The thermal expansion coefficients of pure fullerite are determined on the basis of powder x-ray studies in the temperature range 30–293 K. The obtained results are in good agreement with dilatometric and neutron-scattering data. The data on thermal expansion are used to analyze the heat capacity at constant volume CV. The intramolecular component of CV is analyzed consistently and accurately taking into account the complete set of temperature-dependent intramolecular eigenfrequencies. The rotational component of heat capacity is obtained by subtracting the intramolecular and phonon contributions from the total CV. The phonon component is evaluated on the basis of the Debye model using the Debye temperature (ΘD(0)=55.4 K) calculated from the known sound velocities. The general and partial Grüneisen parameters are calculated as functions of temperature. The results obtained for the high-temperature phase indicate that rotations of C60 molecules are strongly hindered and intercorrelated.

X-ray diffraction studies of the two low-temperature phases of especially pure polycrystalline C70 fullerite are carried out in the temperature range 20–310 K. At room temperature a rhombohedral structure is established, and at T<276 K a monoclinic structure. The volume jump at the low-temperature phase transition is determined to be ∼8.5 cm3/mole, or 1.7%. The temperature dependence of the lattice parameters and of the linear and volume thermal expansion coefficients is investigated in the two phases. For both the intermediate and the low-temperature modifications a substantial anisotropy of the thermal deformation of the crystals along individual crystallographic directions is observed. The results are analyzed with the use of previous structural studies. It is shown that in the absence of rotations at low temperatures the lattice is unstable to monoclinic distortions, the estimated sign and order of magnitude of which agree with the observations.

Powder x-ray and neutron diffraction studies of the crystalline structure, lattice parameters, and thermal expansion coefficients of sulfur hexafluoride SF6 are performed in the temperature range 1.64–110 K. It is shown that the low-temperature phase (at T<94.3 K) is monoclinic, space group C2/m(C2k3), with Z=6, in which 1/3 of the SF6 molecules occupy the positions of higher symmetry (2/m) and 2/3 of the molecules the lower one (m). As follows from the analysis of the structural results obtained, the availability of two types of molecular local symmetry positions is responsible for the anisotropic character of molecular rotation and the presence of features on the temperature dependences of the structural and thermodynamic properties of SF6 crystals in the low-temperature phase, especially near Tc of the orientational phase transition. A detailed comparison the present results with the known data in the literature is carried out.