Quasielastic neutron scattering experiments near the order–disorder phase transition on NH4Cl single crystals

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Abstract
Quasielastic neutron scattering experiments on NH4Cl single crystals near the order–disorder phase transition were interpreted in terms of a model with statistically independent three‐ and fourfold rotational jumps. By proper choice of the orientation of the scattering vector, the corresponding jump probabilities could be determined separately. At the phase transition, the fourfold jump rate, which introduces rotational disorder, changes stepwise by a factor of ∼3, whereas the threefold jump rate remains nearly unchanged. The results are consistent with measurements applying the ’’fixed‐window method’’ (high resolution experiment at zero energy transfer), and with a number of existing NMR T1 experiments. A model of rotations without a preferred rotational axis could be excluded. Results for the order parameter in the vicinity of the phase transition were also obtained. Good agreement was obtained between the activation energy for fourfold jumps from the experiments and the calculations by Hüller et al.