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

Abstract

For pt.I see ibid., vol.12, p.209 (1979). Neutron scattering from NH₄I at room temperature and (NH₄)_0.16K_0.84I and (NH₄)_0.16K_0.84Br 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.