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

Abstract Raman spectra of molten binary mixtures of ZnF 2 –AF (A = K, Cs) and of PbF 2 –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 K 2 ZnF 4 and K 4 PbF 6 have been also measured. For the ZnF 2 –AF melt mixtures rich in AF a strong polarized band at ∼471 cm −1 is assigned to the ZnF 4 2 - ‘tetrahedral’ species. With increasing ZnF 2 mole fraction the polarized band shifts to high frequencies by ∼10 cm −1 indicating the formation of either a trigonal ( ZnF 3 - ) and/or bridged ‘tetrahedral’ ( Zn 2 F 6 2 - ) species. These species are also present in ZnF 2 -rich melt mixtures in equilibrium with a variety of distorted ‘octahedral’ ( ZnF 6 4 - ) species with a characteristic polarized band at ∼364 cm −1 . Most probably the structure of very rich in ZnF 2 melts and of pure molten ZnF 2 is predominated by bridged ‘octahedra’ forming a loose network. The spectra of PbF 2 –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 Pb m F 2 m + n n - of unknown stoichiometry and symmetry. With increasing PbF 2 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 ZnF 2 –KF and PbF 2 –KF binaries have been calculated and are correlated to cation shielding effects and to dipole-induced dipole interactions in these melts.