Thermoelastic and dynamical properties of C2F6

Abstract

Neutron scattering and diffraction were used to measure momentum distributions of carbon and fluorine atoms and diffraction patterns of solid C2F6 from 20 K up to its melting point of 173.3 K, and the momentum distributions of carbon and fluorine in liquid C2F6 at 180 K. At high temperatures, heavy diffuse scattering indicates much molecular orientational disorder. On cooling, a structural phase transformation from bee crystal to triclinic crystal was verified at 103 K. The mean square momentum of fluorine atoms increases with temperature in the lower temperature solid phase but shows little temperature dependence in the higher temperature solid phase, with no significant change below and above the melting temperature. X-ray diffraction was used to measure temperature dependence of lattice parameter for the bee solid and to characterize crystalline perfection. A measurement of longitudinal elastic wave velocity in the bee phase was made, and a set of thermoelastic properties, including longitudinal and transverse velocities of sound, adiabatic and isothermal compressibility, elastic Debye temperature, Grtineisen parameter, heat capacity at constant volume, intramolecular and librational components of heat capacity were calculated for a range of temperatures for the bee phase. Comparisons of the data to a model with hindered rotation of the molecules were made. Many C2F6 properties were found to be similar to those of SF6.