Structure, lattice parameters, and thermal expansion anisotropy of $C70$ fullerite

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.