Chemistry Reference
In-Depth Information
(see discussion on page 67). On the contrary, the quasi-regular zig-zag stacking
pattern and electronic structure of the third are essentially similar to those of the
prototypical triclinic metal and superconductor (TMTSF)
2
ClO
4
.
The CEC was carried out between two glass substrates held close together. The
working electrode was a gold deposit on one of the glass slides. As discussed in
Chapter 3, the CEC technique is well suited for the synthesis of single crystals of
known compounds already obtained by classical EC, as well as for the preparation
of new materials such as (TMTSF)
2
[W
6
O
19
] (Deluzet
et al.
, 2002b).
Let us start first with thin single crystals of (TMTTF)
2
ReO
4
. When prepared
in the confined geometry, single crystals with two different shapes are obtained:
rectangular and square platelets. The rectangular crystals are identified as the clas-
sical
P
1 phase with cell parameters identical to those reported in the literature
for (TMTTF)
2
ReO
4
(Kobayashi
et al.
, 1984). The crystal structure of the square-
shaped crystals is different, as determined by XRD. In Fig. 6.27 the structure for
both the classical, (TMTTF)
2
ReO
4
, and the new phase,
µ
-(TMTTF)
2
ReO
4
, are
reported.
Both salts have the same composition with a stoichiometry of two TMTTF
molecules per one ReO
4
anion. In each structure the organic molecules form slabs
but in the new one each molecular plane is rotated with respect to the next due to a
glide plane
c
in the crystal symmetry
C
2
/
c
. The anion is located on a
C
2
axis and
not on an inversion centre. The new phase
µ
-(TMTTF)
2
ReO
4
is thus a polymorph
of the (TMTTF)
2
ReO
4
salt. The molecular slabs are quite different between the
two structures. The TMTTF columns are more dimerized with a larger longitudinal
displacement of the molecule in
µ
-(TMTTF)
2
ReO
4
as shown in Fig. 6.28.
Overlap between sulfur atoms is less favourable, so interactions between
molecules are weaker in
µ
-(TMTTF)
2
ReO
4
than in (TMTTF)
2
ReO
4
. The elec-
µ
-(TMTTF)
2
ReO
4
is reported in Fig. 6.29. With the cell axis
chosen,
a
and
b
are the intra- and interstack directions, respectively. The band
structure of (TMTTF)
2
ReO
4
has been calculated using the same cell axis system
in order to facilitate the comparison (see Fig. 1.30 and discussion about the ori-
gin of the bands). The main differences between the two band structures are the
larger dimerization gap and thus the smaller dispersion of the upper band for
tronic structure of
µ
-
(TMTTF)
2
ReO
4
. These are ascribed to the decrease of the interdimer interaction
in
µ
-(TMTTF)
2
ReO
4
as a result of the considerably larger displacement along
the long molecular axis (see Fig. 6.28), whereas the intradimer interaction remains
almost identical in both polymorphs. Consequently, electronic repulsion can take
over and lead to a localized system with activated conductivity.
In agreement with this analysis, single-crystal conductivity measurements using
the four-probe technique reveals semiconducting behaviour for
µ
-(TMTTF)
2
ReO
4
,
−
1
cm
−
1
as shown in Fig. 6.30. In this case
σ
RT
0
.
011
and
E
a
0
.
17 eV.
