Raman and Infrared Reflectivity Determination of Phonon Modes and Crystal Structure of Czochralski-Grown NaLnF₄ (Ln = La, Ce, Pr, Sm, Eu, and Gd) Single Crystals

Optical quality NaLnF 4 single crystals have been obtained from nonstoichiometric melts through the Czochralski technique, for Ln = La, Ce, Pr, Sm, Eu, and Gd. These crystals are potential laser host materials because of their high luminescence efficiency when doped with rare earth ions. In view of this application, the knowledge of the optical phonon modes of the crystals is mandatory. In this work, we present our polarized Raman scattering and infrared reflectivity studies of NaLnF 4 single crystals for large ionic radius lanthanides. For the different Ln ions, the spectra in each polarized configuration appeared to be very similar; the effect of Ln ion substitution on the crystal's vibrational modes was generally the hardening of the modes with decreasing ionic radii, with no important symmetry evolution. For all investigated crystals, the symmetries and number of observed phonon modes agree well with group theory predictions for the hexagonal P 6̄ group. The rather broad phonon bands and the presence of defect modes evidence the cationic disorder allowed by this structure.