Abstract
Proton, fluorine, and deuteron magnetic resonance absorption line shapes, as well as 1H and 19F spin—lattice relaxation times, have been measured in polycrystalline cubic (NH4)2SiF6 and (ND4)2SiF6 from −170° to +100°C. The results demonstrate that the threefold proton disorder in the NH4 groups is indeed dynamic and not static, and suggest the following mode of reorientation: The NH4 tetrahedron is first excited from the ground torsional—vibrational state to nearly continuous levels above the potential barrier, and after a short interval of nearly free rotation again drops to one of the three possible equilibrium orientations in the ground state. On the average, each proton spends the same amount of time in each of the 12 equilibrium sites. Large-amplitude rotational oscilations in the ground state are also consistent with the experimental data. The existence of a 19F linewidth transition, as well as spin—lattice relaxation data, demonstrate the onset of hindered rotation of the SiF6 ions above room temperature.
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