Chemistry Reference
In-Depth Information
Fig. 7.1 Crystal structures of
Ni-X chain compounds;
(a) [Ni(chxn)
2
Br]Br
2
and
(b) [Ni(
L
)
2
Br]Br
2
(from [
5
]).
(c) The electronic structure of
Ni-X chain compounds
a
b
Ni
Br
C
N
Br
C
N
a
c
b
c
U
CT
Ni 3
d
lower Hubbad
band
Br 4
p
band
Ni 3
d
upper Hubbard
band
They showed that the lowest optical transition was due to 1D excitons with large
electron-hole-binding energy of about 0.5 eV. Subsequently, Iwasa et al. reported the
third-order nonlinear susceptibility
(3)
in the similar Pt-X chain compounds by using
the third-harmonic generation (THG) method [
2
]. The evaluated
w
(3)
values were,
however, not so large being the order of 10
11
esu. In 2000, it was found that Ni-X
chain compounds show very large
w
(3)
[
3
] as compared to Pt-X chain compounds and
other 1D semiconductors such as conjugated polymers. Since this discovery, a number
of studies have been performed to clarify the origin of the enhancement of
w
(3)
in Ni-X
chain compounds. Those studies also activated time-resolved studies on the photo-
responses in the Ni-X chain compounds and related materials [
4
-
6
].
Ni-X chain compounds (X
w
Cl and Br) are the 1D Mott insulators as discussed
¼
¼
cyclohexanediamine), which is a representative of Ni-X chain compounds. In this
compound, Ni
3+
ions and Br
ions are arranged alternately along the
b
axis [
7
].
In the Ni
3+
ions, an unpaired electron exists in the d
z
2
orbital. The d
z
2
orbital of Ni
3+
and p
z
orbital of Br
form a purely 1D electronic state. Due to the large
electron-electron Coulomb repulsion energy
U
on the Ni site, this compound is a
Mott insulator [
8
,
9
]. More strictly, the occupied Br 4p-band is located between the
Ni 3d upper-Hubbard band and the lower Hubbard band as shown in Fig.
7.1c
.
Therefore,
this compound is classified to the charge-transfer (CT) insulator,
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