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 C2/m, with lattice parameters a = 17.9835(21) Å, b = 10.3642(11) Å, c = 6.3014(8) Å, and β = 94.410(5)° at 323 K, Z = 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 (Cmca, Z = 4) with lattice parameters a = 6.305(4) Å, b = 10.177(12) Å, c = 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, Im3̅m), a consequence of the pseudospherical molecular shape of halogeno-ethane C2X6–nYn (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