Chemistry Reference
In-Depth Information
Figure 1.16. Crystal structures of TMTSF-TCNQ. (a) Black phase,
P
1,
a
=
34
◦
,
67
◦
,
63
◦
.
0
.
388 nm,
b
=
0
.
764 nm,
c
=
1
.
885 nm,
α
=
77
.
β
=
89
.
γ
=
94
.
Crystallographic data from Bechgaard
et al.
, 1977. (b) Red phase,
P
1,
a
=
0
.
810
91
◦
. Crys-
tallographic data from Kistenmacher
et al.
, 1982. C, S and N are represented by
black, medium grey and light grey balls, respectively. H atoms are not represented
for clarity.
78
◦
,
49
◦
,
nm,
b
=
1
.
046 nm,
c
=
0
.
700 nm,
α
=
103
.
β
=
98
.
γ
=
94
.
favour incomplete CT. The TTF molecule is a good donor because its first
I
E
is low
(
0 eV), identical to heptafulvalene, and TCNQ a good acceptor because of its
low
E
A
value (
7
.
8 eV) (Klots
et al.
, 1974).
Table 1.7 summarizes some relevant organic conductors ordered by their
2
.
σ
RT
val-
ues at ambient pressure. HMTSF-TNAP exhibits to date the highest
σ
RT
value fol-
lowed byHMTSF-TCNQ. HMTSF-TCNQ is isomorphous withHMTSF-TCNQF
4
.
However, HMTSF-TCNQF
4
is an insulator, with
−
1
cm
−
1
, almost
nine orders of magnitude lower that HMTSF-TCNQ. This clearly differentiated
behaviour is related to the corresponding
10
−
6
σ
RT
∼
values. For HMTSF-TCNQ
=
0
.
74,
while for HMTSF-TCNQF
4
1, and therefore CT is complete, because TCNQF
4
is a far stronger electron acceptor than TCNQ, resulting in a fully ionic salt. For
comparison let us mention that for
trans
-polyacetylene doped with AsF
5
=
σ
RT
−
1
cm
−
1
(Chiang
et al.
, 1977).
It is interesting to note that the only
single component
molecular metal known
to date is Ni(tmdt)
2
, with
200
−
1
cm
−
1
(Tanaka
et al.
, 2001). The possi-
bility of obtaining such metals was predicted by theoretical work on the so-called
two-bands systems by showing that electron transfer could be induced internally
between two types of bands of the same component and that metal bis(dithiolene)
molecules could lead to single-component molecular metals (Canadell, 1997). This
is an example of internal doping.
(FA)
2
PF
6
is also an interesting material because its structure strongly resembles
that of the Bechgaard-Fabre salts (BFS), as will discussed next. It is composed of
slightly dimerized donor FA stacks piled in a zig-zag manner along the
a
-direction
σ
RT
400
