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

Raman spectra of a series of LnF(3)-KF (Ln = La, Ce, Nd, Sm, Dy, Yb) binary melt mixtures have been measured at temperatures up to 1000 degrees 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 less than or equal to 0.25 the LnF(6)(3-) octahedra are the predominant species giving rise to two main bands, one polarized, the other depolarized, which are assigned to the nu(1)(A(1g)) and nu(5)(F-2g) vibrational modes of the octahedra, respectively. The nu(1) 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 Ln(3+) 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. Borresen, G. Photiadis and G. N. Papatheodorou, J. Chem. See., Faraday Trans., 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 LnF(6)(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 Ln(3+) cation and its relative shielding by the F- anions.