Chemistry Reference
In-Depth Information
1D chains composed of repeating metal-metal-halogen (M-M-X) units with M
¼
Pt and X
I (the MMX chain). In the MMX chain, a 1D electronic state is formed
by d
z
2
orbitals of M and p
z
orbitals of X as schematically illustrated in Fig.
11.2b
.
The average valence of M is 2.5, and one unpaired electron exists per two d
z
2
orbitals. Four charge-ordering states shown below are theoretically expected to
exist in this system [
2
,
6
-
9
].
¼
M
2
:
5
þ
M
2
:
5
þ
X
M
2
:
5
þ
M
2
:
5
þ
X
Average valence state(AV)
:
M
2
þ
M
2
þ
X
M
3
þ
M
3
þ
X
Chargedensitywavestate(CDW)
:
M
2
þ
M
3
þ
X
M
2
þ
M
3
þ
X
Chargepolarizationstate(CP)
:
M
2
þ
M
3
þ
X
M
3
þ
M
2
þ
X
:
Alternating charge polarization state(ACP)
The CDW and CP states are stabilized by the displacements of X, while the ACP
state is stabilized by the displacements of Mdimers. Two classes of MMX chains with
different ligand molecules, R
4
[Pt
2
(pop)
4
X]
n
H
2
O[(pop)
¼
P
2
O
5
H
2
2
,R
¼
K, NH
4
,
and X
CH
3
CS
2
)[
11
,
12
] have been studied so
far. The ground state of R
4
[Pt
2
(pop)
4
X]
n
H
2
O(R
¼
Cl, Br] [
3
,
10
]andPt
2
(dta)
4
I(dta
¼
Cl, Br) has been
revealed to be CDW [
13
,
14
]. For Pt
2
(dta)
4
I, it has been suggested that ACP is
stabilized at low temperature [
12
]. As a general trend, the hybridization between
the p orbital of halogens and the d orbital of metals in the I-bridged compounds is
larger than that in the Cl- or Br-bridged compounds. The larger pd hybridization will
stabilize the various oxidation states (or charge-ordering states) of the metals.
In this chapter, first we show that the ground states on the PtPtI chains
(R
4
[Pt
2
(pop)
4
I]
n
H
2
O and R
0
2
[Pt
2
(pop)
4
I]
n
H
2
O) can be controlled between a dia-
magnetic CDW state and a paramagnetic charge polarization (CP) state by modifi-
cation of the counter ions (R, R
0
) located between chains. After that, we show that in
[(C
2
H
5
)
2
NH
2
]
4
[Pt
2
(pop)
4
I], the pressure-induced transition from CP to CDW
accompanied by a large hysteresis loop occurs. In the last part of this chapter, we
report that in the hysteresis region of the pressure-induced CP to CDW transition of
[(C
2
H
5
)
2
NH
2
]
4
[Pt
2
(pop)
4
I], the photoinduced phase transition from the CDW state
to the CP state could be driven.
¼
K, NH
4
and X
¼
11.2 Experimental Methods
11.2.1 Steady-State Optical Spectroscopy Under High Pressure
Applying external pressure to materials is one of the powerful tools for altering
their electronic states. To achieve the electronic phase control of MMX-chain
compound, we adopted the steady-state optical spectroscopies (polarized Raman
scattering and reflection spectroscopy) under high pressure using a diamond anvil
cell (DAC). DAC is a typical device used in high-pressure measurements. It allows
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