Raman-spectroscopic study of lanthanide trifluorides with the -YF3structure

We have performed polarized Raman scattering measurements on several Czochralski-grown lanthanide trifluoride (LnF3) crystals presenting the β-YF3 structure (space group: Pnma). The phonon mode assignments for the DyF3 and LuF3 spectra are given for the first time. Besides this, the Raman spectra of TbF3, ErF3 and YbF3 complete and extend previous studies on these materials. The vibrational modes of LnF3 compounds are then correlated with those of the archetype β-YF3 crystal. The totally symmetrical fundamentals present a strong dependence on the scattering geometry, similar to what is shown by molecular crystals with weakly interacting molecules. Most of the Raman features are well behaved with the lanthanide ion substitution. Although the observed modes always involve all ions, it was possible to discern modes that are dominated by the motion of either the lanthanide or the fluorine ions. The intensity of some low frequency modes has proved to be quite dependent on the orthorhombic distortion of the quasi-hexagonal structure.