The high resolution inelastic neutron scattering spectrum of ammonium fluoride

The measured high resolution (Delta E/E similar to 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T-d. Because of LO-TO splitting the ammonium ion asymmetric stretch, v(3), has components calculated to be at higher frequencies than those of the symmetric stretch, v(1), which contradicts the assignment scheme produced from optical data.