Chemistry Reference
In-Depth Information
Table 4.2.
Examples of BFS salts containing non-centrosymmetric
anions exhibiting anion ordering transitions
Formula
T
ao
q
r
References
1
(TMTSF)
2
ClO
4
24
(0,
2
, 0)
Pouget
et al.
, 1983
(
2
,
1
1
(TMTSF)
2
ReO
4
177
2
,
2
)
Moret
et al.
, 1982
(
2
, 0, 0)
(TMTSF)
2
NO
3
41
Moret
et al.
, 1983
(
2
,
1
1
(TMTTF)
2
ClO
4
70
2
,
2
)
Pouget
et al.
, 1982
(
2
,
1
1
(TMTTF)
2
ReO
4
160
2
,
2
)
Parkin
et al.
, 1983b
(
2
, 0, 0)
(TMTTF)
2
NO
3
50
Moret
et al.
, 1983
ab
-plane of (TMTSF)
2
PF
6
is depicted in Fig. 4.21. Along
c
alternate organic
(TMTSF) and inorganic (PF
6
) planes are stacked. The periodicity is OIOIOI
...
,
where O and I stand for organic and inorganic, respectively. If the lattice vectors are
defined by the PF
6
sublattice, there are two independent TMTSF sublattices, which
order according to the identity 1
1 superstructure (or identity transformation ma-
trix
E
). The examples given in Figs. 1.9 and 1.13, corresponding to herringbone
structures, can also be described by two independent 1
×
1 superstructures.
Note that PF
6
is centrosymmetric. If the anions were non-centrosymmetric, an
additional degree of freedom would be available in the case that such anions order.
In fact BFS with non-centrosymmetric anions exhibit anion ordering below a given
temperature, which can be classified as an order-disorder transition. Above the
transition temperature
T
ao
anions exhibit random orientations, but for sufficiently
low temperatures (low activation energy) they become ordered.
Table 4.2 shows some examples of BFS exhibiting such order-disorder transi-
tions with the measured
T
ao
and the reduced wave vectors
q
r
(Moret & Pouget,
1986). The anion ordering transitions have important consequences for the low-
temperature properties and ground states of these materials. This can be understood
by realizing that when the
a
∗
component of
q
r
is equal to 1/2, which means that the
period along
a
is doubled, the anion potential can open an energy gap at
E
F
thus
inducing a metal-insulator transition, as discussed in Section 1.7. This is the case
for many of the transitions given in Table 4.2. As shown in Fig. 1.30 the quasi 1D
character of the BFS is expressed by the preferential band dispersion along the
×
X
direction, which corresponds to the real-space
a
-direction. Doubling
a
induces a
gap opening at
−
2
a
. On the other hand, when the
a
∗
component of
q
r
is zero there
is no change of periodicity along
a
and the metallic character can be kept below
the transition.
π/
