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

Molecular crystals of halogen–ethane derivatives C₂X_6–<i>n</i>Y_<i>n</i> (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 <i>P</i>2₁/<i>n</i> structure (<i>Z</i> = 4), while that of CCl₃–CF₂Cl is found to be orthorhombic <i>Pmna</i> (<i>Z</i> = 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