Isotropic and anisotropic Raman scattering from molten alkali-metal fluorides

Abstract

Isotropic and anisotropic Raman scattering spectra of molten LiF, KF, and CsF and of molten mixtures KF–LiF and CsF–LiF have been measured at temperatures up to 900°C. For the single component molten salts the reduced representations of the spectra show one anisotropic and two isotropic bands. The data are discussed and interpreted in terms of interaction induced polarisability fluctuations. The dipole–induced-dipole interactions predominate the CsF anisotropic spectra while polarisability fluctuation due to anion–cation overlap interactions give rise to the strong isotropic scattering in LiF. The anisotropic Raman scattering intensity from the binary mixtures appears to be a superposition of the corresponding intensities of the component salts, while the isotropic spectra are characterised by a new band at ∽325 cm⁻¹ which is common for both the KF–LiF and the CsF–LiF binaries. The origin of this band is attributed to the formation of long lived associated configurations of the type (LiF_x)A, (A=K, Cs) in the melt mixtures.