The oriented gas approximation and the lattce vibrations of molecular crystals: Theory and application to perfluorobiphenyl

The oriented gas model of the polarisability of a molecular crystal provides a basis for the discussion of its Raman spectrum. This model is examined with a view to deriving information about eigenvectors of the zone centre lattice vibrations in these crystals. The theory of the general case is much simplified by symmetry conditions in certain instances. This is applied to the molecular crystal perfluorobiphenyl, C 12 F 10 , of recent interest. For this crystal it is found necessary to extend the theory to include the coupling of internal vibrations with the lattice vibrations. A simple model is found to account for this and the extent of the coupling is thereby estimated.