NEUTRON-SCATTERING FROM AMMONIUM-SALTS .2. REORIENTATIONAL MOTION OF AMMONIUM-IONS IN OCTAHEDRAL ENVIRONMENTS

For pt.I see ibid., vol.12, p.209 (1979). Neutron scattering from NH 4I at room temperature and (NH4)0.16K 0.84I 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.