Quasielastic neutron scattering study of ammonium-ion reorientations in ammonium perchlorate

An investigation of the rotational motion of ammonium ions in NH4ClO4 is reported. Quasielastic neutron scattering measurements were made on a polycrystalline sample at temperatures between 66 and 150 °K. The shapes and widths of the quasielastic scattering peaks as a function of momentum transfer are consistent with instantaneous reorientation about the four C3 axes. Residence times are derived as a function of temperature yielding an activation energy of 2.3 kJ/mole and a frequency factor 1/τ0 of 3.7×1012 s−1.