Vibrational modes and structure of lanthanide fluoride–potassium fluoride binary melts LnF3–KF (Ln=La, Ce, Nd, Sm, Dy, Yb)

Abstract

Raman spectra of a series of LnF₃–KF (Ln=La, Ce, Nd, Sm, Dy, Yb) binary melt mixtures have been measured at temperatures up to 1000°C and at compositions up to 40 mol.% for the mixtures with La and Ce and up to 25 mol.% for the remaining mixtures. The data indicate that at mole fractions X_LnF3⩽0.25 the LnF₆^3- octahedra are the predominant species giving rise to two main bands, one polarized, the other depolarized, which are assigned to the ν₁(A_1g) and ν₅(F_2g) vibrational modes of the octahedra, respectively. The ν₁ frequency varies almost linearly with the polarizing power of the lanthanide cation increasing from 445 cm^-1 (Yb) to 400 cm^-1 (La). Such a variation has not been observed in the corresponding binary melts involving chlorides and bromides; it is argued that the relative shielding of the Ln³⁺ by the anions are responsible for this behaviour. At mole fractions X_LnF3>0.25 the features of the reduced isotropic and anisotropic spectra are similar to those of the YF₃–KF (V. Dracopoulos, B. Gilbert, B. Børresen, G. Photiadis and G. N. Papatheodorou, J. Chem. Soc., FaradayTrans., 1997, 93, 3081). One polarized and two depolarized bands appear in the spectra which are interpreted to indicate that the predominant vibrations in these melts arise from distorted LnF₆^3- octahedra bound by common fluorides (edge sharing). The anisotropic scattering intensity was found to increase, relative to the isotropic intensity, with both increasing the size and mole fraction of the lanthanide cation. This unusual effect is attributed to dipole-induced-dipole interactions between cations in the melt and its variation from system to system is related to the size and polarizability of the Ln³⁺ cation and its relative shielding by the F^- anions.