Lattice vibrations in a disordered crystal: adamantane in plastic phase

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.