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amplitude. The increase of
l
1
(the deviation from the constant value of
l
1
) in [Pd
(chxn)
2
Br]Br
2
(No. 11 in Fig.
2.3
) located near the CDW-MH phase boundary is
attributable to such an increase of
t
[
28
].
2.2.2 Control of CDW Degeneracy
MX compounds have the doubly degenerate CDW ground states, which are
expressed as follows.
X
M
4
þ
X
M
2
þ
X
M
4
þ
X
M
2
þ
X
X
M
2
þ
X
M
4
þ
X
M
2
þ
X
M
4
þ
X
This degeneracy of CDW can also be modified by the choice of constituent
elements.
In the CDW compounds with halogen for the counteranion (Y), a direction of
each halogen displacement is two-dimensionally ordered in the
bc
plane [
28
,
32
], as
shown below.
X
M
4
þ
X
M
2
þ
X
M
4
þ
X
M
2
þ
X
X
M
4
þ
X
M
2
þ
X
M
4
þ
X
M
2
þ
X
X
M
4
þ
X
M
2
þ
X
M
4
þ
X
M
2
þ
X
This is not due to the overlapping of the electronic states between the neighboring MX
chains but due to the interchain coupling through the tight network of the H-bonds
between the aminogroups of the ligands (A) and the counter halogen ions (Y). Thus,
although the electronic states are still one-dimensional, a 2D ordering of CDW is
formed.
Degeneracy of CDW states can be excluded more clearly by the metal alterna-
tion method. By substituting a half of Pt ions with Pd ions in M
Pt compounds,
we can obtain the heterometal compounds in which Pt and Pd ions are arranged
alternatively [
26
]. In those compounds, the 1D electronic state consists of the 4d
orbitals of Pd and the 5d orbitals of Pt. As the energy level of the former is lower
than the latter, the CDW state is formed by Pd
2
þ
ions and Pt
4
þ
ions, as shown
below.
¼
X
Pt
4
þ
X
Pd
2
þ
X
Pt
4
þ
X
Pd
2
þ
X
In this type of the heterometal compounds, CDW states are nondegenerate. The
lattice parameters,
L
,
l
1
and
l
2
for the heterometal compounds were also plotted in
Fig.
2.3
by diamonds. The deviation of the data points from those of the homometal
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