The far-infrared spectra of thin films of NH4F and ND4F and the Raman spectra of polycrystalline samples of both compounds have been recorded at -170°C. Two low-frequency lattice vibrations were observed in each spectrum and these have been assigned as optical translations on the basis of their isotopic shift factors. Librational modes were not observed in any of the spectra.
J. R. Durig and D. J. Antion, "Far-Infrared and Raman Spectra of Ammonium Fluoride and Ammonium Fluoride–d4," Appl. Spectrosc. 24, 16-20 (1970)