Single-crystal X-ray diffraction data have been collected for disordered adamantane at room temperature (T = 293 K). The crystal structure is face-centred cubic with a = 9.445 (4),/k, Z = 4 and space group Fm3m. On the assumption of a rigid molecular skeleton, two kinds of orientational disorder have been investigated: isotropic free rotation of molecules around their centre of mass and hindered reorientations between equally weighted orientations. The second model gives the best fit and good agreement is found between observed and calculated structure factors (R w = 14.0%, R = 11.0%). The molecular packing then shows tertiary carbon atoms aligned in the I 1111 directions.

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Coherent inelastic neutron has been used to measure the high-symmetry dispersion curves in deuterated adamantane (C10D16) at room temperature. The single crystal was grown by slow sublimation. The essential points for a dynamical study of a plastic crystal are briefly summarised and a group-theoretical analysis of the normal modes is given. The existence of librations in adamantane is established. They are found to be severely damped at all wavevectors. A force constant model, allowing for atom-atom interactions between disordered molecules, is then presented in the rigid molecule quasi-harmonic approximation. It accounts quite well for neutron results even for the broad librations and gives interesting information about the interaction mechanism between neighbouring molecules in this orientationally disordered crystal.

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