Polymorphism in Halogen-Ethane Derivatives: CCl₃–CCl₃ and ClF₂C–CF₂Cl

The polymorphism of hexachloroethane, Cl₃C–CCl₃, has been reinvestigated by means of X-ray and neutron scattering. The long time unknown structure of the intermediate phase II of Cl₃C–CCl₃ has been determined as monoclinic <i>C</i>2/<i>m</i>, with lattice parameters <i>a</i> = 17.9835(21) Å, <i>b</i> = 10.3642(11) Å, <i>c</i> = 6.3014(8) Å, and β = 94.410(5)° at 323 K, <i>Z</i> = 6. The polymorphism of 1,2-dichloro-1,1,2,2-tetrafluoro-ethane, ClF₂C–CF₂Cl, has also been investigated, and the structure of the intermediate phase II has been found to be orthorhombic (<i>Cmca</i>, <i>Z</i> = 4) with lattice parameters <i>a</i> = 6.305(4) Å, <i>b</i> = 10.177(12) Å, <i>c</i> = 8.714 (7) Å. The high-temperature phase I of ClF₂C–CF₂Cl has been found to be isomorphous with the orientationally disordered phase I of Cl₃C–CCl₃ (body centered cubic, <i>Im</i>3̅<i>m</i>), a consequence of the pseudospherical molecular shape of halogeno-ethane C₂X_6–<i>n</i>Y_<i>n</i> (X = Cl, Y = F) molecular crystals. The similarities of the intricate disorder of the intermediate phases II of both Cl₃C–CCl₃ and ClF₂C–CF₂Cl compounds are analyzed, together with the influence of the conformational disorder appearing in the last compound