A structural study of the hexafluorobenzene from liquid to supercritical conditions using neutron diffraction and molecular dynamics

This paper is devoted to an investigation of the local order in hexafluorob enzene by neutron diffraction and molecular-dynamics simulations. Experimen tally, the fluid has been studied under isobaric (P similar to 16 MPa) and isothermal (T similar to 573 K) conditions in a broad density domain rangin g from the liquid value (rho similar to 1600 kg m(-3)) down to typical dens ities on the supercritical domain (rho similar to 60 kg m-3). It is found t hat the translational ordering, which is characterized by two well defined shells of neighboring molecules at ambient conditions, is gradually weakene d and extends only to the first shell as the density decreases in the super critical domain. This behavior is in full agreement with the general trend reported for the positional ordering in our previous investigations for som e cyclic molecules. In marked contrast. the short range orientational order ing existing in the first shell of molecule is almost preserved. It is foun d that parallel and perpendicular configurations of a pair of neighboring m olecules coexist, although parallel configurations are found predominant at the shortest intermolecular distances. Finally, the structural evolution i n the three fluids, hexafluorobenzene, benzene and 1,3,5-trifluorobenzene a re comparatively discussed. It is argued that the orientational ordering in the hexafluorobenzene is closer to that existing in 1,3,5-trifluorobenzene than in liquid benzene. However, no presence of stacked dimers (C6F6)(2) h ave been revealed from both experimental and simulated structural analyze. (C) 2001 American Institute of Physics.