Polymorphism in 2‑X-Adamantane Derivatives (X = Cl, Br)

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group <i>P</i>2₁2₁2₁, <i>Z</i> = 4) and a high-temperature plastic phase (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 4) from 277.9 ± 1.0 K to the melting point at 413.4 ± 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (<i>P1̅</i>, <i>Z</i> = 2), which transforms to a monoclinic phase (<i>C</i>2/<i>c</i>, <i>Z</i> = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (<i>P6</i>₃<i>/mcm</i>, <i>Z</i> = 6) at ca. 241 K and the highest one, cubic (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 4), being stable from 244 ± 1.0 K up to the melting point at 467.5 ± 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br···Br as well as C–Br···H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks

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

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

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

Polymorphism in 2‑X-Adamantane Derivatives (X = Cl, Br)

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group <i>P</i>2₁2₁2₁, <i>Z</i> = 4) and a high-temperature plastic phase (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 4) from 277.9 ± 1.0 K to the melting point at 413.4 ± 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (<i>P1̅</i>, <i>Z</i> = 2), which transforms to a monoclinic phase (<i>C</i>2/<i>c</i>, <i>Z</i> = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (<i>P6</i>₃<i>/mcm</i>, <i>Z</i> = 6) at ca. 241 K and the highest one, cubic (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 4), being stable from 244 ± 1.0 K up to the melting point at 467.5 ± 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br···Br as well as C–Br···H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks

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

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

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

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

Double Primary Relaxation in a Highly Anisotropic Orientational Glass-Former with Low-Dimensional Disorder

The freezing of the cooperative reorientational motions in orientationally disordered (OD) molecular crystals marks the so-called “glassy” transition, which may be considered a lower-dimensional version of the structural glass transition. Although structural glasses display both positional and orientational disorder, in orientational glasses, however, the disorder involves exclusively the orientational degrees of freedom of the constituent molecules, while the molecular centers of mass form an ordered lattice. We report here on a glass-forming system with even fewer degrees of freedom, namely, the OD phase of a dipolar benzene derivative, pentachloronitrobenzene (C₆Cl₅NO₂). We probe the orientational dynamics of pentachloronitrobenzene as a function of temperature and pressure by means of dielectric spectroscopy (and high-pressure density measurements), and we show that, due to its anisotropy, the system exhibits a double primary relaxation feature associated with two distinct motions of the molecular dipole moment. This complex relaxation scenario shows a scaled dependence on the thermodynamic variables (<i>P</i>,<i>T</i>), with all relaxation times collapsing onto a single curve for each relaxation when plotted versus a specific-volume-dependent scaled variable <i>TV</i>^γ. Our findings are in line with the recent prediction by Dyre and co-workers of the existence of a hidden-scale invariance also in van der Waals crystalline materials