- ions with vibrational frequences at 626 (p), 371 (dp) and 290 cm-1 (dp). With the addition of small amounts of oxide, vibrational frequences due to a NbOF.,(n-3)- species are observed at 921 (p), 583 (p) and 307 cm-1 (dp) for oxide to niobium(V) mole ratios up to approximately 2. The most probable value of n is 5. Infrared spectra of solidified melts support the existence of a niobium-oxygen double bond in this complex. The vibrational spectrum is in accord with the band pattern of monomeric NbOF52- with a C4v symmetry. Further additions of oxide lead to formation of NbO2Fn(n-1)- complexes. Bands in the Raman spectrum of the melt at 878 and 815 cm-1 and in the infrared spectrum of the solidified melt at 879 and 809 cm-1 are ascribed to stretching vibrations of the NbO2 entity. The Raman spectrum and the infrared spectra are consistent with the presence of NbO2F43- ions of C4v symmetry. Melts saturated with oxide show vibrational frequencies that are likely to be due to [NbO3Fn](1+n)--containing species which, furthermore, seem to polymerize.