Lattice Vibrations and Structure of Rare-Earth Fluorides

The Raman spectra of La${\mathrm{F}}_3$, Ce${\mathrm{F}}_3$, Pr${\mathrm{F}}_3$, and Nd${\mathrm{F}}_3$ have been obtained with an argon-ion laser at room temperature and low temperatures. On the basis of polarizations and intensities of the lines an assignment is made of the Raman-active modes. The infrared-reflectance spectrum was measured with electric vector parallel to the crystal axis. The spectra are consistent with a hexamolecular unit cell, of ${D_{3d}}^4$ symmetry, with small distortions from the more symmetrical hexamolecular ${D_{6\mathrm{h}}}^3$ and bimolecular ${D_{6\mathrm{h}}}^4$ unit cells proposed earlier. The ${D_{3d}}^4$ symmetry appears to offer an explanation of apparent disagreements among earlier studies of these crystals.