Neutron powder diffraction analysis and constrained refinement of perfluorodiphenyl

It is shown that with the small amount of data available in the neutron powder diffraction method, it is possible, by means of reasonable constraints, to refine a molecular structure involving a large number of atoms. The method has been used to refine the crystal structure of perfluorodiphenyl. Although the asymmetric unit contains 11 atoms, only three parameters are used to govern their positions, namely the central bond length, the orientation of the two rings about this bond, and the orientation of the whole molecule about the crystallographic c axis. The refinement procedure proved to be quite stable and a good fit to the observations was obtained. This procedure is clearly of value in the study of phase transitions in molecular crystals.