Dynamics and structure of solid hexafluoroethane

Neutron Compton profiles were measured of atomic momentum distributions in solid hexafluoroethane (C2F6) at saturated vapor pressure from 20 to 170 K, and of the liquid phase at 180 K. There are conflicting reports in the lit erature, from different kinds of measurements, about the nature and extent of solid phases of C2F6. Present neutron diffraction and thermal analysis s hows that there is a clear crystallographic transformation near 103 K from a higher-temperature body-centered-cubic (bcc) phase which has considerable orientational disorder to a lower-temperature phase of different symmetry, which has relatively little disorder. The C and F atomic momentum profiles are found to have only small dependencies upon temperature and upon conden sed phase. They are analyzed in terms of a dominant contribution from intra molecular vibrations plus contributions from translational and rotational m otions of molecules. The nature of rotational contributions is of special i nterest, because of questions about the extent of static versus dynamic dis order in this kind of crystal. Brief comparisons to related solids such as SF6 are made. (C) 1999 American Institute of Physics. [S0021-9606(99)51303- 2].