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

• Zirconium and hafnium fluorides are present as MF 6 2– and MF 7 3– in the KF melts. • The influence of O2– on the speciation in melts has been studied by Raman and DFT. • Oxyfluorides exist in the form of MOF 5 3– with terminal oxygen structure. The influence of oxide ions on the speciation in molten KF containing ZrF 4 and HfF 4 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 MF 6 2– and MF 7 3– (M = Zr, Hf) in the KF melts with low MF 4 concentrations, whereas MF 6 2– dominate at high MF 4 concentrations. With the addition of 3–5 mol% Li 2 O into the KF-MF 4 (10 mol%) melts, oxyfluorides in the form of MOF 5 3– 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% Li 2 O was added into the KF melts with 30–40 mol% ZrF 4 and HfF 4. The formation of MOF 5 3– anion is consistent with the much lower average association energies of MF 7 3− than MF 6 2−. This makes it a lot more favourable to replace F− by O2− so as to generate MOF 5 3–, and the free F- ions released can further react with MF 6 2− to give MF 7 3−. As the Li 2 O 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. [ABSTRACT FROM AUTHOR]