Influence of oxide ions on the speciation in molten KF-ZrF4 and KF-HfF4: A Raman spectroscopic and theoretical investigation

Abstract The influence of oxide ions on the speciation in molten KF containing ZrF4 and HfF4 has been studied by using Raman spectroscopy, and identification of the structures is carried out by density functional theory calculations within the polarization continuum model. Zirconium and hafnium fluorides are present as MF62– and MF73– (M = Zr, Hf) in the KF melts with low MF4 concentrations, whereas MF62– dominate at high MF4 concentrations. With the addition of 3-5 mol% Li2O into the KF-MF4 (10 mol%) melts, oxyfluorides in the form of MOF53– were observed with characteristic terminal M-O stretching bands at 775 (Zr) and 763 (Hf) cm–1. However, these oxyfluorides were not observed when 5 mol% Li2O was added into the KF melts with 30-40 mol% ZrF4 and HfF4. The formation of MOF53– anion is consistent with the much lower average association energies of MF73− than MF62−. This makes it a lot more favourable to replace F− by O2− so as to generate MOF53–, and the free F- ions released can further react with MF62− to give MF73−. As the Li2O concentration is increased to 15 mol%, the melts are no longer homogeneous and the Raman bands due to zirconium and hafnium oxides coming into exist.