Polymorphism in Halogen-Ethane
Derivatives: CCl<sub>3</sub>–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl
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Abstract
The polymorphism of hexachloroethane, Cl<sub>3</sub>C–CCl<sub>3</sub>, has been reinvestigated by means of X-ray and neutron scattering.
The long time unknown structure of the intermediate phase II of Cl<sub>3</sub>C–CCl<sub>3</sub> 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,
ClF<sub>2</sub>C–CF<sub>2</sub>Cl, 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 ClF<sub>2</sub>C–CF<sub>2</sub>Cl has been found to be isomorphous with the orientationally disordered
phase I of Cl<sub>3</sub>C–CCl<sub>3</sub> (body centered cubic, <i>Im</i>3̅<i>m</i>), a consequence of the pseudospherical
molecular shape of halogeno-ethane C<sub>2</sub>X<sub>6–<i>n</i></sub>Y<sub><i>n</i></sub> (X = Cl, Y = F) molecular
crystals. The similarities of the intricate disorder of the intermediate
phases II of both Cl<sub>3</sub>C–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl compounds are analyzed, together
with the influence of the conformational disorder appearing in the
last compound