208
10 Importance of Molecular Crystals
for TP is ca. 50 meV [56] in contrast to its transition temperature, T trs = 193.5 K.
These facts are consistent with the location of DFTP in Fig. 10.5, because the unified
model [32, 33] classifies the transition based on whether k B T trs is much smaller than
the barrier height or not.
We can assign an effective decrease in n, the number of phenyl groups, for the
decrease in the transition temperature. Interestingly, the same decrease (an arrow
in Fig. 10.5) reasonably reproduces the transition temperatures for DFTP (n = 3)
and DFQP (n = 4). The same shift also rationalizes the disappearance of the twist
transition in DFBP. These results exemplify the successful and artificial control of
the transition mechanism of a structural phase transition. It is noteworthy that the
twist transition also disappears in dihydroxybiphenyl [HO–(C 6 H 4 ) 2 –OH] [57, 58],
for which we expect a similar intramolecular potential for the twisting [58]. In this
case, however, the intermolecular interaction enhanced by hydrogen bonds plausibly
contributes to stabilizing the planar form of molecules.
As mentioned earlier, the pressure can be an external parameter to control the
nature of phase transitions. Since the pressure stabilizes the planar conformation of
p-polyphenyls in crystal, resulting in the notable change in the single-particle potential, it would effectively decrease n. Indeed, the pressure coefficients of the transition
temperature of BP and TP crystals are negative [59, 60]. The incommensurate structure of DFTP crystal [52] is compatible with such instability predicted theoretically
for TP crystal under pressure [61]. This compatibility is another support for the conclusion that the twist transition of DFTP crystal is in between the typical displacive
and order-disorder transitions [52].
10.2.3 Impurity Effects on Structural Phase Transitions
Having learned the systematics of the twist transition in crystalline p-polyphenyls, we
can examine impurity effects on structural transitions for this series. This systematic
examination is distinct in, at least, two aspects from general studies. One is the
possibility of simultaneous examinations on two limiting cases, the displacive type
accompanying the soft mode in biphenyl (BP) to the order-disorder one for others
in non-substituted p-polyphenyls, in a single series of compounds. The other is that
we can utilize well-characterized impurities.
For the examination of the impurity effect on an order-disorder transition, utilized
as an impurity is a compound that has a similar molecular structure and significantly
different single-particle potential. The molecule is a derivative of 1,2,4,5-tetrazine
(C 2 H 2 N 4 ), which has a planar hexagon structure (similar to benzene) with two carbon
atoms most apart on the ring. The substitution of phenyl groups for two hydrogen
atoms yields a molecule (diphenyltetrazine, DPTZ) similar to that of p-terphenyl
(TP). Despite the similarity in the molecular shape, however, the absence of hydrogen
atoms attached to the central tetrazine ring guarantees the absence of the repulsion
between ortho hydrogen atoms. Since the repulsion is the exclusive contribution to the
hump in the single-particle potential at the planar molecular form of p-polyphenyls, a
10 Importance of Molecular Crystals
for TP is ca. 50 meV [56] in contrast to its transition temperature, T trs = 193.5 K.
These facts are consistent with the location of DFTP in Fig. 10.5, because the unified
model [32, 33] classifies the transition based on whether k B T trs is much smaller than
the barrier height or not.
We can assign an effective decrease in n, the number of phenyl groups, for the
decrease in the transition temperature. Interestingly, the same decrease (an arrow
in Fig. 10.5) reasonably reproduces the transition temperatures for DFTP (n = 3)
and DFQP (n = 4). The same shift also rationalizes the disappearance of the twist
transition in DFBP. These results exemplify the successful and artificial control of
the transition mechanism of a structural phase transition. It is noteworthy that the
twist transition also disappears in dihydroxybiphenyl [HO–(C 6 H 4 ) 2 –OH] [57, 58],
for which we expect a similar intramolecular potential for the twisting [58]. In this
case, however, the intermolecular interaction enhanced by hydrogen bonds plausibly
contributes to stabilizing the planar form of molecules.
As mentioned earlier, the pressure can be an external parameter to control the
nature of phase transitions. Since the pressure stabilizes the planar conformation of
p-polyphenyls in crystal, resulting in the notable change in the single-particle potential, it would effectively decrease n. Indeed, the pressure coefficients of the transition
temperature of BP and TP crystals are negative [59, 60]. The incommensurate structure of DFTP crystal [52] is compatible with such instability predicted theoretically
for TP crystal under pressure [61]. This compatibility is another support for the conclusion that the twist transition of DFTP crystal is in between the typical displacive
and order-disorder transitions [52].
10.2.3 Impurity Effects on Structural Phase Transitions
Having learned the systematics of the twist transition in crystalline p-polyphenyls, we
can examine impurity effects on structural transitions for this series. This systematic
examination is distinct in, at least, two aspects from general studies. One is the
possibility of simultaneous examinations on two limiting cases, the displacive type
accompanying the soft mode in biphenyl (BP) to the order-disorder one for others
in non-substituted p-polyphenyls, in a single series of compounds. The other is that
we can utilize well-characterized impurities.
For the examination of the impurity effect on an order-disorder transition, utilized
as an impurity is a compound that has a similar molecular structure and significantly
different single-particle potential. The molecule is a derivative of 1,2,4,5-tetrazine
(C 2 H 2 N 4 ), which has a planar hexagon structure (similar to benzene) with two carbon
atoms most apart on the ring. The substitution of phenyl groups for two hydrogen
atoms yields a molecule (diphenyltetrazine, DPTZ) similar to that of p-terphenyl
(TP). Despite the similarity in the molecular shape, however, the absence of hydrogen
atoms attached to the central tetrazine ring guarantees the absence of the repulsion
between ortho hydrogen atoms. Since the repulsion is the exclusive contribution to the
hump in the single-particle potential at the planar molecular form of p-polyphenyls, a