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.
Solid solutions of (KI)i-t(NH4l)x with concentrations x=0.14, 0.43, and 1 were investigated using dielectric spectroscopy. The high-temperature rocksalt structure of these compounds exhibits a dipole moment of 1.4 D. For x-0.14 and 0.43 the NaCl structure is stable down to the lowest temperatures and the reorienting dipoles undergo a cooperative freezing transition devoid of long-range orientational order.
In the inelastic neutron scattering spectra of Ni-Ni-Bz(d6) obtained at 1.8 K the authors observed transitions differing only slightly, but systematically, from those between uniaxial free rotator levels of ammonia groups. They interpreted the deviations as the influence of a weak hindering potential. A twelve-fold cosine potential with a barrier height V12 of 16+or-3 meV together with a rotational constant of 0.71+or-0.02 meV for NH3 gave a satisfactory description of the experimental results. Spectra obtained at higher temperatures (up to 51 K) showed qualitatively the transition from quantum to classical behaviour.
Substitutional disorder in an orientationally disordered crystal generates static random strain fields. Strains cause scattering of orientational and displacive waves. The resulting friction processes may lead to structural arrest (nonergodic instability) at a threshold temperature T&. The slow dynamics associated with structural arrest is studied. The scattering law exhibits a dynamic central peak. The infiuence of microscopic relaxation processes on the dynamics of structural arrest is investigated. Phonon resonances and ultrasonic attenuation are calculated. A comparison with experiments in K(CN) Brl ranging from high-to low-frequency methods is made.
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