The high resolution inelastic neutron scattering spectrum of ammonium fluoride

Abstract

The measured high resolution (ΔE/E ∼ 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, ν₃, has components calculated to be at higher frequencies than those of the symmetric stretch, ν₁, which contradicts the assignment scheme produced from optical data.