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Raman spectra of N&ReO, and ND4Re04 have been recorded at several temperatures between 77 K and 353K. As the temperature is increased, several bands due to both internal and external modes of the perrhenate ion are shifted to lower wavenumbers and there is a discontinuity in the slope of the wavenumber versus temperature curves near 200 K. This behaviour is correlated with recent NQR and X-ray diffraction results and supports the suggestion of a higher order phase transition near 200K. The bands due to the ammonium ion vibrations are broadened as the temperature is increased and the librational lattice mode is no longer observable above about 200 K. The nature of the motion of the ammonium ion in the lattice is discussed and it is pointed out that the presence of a librational lattice mode does not necessarily indicate an absence of reorientational motion.

Raman spectra of polycrystalline ammonium fluoride and fully deuterated ammonium fluoride were recorded at various temperatures between 80 and 300 K. The spectra were obtained using a cold stage attached to a Raman microscope. At low temperatures the Raman lines were sharper and shifted to higher wavenumbers compared with their room temperature counterparts. However, no indications of any crystalline phase change were observed at low temperatures. A strong line at 2876 cm −1 was assigned to the infrared inactive symmetric NH stretching mode n 1 .A 1 /. A very strong line at lower wavenumbers (2818 cm −1 ) was assigned to the antisymmetric stretching mode n 3 .F 2 /. Observed deuterium isotope shifts were in agreement with those calculated using the Redlich-Teller isotope product rule. The degenerate bending modes n 2 .E/ and n 4 .F 2 / were observed in the Raman spectra of NH 4 F and ND 4 F for the first time. The n 2 fundamental was split into doublets in the low-temperature spectra, as predicted from a factor group analysis based on the known crystal structure. Three bands due to translational lattice modes were observed between 400 and 200 cm −1 in the spectra of both isotopic molecules. Several bands in both spectra are assigned to overtones and combinations involving an ammonium ion libration, but no bands directly attributable to ammonium ion librations were observed.

Ab initio density functional calculations have been carried out on ammonium fluoride to determine the equilibrium structure and the transition state for rotation of the ammonium ion. The calculated equilibrium geometry agrees satisfactorily with crystallographic data. Optimization of the crystal geometry in the transition state for rotation results in significant distortion and displacement of the ammonium ion within the unit cell. Upon reexamination of the rotational transition states in ammonium chloride, similar distortion and displacement of the ammonium ion are found. The rotation process is similar to a carousel motion, in which the ion moves along the rotation axis and changes shape as it rotates. These results show that rigid ion models for ammonium ion rotational dynamics in crystals are, at best, incomplete.