Structural and dynamical properties of a number of molecular crystals
have been investigated by coherent inelastic neutron scattering,Raman
scattering and neutron powder diffraction.
Measurements of the phonon dispersion relations in perdeuteronaphthalene
have been assigned with the aid of a rigid molecule lattice
dynamics calculation. The intermolecular forces were derived from semi -
empirical potential functions which represented the interactions between
non -bonded atoms. An attempt was made to improve the model by altering
the potential function parameters to get a closer correspondence between
observed and calculated frequencies.
The method of total profile refinement has been used in the analysis
of neutron powder diffraction data from perfluorodiphenyl, octafluoro-
naphthalene and para- .diiodo and para -dibromo -tetrafluorobenzene. It has
been shown that it is possible, by means of reasonable constraints, to
refine a molecular structure involving a large number of atoms. The
Rietveld program for structure refinement was used for perfluorodiphenyl
but the constraint facilities of this program have been found to be
inadequate for the lower symmetry systems of interest. The development
and use of a new refinement program, embodying more versatile constraint
facilities is described.
The Raman spectrum of perfluorodiphenyl has been interpreted in
terms of a simple polarisability model. The low frequency totally
symmetric spectra are explained by the coupling of a lattice vibration
with a torsional mode of the molecule.
The phase transition in octafluoronaphthalene has been investigated
by Raman scattering and neutron powder diffraction. The weight of
experimental evidence points to a unit cell doubling with no change in
space group symmetry. Lattice dynamics calculations support this, and
indicate that an instability in a zone boundary acoustic mode, may be
associated with the transition.