9.3 Phase Transitions Related to Molecular Deformation
179
(PE) phase above the higher transition temperature versus P2 1 of the ferroelectric
(FE) phase. The orientational dynamics of trichloromethyl and amino groups and the
partial orientational disorder of the trichloromethyl group of molecule B have been
reported. The report of the disorder and dynamics suggests that the phase transitions
are basically of order-disorder type. The excess entropy acquired in the course of
phase transitions confirmed this expectation [14]. It is noteworthy that the magnitude
of the excess entropy distinguishes whether the motional correlation is significant or
not in this disordering process (as will be discussed in Sect. 10.1). The experimental
result indicated that not a dimer but a molecule is the disordering entity: the PE phase
is a “mixture” of four types of dimers A:B, (A:B) inv
1 (this is equivalent to A inv :B inv ),
A:B inv , and A inv :B.
Crystalline TCAA serves as an example of the disordering process involving
molecular deformation and a novel strategy to design molecular ferroelectrics [14].
Although the majority of molecules are certainly polar (asymmetric), only a limited
number of ferroelectrics have been discovered and developed in neat molecular substances. Even if polar molecules crystallize in a polar structure that has a spontaneous
polarization (under ultimate electric field, for example), the polarization reversal,
which is the most important property for ferroelectrics, is hard because of severe
steric hindrance due to molecular anisotropy. The polarization reversal is essential
for achieving a large dielectric constant. In crystalline TCAA, the intramolecular
reorientation of rotors, the trichloromethyl groups (CCl 3 –), promotes the reversal,
as schematically shown in Fig. 9.1. Methyl groups and those substituted with three
identical atoms often reorient themselves in crystals because of their globular shape.
Despite their nearly globular shape, the reorientation can establish the inversion
symmetry, resulting in the disappearance of a finite dipole moment of a dimer. The
ease in reorientation efficiently contributes to the polarization reversal in crystalline
TCAA. A variety of intramolecular deformations such as reorientation, twist, or
crankshaft motion observed in trans-azobenzene (Sect. 9.2) potentially work as an
underlying mechanism of the polarization reversal because they can break/establish
the inversion symmetry of a unit cluster of interacting molecules. Since a dimer is
the easiest to be formed and widely observed in crystals, we reach the following
strategy for the development of molecular ferroelectrics: Design a crystal consisting of dimers of polar molecules that must have, at least, a partial structure easy to
be deformed. Although the saturated polarization realized by this strategy is smaller
than the fully ferroelectric arrangement of the molecular dipole moments, the strategy
automatically implements the mechanism of the polarization reversal in the crystal.
It is noteworthy that the strategy applies solely to molecular crystals because the
intramolecular deformation is intrinsic to molecular systems.
1 The subscript “inv” means the inverted counterpart of the original one.
179
(PE) phase above the higher transition temperature versus P2 1 of the ferroelectric
(FE) phase. The orientational dynamics of trichloromethyl and amino groups and the
partial orientational disorder of the trichloromethyl group of molecule B have been
reported. The report of the disorder and dynamics suggests that the phase transitions
are basically of order-disorder type. The excess entropy acquired in the course of
phase transitions confirmed this expectation [14]. It is noteworthy that the magnitude
of the excess entropy distinguishes whether the motional correlation is significant or
not in this disordering process (as will be discussed in Sect. 10.1). The experimental
result indicated that not a dimer but a molecule is the disordering entity: the PE phase
is a “mixture” of four types of dimers A:B, (A:B) inv
1 (this is equivalent to A inv :B inv ),
A:B inv , and A inv :B.
Crystalline TCAA serves as an example of the disordering process involving
molecular deformation and a novel strategy to design molecular ferroelectrics [14].
Although the majority of molecules are certainly polar (asymmetric), only a limited
number of ferroelectrics have been discovered and developed in neat molecular substances. Even if polar molecules crystallize in a polar structure that has a spontaneous
polarization (under ultimate electric field, for example), the polarization reversal,
which is the most important property for ferroelectrics, is hard because of severe
steric hindrance due to molecular anisotropy. The polarization reversal is essential
for achieving a large dielectric constant. In crystalline TCAA, the intramolecular
reorientation of rotors, the trichloromethyl groups (CCl 3 –), promotes the reversal,
as schematically shown in Fig. 9.1. Methyl groups and those substituted with three
identical atoms often reorient themselves in crystals because of their globular shape.
Despite their nearly globular shape, the reorientation can establish the inversion
symmetry, resulting in the disappearance of a finite dipole moment of a dimer. The
ease in reorientation efficiently contributes to the polarization reversal in crystalline
TCAA. A variety of intramolecular deformations such as reorientation, twist, or
crankshaft motion observed in trans-azobenzene (Sect. 9.2) potentially work as an
underlying mechanism of the polarization reversal because they can break/establish
the inversion symmetry of a unit cluster of interacting molecules. Since a dimer is
the easiest to be formed and widely observed in crystals, we reach the following
strategy for the development of molecular ferroelectrics: Design a crystal consisting of dimers of polar molecules that must have, at least, a partial structure easy to
be deformed. Although the saturated polarization realized by this strategy is smaller
than the fully ferroelectric arrangement of the molecular dipole moments, the strategy
automatically implements the mechanism of the polarization reversal in the crystal.
It is noteworthy that the strategy applies solely to molecular crystals because the
intramolecular deformation is intrinsic to molecular systems.
1 The subscript “inv” means the inverted counterpart of the original one.