Chemistry Reference
In-Depth Information
Fig. 2.2 Schematic electronic
structure of the MX chain.
(a) The mono-valence
(Mott-Hubbard) state and
(b) the mixed-valence (CDW)
state
a
X
-
X
-
X
-
X
-
M
3+
M
3+
M
3+
M
3+
M(
d
z
)
X(
d
z
)
b
X
-
X
-
X
-
X
-
M
4+
M
2+
M
4+
M
2+
E
s
U
the heterometal compounds in which Pt and Pd ions arrange alternatively [
26
,
52
].
Such a controllability of CDW in MX-chain compounds allows us to make an
advanced study of dynamics of excitons, solitons, and polarons.
In this chapter, firstly, we review how the CDW ground state can be controlled
by the choice of constituent elements (M, X, A, and Y). Secondly, we summarize
the fundamental optical properties of the CDW compounds, focusing on the optical
absorption and photoluminescence (PL) properties, which are dominated by the
transition to the lowest charge-transfer (CT) exciton and the emission from the self-
trapped exciton (STE), respectively. After that, we detail nature of solitons and
polarons investigated by the photoinduced absorption (PA) spectroscopy. From the
comparison of PL properties from STEs and PA spectra due to solitons and
polarons, we discuss the relaxation dynamics of photoexcited states in MX-chain
compounds in the CDW phase.
2.2 Control of CDW Ground States [
5
,
32
]
In MX-chain compounds, 1D electronic state of an MX chain consists of d
z
2
orbitals
of M and p
z
orbitals of X. The ground state is the mono-valence M(III) state or the
mixed-valence M(II) and M(IV) state (or equivalently the commensurate CDW
state) as shown in Figs.
2.2a, b
, respectively. In Pt and Pd compounds, the mono-
valence state is usually unstable due to the site-diagonal-type e-l interaction
overcoming the on-site Coulomb repulsion energy
U
on the metal site [
8
,
9
]. In
Ni compounds, the mono-valence state is stabilized due to the large
U
on the Ni
sites, so that their electronic structure is considered to be the Mott-Hubbard (MH)
insulator state [
53
,
54
]. More precisely, the Ni compounds belong to the CT
insulators in which the transition from the halogen p valence band to the Ni 3d
upper Hubbard band corresponds to the optical gap [
55
]. CDW states are never
stabilized in the Ni compounds. In this section, we show that CDW states
(amplitudes of CDW, degeneracy of CDW, and optical gap energies) in Pt and Pd
compounds can be widely controlled by the choice of constituent elements in
MX-chain compounds.
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