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isolated Pt
3+
state (or [Pt
2+
-Pt
3+
] unit) locating at the mismatch of the doubly
degenerate valence ordering phases of
Pt
2
þ
Pt
2
þ
I
Pt
3
þ
Pt
3
þ
I
½
I
I
½
Pt
3
þ
Pt
3
þ
Pt
2
þ
Pt
2
þ
. The formation of solitons has
been predicted theoretically in the case of CDW and ACP states in the MMX chains
by using Peierls-Hubbard model [
68
-
70
]. However, it has been actually
demonstrated only in the case of the ACP state of a dta system Pt
2
(
n
-pentylCS
2
)
4
I
by the ESR technique [
38
,
44
,
45
]. Therefore, the solitons in CDW state and those
in pop system are demonstrated for the first time in K
2
(NC
3
N)[Pt
2
(pop)
4
I]. Similar
phenomena are also found in K
2
(Me-NC
3
N)[Pt
2
(pop)
4
I]
and
4H
2
O and K
2
(Cl-NC
3
N)
[Pt
2
(pop)
4
I]
4H
2
O (Fig.
10.14a
inset) whereas the crystal becomes polycrystal after
the dehydration in these compounds.
Taken together, reversible single-crystal-to-single-crystal transformation
accompanied by changes in the optical gap and electronic state was realized in
K
2
(NC
3
N)[Pt
2
(pop)
4
I]
4H
2
O by the dehydration and rehydration.
10.3.3 Dependence of Water-Vapor-Induced Switching in MMX
Chains on the Length of Alkyldiammonium Ions
To reveal the dependence of the dehydration-rehydration of the complexes
on alkyldiammonium ion, XRD patterns of K
2
(B)[Pt
2
(pop)
4
I]
4H
2
O were
measured at three stages (1) Initial hydrated complex. (2) Dehydrated complex.
(3) After the exposure of dehydrated complex to H
2
O vapor for a day. As a result,
for B
2+
NC
3
N
2+
, Me-NC
3
N
2+
, Cl-NC
3
N
2+
,
cis
-NC
4
N
2+
, and
trans
-NC
4
N
2+
,
structural changes by dehydration-rehydration were reversible (Fig.
10.15a,
K
2
(NC
3
N)[Pt
2
(pop)
4
I]
¼
4H
2
O as a example). The XRD patterns of dehydrated
complexes can be fitted as orthorhombic
C
-lattice. Because the unit cell parameter
c
(chain axis) of dehydrated complexes are almost equal to that of K
2
(NC
3
N)
[Pt
2
(pop)
4
I], the
d
(Pt-I-Pt) of the dehydrated complexes (B
2+
Me-NC
3
N
2+
,
Cl-NC
3
N
2+
,
cis
-NC
4
N
2+
,
trans
-NC
4
N
2+
) are estimated to be close to that of
K
2
(NC
3
N)[Pt
2
(pop)
4
I]. In contrast, for B
2+
¼
NC
5
N
2+
and NC
2
OC
2
N
2+
, the
changes were only partly reversible and perfectly irreversible, respectively
(Fig.
10.15b, c
). Moreover, the XRD patterns of dehydrated K
2
(NC
5
N)[Pt
2
(pop)
4
I]
and K
2
(NC
2
OC
2
N)[Pt
2
(pop)
4
I] were quite different from their hydrated state. The
detail structural analysis is remaining issue that need to be solved.
The temperature dependence of the electrical conductivity parallel to chain axis
(
c
)ofK
2
(B)[Pt
2
(pop)
4
I]
¼
4H
2
O in each stage are shown in Fig.
10.16
[
31
,
46
]. The
s
RT
and activation energy (
E
a
) in each stage are summarized in Table
10.4
. The
conductivity of K
2
(NC
2
OC
2
N)[Pt
2
(pop)
4
I]
4H
2
O at stage 2 and 3 were unavailable
due to the quite low conductivity.
Except K
2
(NC
2
OC
2
N)[Pt
2
(pop)
4
I]
4H
2
O,
E
a
decreased by the dehydration due
to the shrinking of
d
(Pt-I-Pt), that is, increase of orbital overlap between the Pt
ð
5
d
z
2
Þ
and I (5p
z
) atoms.
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