Polymorphism in Halogen–Ethane Derivatives: CCl3–CF2Cl and CF3–CF2Cl

International audienceMolecular crystals of halogen-ethane derivatives C2X6-nYn (X = Cl, Y = F) are known to display order-disorder phase transitions involving changes of the translational, orientational, and conformational order. The appearance of a high-temperature orientationally disordered phase with a high symmetry lattice is expected for this set of compounds in view of their "pseudospherical" molecular geometry. In this work, we present a study of polymorphism of the compounds 1,1,1,2-tetrachloro-2,2-difluoroethane (CCl3-CF2Cl) and 1-chloro-1,1,2,2,2-pentafluoroethane (CF3-CF2Cl), in which conformational disorder is not present, by combining neutron (D2B and D1B instruments at the Laue-Langevin Institute) and X-ray scattering experiments. We show that despite the close molecular shapes and molecular structures of both compounds, strong differences concerning the disorder appear in the low-temperature phase. The low-temperature phase for CF3?CF2Cl is found to be fully ordered, with a monoclinic P21/n structure (Z = 4), while that of CCl3-CF2Cl is found to be orthorhombic Pmna (Z = 4) with a disorder concerning one Cl and one F sites, each one with a fractional occupancy of 50%. Details related to the high-temperature orientationally disordered phases (both body-centered-cubic) are also given