Structure and thermal expansion of the low-temperature phase of SF6

Powder x-ray and neutron diffraction studies of the crystalline structure, lattice parameters, and thermal expansion coefficients of sulfur hexafluori de 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(C -2k(3)), with Z=6, in which 1/3 of the SF6 molecules occupy the positions o f higher symmetry (2/m) and 2/3 of the molecules the lower one (m). As foll ows from the analysis of the structural results obtained, the availability of two types of molecular local symmetry positions is responsible for the a nisotropic 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 T-c of the or ientational phase transition. A detailed comparison the present results wit h the known data in the literature is carried out. (C) 2000 American Instit ute of Physics. [S1063-777X(00)01204-4].