The infrared lattice spectra of a large number of rare-earth garnets have been recorded. Of the seventeen predicted lattice modes, we observed 15 for the aluminum series, 12 for the gallium, and 10 for the iron. A set of three bands in the high-frequency region of each spectrum has been assigned to the motion of the oxygen anions bonded to the rare-earth cation. The lowest-frequency bands in each spectrum have been assigned to the translational motions of the rare-earth ion. All of the frequencies were assigned to external motions of the crystal lattice.
NEIL T. MCDEVITT, "Infrared Lattice Spectra of Rare-Earth Aluminum, Gallium, and Iron Garnets," J. Opt. Soc. Am. 59, 1240-1244 (1969)