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Fig. 9.25 Portions of X-ray
oscillation photographs of
[Pt
2
(
n
-PenCS
2
)
4
I]
1
(5)in
each phase taken with an
oscillation angle of 3
around
c
axis [
35
]. Axis
c
* (1D chain
direction) is vertical. (a) The
X-ray diffraction experiment
of the supercooled HT phase
was carried out at 300 K using
a crystal once heated to 350 K
ordering with sixfold periodicity develops in the LT phase, similarly to the semicon-
ducting region of 3 [
34
]. However, the crystal structure determination of the super-
structure corresponding to sixfold of a -Pt-Pt-I- period were unsuccessful.
9.2.7 Magnetic Properties
9.2.7.1
[Pt
2
(MeCS
2
)
4
I]
1
(1)
The temperature dependence of the magnetic susceptibility of [Pt
2
(MeCS
2
)
4
I]
1
(1)
is shown in Fig.
9.26
[
31
]. In the metallic region of 300
< T <
400 K, the
susceptibility decreases with lowering temperature on the order of 20 % as are
often observed in 1D molecular conductors. In most of 1D molecule-based
conductors, the temperature dependence of the susceptibility deviates consider-
ably from the Pauli temperature-independent paramagnetism. Kitagawa et al.
have stated that one of the most important effects on the deviations from the
Pauli-like susceptibility in 1 is the Coulomb interactions which play an important
role in the MMX-chain system considered as the Mott-Hubbard system with
U
300 K, on the other hand, the magnetic susceptibility
increases slightly with a slight convex in the
4
t
[
75
]. Below
T
M-S
ΒΌ
w
M
vs.
T
curve upon cooling to 90 K.
If a transition relates to Peierls (CDW) instability, the magnetic susceptibility
should become activated below the transition because of the freezing of both
charge and spin degrees of freedom. However, the opposite behavior has been
observed below 300 K in the susceptibility data. As described in the transport
properties, the MMX-chain system is strongly suggested to be the electronic
system having an effectively half-filled conduction band. The most important
effect especially for the 1D half-filled system is on-site Coulomb repulsion (
U
)
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