CRYSTAL-STRUCTURES OF THE LOW-TEMPERATURE PHASES OF TEF6

The crystal structure of tellurium hexafluoride in its low temperature phase has been solved from neutron powder diffraction measurements. At both 15 K and 190 K the structure is orthorhombic, space group Pnma, Z = 4. Molecules in the crystal are deformed very little from their structures in the vapour phase. A molecular packing analysis gives results in good agreement with the observed crystal structure. It suggests that the structure is determined primarily by steric interactions, with the ionic character of the interatomic contacts playing only a minor role. In previous electron diffraction studies on cold clusters of TeF6 a third phase interpreted as a monoclinic structure has been observed. Monte Carlo and molecular dynamics calculations on clusters also yield this phase, but suggest that it is metastable with respect to the orthorhombic structure in the low temperature phase regime of TeF6. The orthorhombic structure appears to be adopted preferentially when the inter- and intra-F-F distances are almost equal (TeF6 and transition metal hexafluorides) while, when these are unequal, the monoclinic structure appears to be preferred (SF6).