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Fig. 4.5 Temperature
dependence of (a) optical
conductivity spectra and
(b) their peak energy
and MH states occurs at 206
2 K. This phase transition was also confirmed using
heat capacity measurements (
0.011 kJ mol 1 ,
D H ¼
1.554
D S ¼
7.48
0.05 J K 1 mol 1 ).
4.2.4 Alkyl Chain Length Dependency
In order to clarify the origin of the phase transition, we synthesized a series of
Pd-Br complexes [Pd(en) 2 Br](C n -Y) 2
4, 5, 6, 7, 8, 9, and 12) with
different alkyl chain lengths. We determined the nearest neighbor Pd
H 2 O( n ¼
Pd distance
of these complexes from powder X-ray diffraction (PXRD) patterns. Increasing the
alkyl chain length caused the Pd
Pd distance to decrease dramatically even at
room temperature, as shown in Fig. 4.6 , suggesting that the attractive force between
the alkyl chains of the counterions becomes stronger with an increase in the alkyl
chain length. In other words, the attractive force acting between the alkyl chains is
the origin of the chemical pressure.
Figure 4.7 shows Raman spectra of the series of Pd-Br complexes at room
temperature. An intense Raman peak attributed to
n
(Br-Pd-Br) was observed for
the complexes with short alkyl chains ( n
8), whereas it was not observed for the
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