Neutron quasielastic scattering study of the ammonium ion reorientations in a single crystal of NH4Br at 373 °K

The ammonium ion reorientations in a single crystal of NH4Br in its disordered CsCl phase have been investigated by quasielastic neutron scattering at 373°K. Neutron spectra were measured at four different crystal orientations (with the [110] planes of the crystal in the scattering plane) and data were recorded simultaneously at a variety of scattering angles providing a range of momentum transfers for elastic scattering, 0.5 Å−1 ≤ Q ≤ 2.5 Å−1. The experimental results were fit to models allowing instantaneous random jumps around the C2 and C3 axes of the (NH4)+ ions using a variety of analytical procedures that are described in some detail. The results of these fits establish conclusively that the ammonium ion reorientations are dominated by 90° jumps around the C2 axes with an average time τ between jumps of 3.2±0.4 psec at 373°K. The inelastic part of the measured neutron spectra shows a torsional vibration peak at 305 cm−1, in good agreement with previous values. The results and analysis demonstrate the value of single crystal measurements in neutron studies of dynamic orientational disorder in solids.