VIBRATIONAL SPECTRA AND STRUCTURE OF MOLTEN METAL FLUORIDES

摘要

Raman spectroscopy has been used to elucidate the structure of metal fluorides and their mixtures with alkali metal fluorides. Isotropic and anisotropic Raman scattering spectra of molten alkali fluorides are reported and interpreted in terms of interaction induced polarizability fluctuations. Long lived associated configurations are formed in alkali metal fluoride mixtures. Raman spectral changes occurring upon melting the K_2ZnF_4 and K_4PbF_6 compounds indicate that the divalent cation in both cases is four-fold co-ordinated in the melt. The four-fold co-ordination predominates the structure of the PbF2-KF mixtures at all composition while for the ZnF_2-KF melt mixtures both four- and six- fold co-ordinated species appear to exist. Raman spectra of ZrF_4-KF molten mixtures suggest that the "octahedral" ZrF_6~(2-) species predominate the structure at all compositions. In dilute in ZrF4 melts an equilibrium between ZrF_6~(2-) and ZrF_7~(3-) is established. At high ZrF_4 contents the ZrF_6~(2-) octahedral are bound by corners and/or edges forming chain like structure. The observed structural trends of the metal fluoride melts are discussed and compared with the corresponding chlorides and bromides.