Raman spectra and structure of molten fluorides: The binary mixtures MF2-AF (M = Zn, Pb; A = K, Cs) and the compounds K2ZnF4 and K4PbF6

Raman spectra of molten binary mixtures of ZnF2-AF (A = K, Cs) and of PbF2-KF have been measured at temperatures up to 1000 °C. Composition and temperature dependent measurements are related to the structure of these melts. The changes of vibrational modes upon melting the polycrystalline compound K2ZnF4 and K4PbF6 have been also measured. For the ZnF2-AF melt mixtures rich in AF a strong polarized band at ∼471 cm−1 is assigned to the ZnF42- 'tetrahedral' species. With increasing ZnF2 mole fraction the polarized band shifts to high frequencies by ∼10 cm−1 indicating the formation of either a trigonal (ZnF3-) and/or bridged 'tetrahedral' (Zn2F62-) species. These species are also present in ZnF2-rich melt mixtures in equilibrium with a variety of distorted 'octahedral' (ZnF64-) species with a characteristic polarized band at ∼364 cm−1. Most probably the structure of very rich in ZnF2 melts and of pure molten ZnF2 is predominated by bridged 'octahedra' forming a loose network. The spectra of PbF2-KF melt mixtures show mainly one polarized band at ∼400 cm−1 whose position is practically independent of composition. It appears that in KF-rich melt mixtures the structure is predominated by independent anionic species PbmF2m+nn- of unknown stoichiometry and symmetry. With increasing PbF2 mole fraction the anionic species interact with each other by sharing fluorides and forming larger structures with short lifetimes. The relative intensities of the isotropic and anisotropic spectra of both sets of the ZnF2-KF and PbF2-KF binaries have been calculated and are correlated to cation shielding effects and to dipole-induced dipole interactions in these melts.