Neutron scattering from ammonium salts. II. Reorientational motion of ammonium ions in octahedral environments

For pt.I see ibid., vol.12, p.209 (1979). Neutron scattering from NH 4 I at room temperature and (NH 4 ) 0.16 K 0.84 I and (NH 4 ) 0.16 K 0.84 Br from 98K to room temperature are reported. Measurements using a fixed elastic window technique show that reorientation rates of NH 4 + ions in all the salts are fast ( tau approximately 5 ps) and similar at room temperature. This is a feature typical of the octahedral environments in these salts. Quasi-elastic scattering data has been utilised to derive reorientation rates, the NH 4 + ion form factor and information regarding the reorientational barrier. A number of reorientational models have been considered and it is shown that none of the models proposed in the literature can explain neutron diffraction, quasi-elastic and inelastic scattering data simultaneously. A new model, which has the qualitative features required to explain all these data, is proposed.