Interfaces - Molecular Organic Materials - page 183

Examples of these two behaviours are provided by (TMTSF) 2 ReO 4 and
(TMTSF) 2 ClO 4 . In (TMTSF) 2 ReO 4 the anions order at c . 180 K with a re-
duced wave vector ( 2 ,

1 1

2 , 2 ) and undergo a sharp metal-insulator transition with

16 eV (Jacobsen et al. , 1982). The anion ordering transition of
(TMTSF) 2 ClO 4 occurs at 24 K with q r =

E a = 0 .

1
2

0). This q r value implies that the
ClO 4 tetrahedra have the same orientation in a given ac -plane while the orientation
alternates in the b -direction. Since the period along a is preserved, no gap opening
is expected at T ao , as experimentally verified (Gubser et al. , 1982). This is the rea-
son why (TMTSF) 2 ClO 4 becomes a superconductor in spite of the anion ordering
transition.
(TMTTF) 2 BF 4 shows T ao

(0 , ,

( 2 ,

1
2

1

2 ) with
the corresponding energy gap opening at E F . However, we saw in Table 1.8 that it
becomes superconducting below 1.4 K, which is ascribed to the application of an
external pressure that modifies the intermolecular distances and thus the interaction
strength and geometry. This represents an additional example of the importance
of specifying the physical parameters when discussing the physical properties of
materials.
A phase transition of a different origin is found for Fabre salts constituted of cen-
trosymmetric anions (Chow et al. , 2000). No structural modifications have been
observed for these salts along the transition, hence deserving the term structureless,
and primarily the charge degrees of freedom are involved. The phase transition
is ascribed to charge ordering, where the electronic equivalence of the TMTTF
molecules is removed below a critical temperature making the charge dispropor-
tionate. This phase transition is ubiquitous for the TMTTF family and should be
included in the generic phase diagram of Fig. 1.19.
Experiments based on 13 C NMR spectroscopy performed on 13 C spin-labelled
(TMTTF) 2 PF 6 and (TMTTF) 2 AsF 6 samples reveal changes in the hyperfine cou-
pling signal belowa given temperature T co (Chow et al. , 2000). At RT, eachmolecule
is equivalent, but the two 13 C nuclei in each molecule have inequivalent hyperfine
coupling, giving rise to two spectral lines separated by

40 K at ambient pressure with q r = ,

10 kHz. Upon cooling,
the NMR spectrum of (TMTTF) 2 AsF 6 remains unchanged down to T co 105 K,

below which both the peaks split. From each molecule there is a signal from the
nucleus with a stronger hyperfine coupling and a signal from the nucleus with a
weaker hyperfine coupling. The doubling arises from two different molecular en-
vironments of roughly equal number, one with slightly greater electron density and
one with a reduced electron density. For (TMTTF) 2 PF 6 , T co 65 K.

Charge ordering below 220 K has also been found, in the organic conductor
(DI-DCNQI) 2 Ag (Hiraki & Kanoda, 1998).
LB films also form well-defined engineered organic/inorganic superlattices.
There are many examples in the literature and we just recall here some of them: