Lattice dynamics of adamantane in the disordered phase

Abstract

Nearly complete transitional phonon dispersion curves have been measured from the orientationally disordered (plastic) phase of crystalline adamantane. The translational modes were observed above a broad diffuse scattering component which is believed to arise from damped librational modes. No discrete librational modes were observed. The transitional modes are fitted by the solution of the dynamical matrix for a force field assumed to arise from a superposition of inter-atomic Buckingham potentials from all pairs of atoms between nearest and next-nearest-neighbour molecules. This model has been used to calculate the phonon dispersion curves for the low temperature, tetragonal phase of adamantane. The elastic and inelastic diffuse scattering have also been measured.