76
4 Molecular Crystals
conductors, exhibit polymorphs, among which the thermodynamic relation cannot
be assessed experimentally.
Different polymorphs appear depending on the process of its formation. The variety of the way of polymorph formation includes deposition of vapor (including the
sublimation), crystallization from solution (the so-called recrystallization), crystallization from its melt, and transition from another crystalline phase. In the case of
crystallization from the solution, the solvent often affects the resulting polymorph.
However, other factors, such as temperature or thermal history of the system, can
be effective. We discussed one underlying aspect with somewhat broad applicability
in Sect. 2.1.3. In this context, it seems valuable to mention the so-called cold crystallization. It is a crystallization much below the equilibrium transition temperature
upon heating. Its realization means that the compound could be supercooled deeply
without crystallization. Usually, the cold crystallization occurs as a subsequent phenomenon to the softening
8 of some liquidus glasses (discussed in Chap. 8).
The crystal structure primarily governs the physical properties of crystalline materials. For example, p-NPNN (an abbreviation of p-nitrophenyl nytronyl nitroxide), the
first compound for purely organic ferromagnetism, exhibits not only the ferromagnetic polymorph but also an antiferromagnetic one [39, 41]. (ET) 2 I 3 is also another
example. The appearance itself of the superconductivity depends on the polymorph
besides the variety of transition temperature to a superconducting state [32–35]. The
polymorphism is thus essential in both fundamental study and practical application
of molecular crystals. Unfortunately, there is no universal rule to control polymorphism. The mechanism of the formation of polymorphs is too specific to the target
polymorph. Finding the condition through many trial and error is practically the only
strategy as long as the bulk material is considered. On the contrary, the identification
of a useful polymorph can give a target local-structure for nanotechnology, such as
molecular manipulation.
4.4 Cohesive Energy
4.4.1 Experimental Information
The state with minimum energy is certainly chosen at the absolute zero. The energy
gain by cohesion, i.e., cohesive energy, is a fundamental quantity to characterize the
crystal, accordingly.
For molecular crystals, estimates of cohesive energy can be obtained as the heat
of sublimation. The heat of sublimation can be estimated by direct calorimetry if the
rate of sublimation is relatively high. It is noteworthy that the heat of sublimation
does not coincide with the enthalpy of sublimation because of the effects of pressure.
Appropriate corrections are necessary. When the direct calorimetry is challenging to
8 The glass transition on heating.
4 Molecular Crystals
conductors, exhibit polymorphs, among which the thermodynamic relation cannot
be assessed experimentally.
Different polymorphs appear depending on the process of its formation. The variety of the way of polymorph formation includes deposition of vapor (including the
sublimation), crystallization from solution (the so-called recrystallization), crystallization from its melt, and transition from another crystalline phase. In the case of
crystallization from the solution, the solvent often affects the resulting polymorph.
However, other factors, such as temperature or thermal history of the system, can
be effective. We discussed one underlying aspect with somewhat broad applicability
in Sect. 2.1.3. In this context, it seems valuable to mention the so-called cold crystallization. It is a crystallization much below the equilibrium transition temperature
upon heating. Its realization means that the compound could be supercooled deeply
without crystallization. Usually, the cold crystallization occurs as a subsequent phenomenon to the softening
8 of some liquidus glasses (discussed in Chap. 8).
The crystal structure primarily governs the physical properties of crystalline materials. For example, p-NPNN (an abbreviation of p-nitrophenyl nytronyl nitroxide), the
first compound for purely organic ferromagnetism, exhibits not only the ferromagnetic polymorph but also an antiferromagnetic one [39, 41]. (ET) 2 I 3 is also another
example. The appearance itself of the superconductivity depends on the polymorph
besides the variety of transition temperature to a superconducting state [32–35]. The
polymorphism is thus essential in both fundamental study and practical application
of molecular crystals. Unfortunately, there is no universal rule to control polymorphism. The mechanism of the formation of polymorphs is too specific to the target
polymorph. Finding the condition through many trial and error is practically the only
strategy as long as the bulk material is considered. On the contrary, the identification
of a useful polymorph can give a target local-structure for nanotechnology, such as
molecular manipulation.
4.4 Cohesive Energy
4.4.1 Experimental Information
The state with minimum energy is certainly chosen at the absolute zero. The energy
gain by cohesion, i.e., cohesive energy, is a fundamental quantity to characterize the
crystal, accordingly.
For molecular crystals, estimates of cohesive energy can be obtained as the heat
of sublimation. The heat of sublimation can be estimated by direct calorimetry if the
rate of sublimation is relatively high. It is noteworthy that the heat of sublimation
does not coincide with the enthalpy of sublimation because of the effects of pressure.
Appropriate corrections are necessary. When the direct calorimetry is challenging to
8 The glass transition on heating.