Optical Phonons and Symmetry of Tysonite Lanthanide Fluorides

The polarized infrared reflectance of LaF₃, CeF₃, PrF₃, and NdF₃ has been measured from 40 to 500 cm^-1 at 295, 78, and 7^∘{}K. Kramers-Kronig analyses of the reflectance spectra yield evidence for eleven infraredactive phonons in the σ polarization (E⊥c) and six in the π polarization (E∥c). These results, together with reported magnetic-resonance measurements, are consistent with a P3̄^'c^'1 magnetic space group. A theory and method for directly observing the longitudinal optical phonon frequencies of strongly anisotropic crystals in transmission are reported.