Phase transition and thermal expansion of hexafluoroethane

Abstract

X-ray studies of the lattice parameters of polycrystalline hexafluoroethane provide data on the linear and volume coefficients of thermal expansion in the low- and high-temperature phases for temperatures of 5–130K. The thermal expansion of the low-temperature monoclinic phase is found to be highly anisotropic. The expansion anisotropy is similar to that usually observed in laminar crystals. Strong damping of the diffraction pattern is observed in the high-temperature phase. It is proposed that this effect may be related to peculiarities in the lattice dynamics of C2F6 owing to a strong rotational-translational interaction. In this case, increased disorder associated with rising temperature, in both the orientational and translational subsystems of the crystal, becomes probable, as does the formation of a “dynamic glass” state at a certain time. A previously observed shift of the temperature of the orientational phase transition to 70K is explained. The contributions of the translational and rotational subsystems to the specific heat are analyzed thermodynamically. Indications of extremely strong disinhibition of torsional rotation of C2F6 are obtained, which ultimately leads to a structural phase transition. The temperature variation in the Grüneisen coefficient for the low-temperature phase of hexafluoroethane is found to be qualitatively similar to that observed for ethane and other simple molecular substances. This suggests that the lattice dynamics and scenarios for orientational disorder in the region of the phase transition are similar for these substances.