10 research outputs found
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>2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>, <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><sub>3</sub><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: CCl<sub>3</sub>–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl
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
Polymorphism in Halogen-Ethane Derivatives: CCl<sub>3</sub>–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl
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
Polymorphism in Halogen-Ethane Derivatives: CCl<sub>3</sub>–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl
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
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>2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>, <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><sub>3</sub><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: CCl<sub>3</sub>–CCl<sub>3</sub> and ClF<sub>2</sub>C–CF<sub>2</sub>Cl
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
Polymorphism in Halogen–Ethane Derivatives: CCl<sub>3</sub>–CF<sub>2</sub>Cl and CF<sub>3</sub>–CF<sub>2</sub>Cl
Molecular crystals of halogen–ethane derivatives
C<sub>2</sub>X<sub>6–<i>n</i></sub>Y<sub><i>n</i></sub> (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 (CCl<sub>3</sub>–CF<sub>2</sub>Cl) and 1-chloro-1,1,2,2,2-pentafluoroethane
(CF<sub>3</sub>–CF<sub>2</sub>Cl), 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 CF<sub>3</sub>–CF<sub>2</sub>Cl is found to be fully ordered, with
a monoclinic <i>P</i>2<sub>1</sub>/<i>n</i> structure
(<i>Z</i> = 4), while that of CCl<sub>3</sub>–CF<sub>2</sub>Cl 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: CCl<sub>3</sub>–CF<sub>2</sub>Cl and CF<sub>3</sub>–CF<sub>2</sub>Cl
Molecular crystals of halogen–ethane derivatives
C<sub>2</sub>X<sub>6–<i>n</i></sub>Y<sub><i>n</i></sub> (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 (CCl<sub>3</sub>–CF<sub>2</sub>Cl) and 1-chloro-1,1,2,2,2-pentafluoroethane
(CF<sub>3</sub>–CF<sub>2</sub>Cl), 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 CF<sub>3</sub>–CF<sub>2</sub>Cl is found to be fully ordered, with
a monoclinic <i>P</i>2<sub>1</sub>/<i>n</i> structure
(<i>Z</i> = 4), while that of CCl<sub>3</sub>–CF<sub>2</sub>Cl 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: CCl<sub>3</sub>–CF<sub>2</sub>Cl and CF<sub>3</sub>–CF<sub>2</sub>Cl
Molecular crystals of halogen–ethane derivatives
C<sub>2</sub>X<sub>6–<i>n</i></sub>Y<sub><i>n</i></sub> (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 (CCl<sub>3</sub>–CF<sub>2</sub>Cl) and 1-chloro-1,1,2,2,2-pentafluoroethane
(CF<sub>3</sub>–CF<sub>2</sub>Cl), 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 CF<sub>3</sub>–CF<sub>2</sub>Cl is found to be fully ordered, with
a monoclinic <i>P</i>2<sub>1</sub>/<i>n</i> structure
(<i>Z</i> = 4), while that of CCl<sub>3</sub>–CF<sub>2</sub>Cl 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 (C<sub>6</sub>Cl<sub>5</sub>NO<sub>2</sub>). 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><sup>γ</sup>. 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