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

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Abstract
For pt.I see ibid., vol.12, p.209 (1979). Neutron scattering from NH4I at room temperature and (NH4)0.16K0.84I and (NH4)0.16K0.84Br from 98K to room temperature are reported. Measurements using a fixed elastic window technique show that reorientation rates of NH4+ 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 NH4+ 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.