Chemistry Reference
In-Depth Information
Figure 6.33. View along the
c
-axis of the crystal structure of TTF-CA: (a)
N-phase (
P
2
1
/
10
◦
) and
n
,
a
=
0
.
740 nm,
b
=
0
.
762 nm,
c
=
1
.
459 nm,
β
=
99
.
60
◦
). C,
Cl, S and O atoms are represented by black, dark grey, medium grey and light grey
balls, respectively. H atoms are omitted for clarity. Crystallographic data from Le
Cointe
et al.
, 1995.
(b) I-phase (
Pn
,
a
=
0
.
719 nm,
b
=
0
.
754 nm,
c
=
1
.
444 nm,
β
=
98
.
It is worth noting that when trying to obtain other polymorphs of Bechgaard
salts incorporating octahedral, fluorine-containing anions as in (TMTSF)
2
PF
6
or
(TMTSF)
2
SbF
6
only the triclinic phase is obtained.
Phase transitions
Some semiconducting organic CT complexes of mixed-stack architecture exhibit
the rather unusual neutral-to-ionic (N-I) phase transition upon variation of an exter-
nal variable of parameter hyperspace, such as
P
or
T
. The transition manifests itself
by a change of
and a dimerization distortion with the formation of donor-acceptor
dimers along the stacking axis in the I-phase.
This situation is different from what we previously saw for the BFS polymorphs
and, e.g., for TMTSF-TCNQ, because in those cases the physical properties are as-
sociated with different structures in a
discrete way
, that is the synthetic routes lead to
the different polymorphs and each polymorph exhibits its specific properties. How-
ever, what we encounter here is the
continuous
transition between two polymorphs,
with the resulting change of properties. TTF-CA is a prototype compound under-
going such a N-I transition (Torrance
et al.
, 1981). At atmospheric pressure and
RT, TTF-CA crystallizes in the N-phase (see Table 1.9). The space group is
P
2
1
/
n
with two symmetry related undimerized donor-acceptor pairs in the unit cell (see
Fig. 6.33(a)). TTF and CA molecules are located on sites with a centre of inversion
i
and form alternating stacks along the crystallographic
a
-axis. For the N-phase
0
.
3. Below
T
N
−
I
81 K the system undergoes a first order transition, making
