Chemistry Reference
In-Depth Information
140 cm
1
).
The Pt compounds become insulator with lowering temperature due to the lattice
dimerization originating from an effectively half-filled metallic band. The Pt
compounds shows not only small magnetic susceptibility corresponding to the itiner-
ancy of the unpaired electrons in the metallic state but also the paramagnetic-non-
magnetic transition originating from the regular electronic Peierls transition. The
valence-ordering state of the Pt MMX compounds of the dta family at the low
temperature is assigned to be the ACP state of -Pt
2+
-Pt
3+
-I
-Pt
3+
-Pt
2+
-I
-. On the
other hand, all the Ni MMX-chain compounds are Mott-Hubbard semiconductor due
to the strong on-site Columbic repulsion on the nickel atom and exhibit very strong
antiferromagnetic interaction. Furthermore, the spin-Peierls transition has been
observed in [Ni
2
(RCS
2
)
4
I]
1
(R
n
-Pen (5)) change from a static state to dynamic one at near 200 K (
Et (8),
n
-Pr (9)) at low temperature.
The partial oxidation of the metal atoms is essential to realize the metallic state
being stable down to the low temperature. In the case of the copper oxide
superconductors, the superconductivity has been realized by the partial oxidation
of the CuO
2
plane in the strongly correlated electronic systems (
U
/
t
¼
1) [
16
].
Therefore, the partial oxidation of the Ni MMX compounds having strong electron
correlation is very interesting from the viewpoint of appearance of the supercon-
ductivity that is never realized in the 1D electronic system.
The elongation of alkyl chain of the dithiocarboxylato ligand not only
eliminated the interchain S
S contacts and enhanced the one-dimensionality
of the chains, but also introduced motional degrees of freedom in the system. As a
consequence, several
interesting properties that were never observed in
[M
2
(MeCS
2
)
4
I]
1
(M
Pt (1), Ni (7)) have appeared. By the elongation of the
alkyl chains in dithiocarboxylato ligands, [Pt
2
(RCS
2
)
4
I]
1
(R
¼
¼ n
-Pr (3),
n
-Bu
(4), and
n
-Pen (5)) underwent two phase transitions at near 210 K and above room
temperature, indicating the existence of the LT, RT, and HT phases. The period-
icity of crystal lattice along 1D chain is threefold of a -Pt-Pt-I- unit, and the
structural disorders were detected for the dithiocarboxylato group and the alkyl
chain belonging to only the central dinuclear units in the threefold periodicity. In the
HT phase, the dithiocarboxylato groups of all the dinuclear units were disordered.
Ikeuchi and Saito have revealed from the heat capacity measurements that the
entropy (disorder) reserved in alkyl groups in the RT phase is transferred to the
dithiocarboxylate groups with the RT-HT phase transition. Whereas, the dithiocar-
boxylate groups of all the dimetal units in the LT phase of [Pt
2
(
n
-BuCS
2
)
4
I]
1
(4)
were ordered. The Pt MMX-chain compounds exhibited a rapid increase in resis-
tivity and a drastic change in the magnetism to the diamagnetic state with the
structural phase transition from the RT phase to LT one at near 200 K. This fact
suggests that the dynamics (motional degrees of freedom) of the dithiocarboxylato
ligands and bridging iodine atoms affects the electronic and magnetic systems
through the electron-lattice interaction. On the other hand, the Ni MMX-chain
compound, [Ni
2
(
n
-PrCS
2
)
4
I]
1
(9), behaved the 1D antiferromagnetic chain in the
both RT and LT phases, and only the coupling constant
J
slightly changed with the
structural phase transition. This fact demonstrates that, unlike the platinum
compounds, the electronic system of the Ni MMX-chain compounds in which the
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