Chemistry Reference
In-Depth Information
Figure 1.18. Crystal structure of the Bechgaard salt (TMTSF)
2
PF
6
.
P
1,
a
=
01
◦
.
C, Se, F and P atoms are represented by black, medium grey, dark grey and light
grey balls, respectively. H atoms are omitted. Crystallographic data from Thorup
et al.
, 1981.
39
◦
,
27
◦
,
0
.
730 nm,
b
=
0
.
771 nm,
c
=
1
.
352 nm,
α
=
83
.
β
=
86
.
γ
=
71
.
ClO
4
ion being smaller than the octahedral PF
6
ion, the TMTSF sheets become
closer to each other,
chemically
inducing the same effect as the
physical
hydrostatic
pressure. When replacing TMTSF by TMTTF one obtains the isostructural Fabre
salts.
Materials with the general formula (TMTCF)
2
X, where C stands for chalcogens
sulfur and selenium and X for monovalent anions, are known as Bechgaard-Fabre
salts (BFS). Because of the extremely high quality of the BFS that can be achieved,
these are, together with TTF-TCNQ and BEDT-TTF salts, the most extensively
studied crystalline MOMs and a matter of intensive research.
The generic
T-P
phase diagram of the BFS is shown in Fig. 1.19. The pressure
axis describes both the externally applied hydrostatic pressure
P
as well as the
internal
chemical
pressure, where the selenium-based materials (TMTSF) would
exhibit a larger pressure than the sulfur-based counterparts (TMTTF). The choice
of the origin on the pressure axis, (TMTTF)
2
PF
6
, will become evident below. Let
us start our discussion with the (TMTTF)
2
X materials, which are Mott-Hubbard
(MH) insulators (semiconductors) at RT and ambient pressure. Increasing pressure
at RT leads to a MH-metal (M) transition, thus in general (TMTSF)
2
X materials
are metals at 300 K. The pressure axis can also be viewed as a decreasing electron
localization axis.
We saw in Section 1.1 for N
2
that electron localization implies insulating ground
states and that localization can be reduced by applying external pressure. When
reducing temperature from RT down to about 20 K, (TMTTF)
2
X and (TMTSF)
2
X
