Chemistry Reference
In-Depth Information
Fig. 4.4 Temperature
dependence of Raman spectra
of [Pd(en)
2
Br](C
5
-Y)
2
H
2
O
4.2.3 Raman and Optical Conductivity Spectra
It is known that the Br-Pd-Br symmetrical stretching mode
(Br-Pd-Br) in Raman
spectrum is allowed in the CDW state and forbidden in the MH state [
3
]. Thus,
Raman spectroscopy is a good probe to determine the electronic states of the MX-
chain compounds. Figure
4.4
shows the temperature dependence of the Raman
spectra. In the high temperature (HT), an intense Raman peak was observed at ca.
130 cm
1
, which was assigned to
n
(Br-Pd-Br). On the other hand, the peak
disappeared in the low temperature (LT) spectra. This result clearly indicates that
this complex undergoes the phase transition from CDW to MH states.
Figure
4.5
shows the temperature dependence of the optical conductivity spectra
and the peak energies (
E
opt
). A clear discontinuity in
E
opt
was observed at
206
n
2 K. This temperature is similar to those obtained from the crystal structure
analysis, ESR, and Raman spectroscopies. In the HT spectra, the peak energy
decreased from 0.86 eV (290 K) to 0.66 eV (210 K). On the other hand, the peak
energy barely changed with a change in the temperature in the LT spectra. This can
be understood by considering the origin of the electronic transition. In the CDW
state, the lowest electronic transition is an IVCT transition from the Pd(II) d
z
2
to Pd
(IV) d
z
2
species. In this case, the CT energy (
E
opt
) can be evaluated as 2
S
-
U
if
V
and
t
are ignored [
4
,
5
]. In the MH state, on the other hand, the lowest electronic
transition is CT transition from averaged Pd(III)-Pd(III) state to disproportioned Pd
(II)-Pd(IV) (or Pd(IV)-Pd(II)) state. The CT energy is evaluated to be
U
. Because
almost no temperature dependence was observed in the LT spectra and there was a
large temperature dependence of the CT energy in the HT spectra,
S
, not
U
,
decreases as the Pd
Pd distance becomes shorter.
From the crystal structures, spin susceptibility, optical conductivity spectra, and
Raman spectra, we concluded that the first-order phase transition between CDW
Search WWH ::
Custom Search