Chemistry Reference
In-Depth Information
Chapter 12
Theory of MMX-Chain Compounds
Kenji Yonemitsu
12.1
Introduction
MMX-chain compounds have a variety of electronic phases owing to their low-
dimensional electronic structures and their strong electron-lattice and
electron-electron interactions. Some of these interactions compete with each
other, while some of them collaborate. To understand the interplay, theoretical
studies based on Peierls-Hubbard models are useful. The electronic properties
depend on transition metal (M) ions, halogen (X) ions, ligands, and counterions.
Observed and suggested electronic phases for MMX-chain compounds are
schematically shown in Fig.
12.1
and classified into an averaged-valence (AV)
phase, a charge-density-wave (CDW) phase, a charge-polarization (CP) phase, and
an alternate-charge-polarization (ACP) phase. From the valences of the metal ions,
the CDW, CP, and ACP phases are also called the 2233, 2323, and 2332 phases,
respectively. Because electrons are not completely localized, the formal valences
2 and 3 mean valences 2.5
d
d
< d <
0.5. The AV
and CP phases are paramagnetic, while the CDW and ACP phases are nonmagnetic.
In the CDW phase, the spin gap is expected to be comparable with the charge gap.
Meanwhile, in the ACP phase, the spin gap is generally much smaller than the
charge gap. In principle, all of the four phases are insulating because of the finite
charge gap.
In this chapter, we consider compounds with M
and 2.5 +
, in reality, with 0
Pt, where competition
between different interactions is manifest. Two classes of MMX compounds
exist. (1) In R
4
[Pt
2
(pop)
4
X]
n
H
2
O with monovalent cations R, four ligands of pop
¼
¼
P
2
O
5
H
2
2
surround the binuclear MM unit. The ground states of K
4
[Pt
2
(pop)
4
X]
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