Chemistry Reference
In-Depth Information
TTF-CA more ionic, increasing
up to 0.7. The space group of the I-phase is
Pn
with two equivalent donor-acceptor dimers related by a glide plane with a ferro-
electric arrangement (see Fig. 6.33(b)). Further examples of mixed-stack organic
CT materials exhibiting N-I transitions are tetramethylbenzidine-TCNQ (
T
N
−
I
205 K) (Iwasa
et al.
, 1990) and DMTTF-CA (
T
N
−
I
65 K) (Aoki
et al.
, 1993).
In addition to the
P
- and
T
-induced N-I transition, such a transformation can
also be induced by laser irradiation (Collet
et al.
, 2003). With the development of
ultrafast lasers and reliable optics, it is now possible to induce a phase transition by
a light pulse and then to change the macroscopic state and the physical properties
of a material. An optical 300 fs laser pulse effectively switches the material from
the N to the I state on a 500 ps time scale, generating long-range structural order.
This kind of study has become possible thanks to time-resolved XRD possibili-
ties offered by latest generation synchrotron sources. The experiments referred to
here were performed with laser pulses of about 300 fs at 1.55 eV, with the light
polarization parallel to the CT stack. After laser irradiation, drastic changes are
observed in the intensity of some of the Bragg reflections. These intensity changes
are a direct signature of a strong structural reorganization in the photoinduced state.
The equilibrium state recovers in much less than 1 ms.
The case of TTTA is also extremely interesting since it shows RT magnetic
bistability (Fujita & Awaga, 1999). As TTTA samples are cooled from RT, the
paramagnetic
shows a slight decrease, which becomes rather abrupt at 230
K, and below 170 K
χ
0. TTTA is thus diamagnetic below this temperature
(LT-phase from Table 1.11). When heated from below this temperature (150 K),
χ
χ
0 up to 230 K and slightly increases above this temperature but a sudden
increase is observed at 305 K, recovering the RT value (HT-phase from Table
1.11). This paramagnetic-diamagnetic transition is of first order and exhibits a
surprisingly wide hysteresis loop (230-305 K). The major structural difference
between both phases is the strong dimerization found in the LT-phase. For the
HT-phase the intra- and intercolumn exchange coupling constants are
−
27.6 and
−
5.2 meV, respectively, while for the LT-phase the intradimer constant is much
larger,
−
112.2 meV.
Extrinsic structure-property relationships
The role of grain boundaries
In this section we illustrate the effect of grain boundaries on the conductivity of
thin films of the TTF-based molecular metals TTF-TCNQ and TTF[Ni(dmit)
2
]
2
.
In general the existence of grain boundaries hinders the metallic behaviour, in-
ducing semiconducting-like activated conduction (
−
1
cm
−
1
) because
at the intergrain interfaces stoichiometry is not necessarily preserved and because
σ
RT
<
10
