Rotational excitations in K1−x(NH4)xI mixed crystals: A neutron scattering study

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Abstract
A neutron scattering study was performed on K1−x(NH4)xI mixed crystals (x≤0.44) at temperatures T≤20 K. Our results show in a direct fashion that the NH+4–NH+4 interaction contributes significantly to the crystal‐field potential. A quantitative discussion in terms of a locally perturbed field is applied to the x=0.02 results. The barrier increase due to the presence of a single NH+4 ion cannot be ascribed to octopole–octopole interactions alone. A qualitative model, which includes dipole–dipole interactions, is presented. The temperature study on a sample with x=0.28 shows a continuous transition from a quantum regime to a classical regime of reorientational motions.