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T. Asaji et al. / Solid State Communications 125 (2003) 171–173
to literature [5]. Bromine-adsorption was performed by
keeping pulverized [Pd(chxn)2][PdBr2(chxn)2]Br4 crystals
coexistent with a methanol solution of bromine over a
period of several hours in a desiccator. Elemental analysis
was made by Center for Organic Elemental Microanalysis,
Kyoto University. Calcd for [Pd(chxn)2][PdBr2(chxn)2]Br4:
C, 25.1%; H, 4.9%; N, 9.8%; Br, 41.7%. Found: C, 24.5%;
H, 4.6%; N, 9.6%; Br, 42.1%. Calcd for [Pd(chxn)2]
[PdBr2(chxn)2]Br4·2Br2: C, 19.6%; H, 3.8%; N, 7.6%; Br,
54.4%. Found: C, 19.3%; H, 3.7%; N, 7.4%; Br, 52.2%. X-
Ray powder diffraction patterns were measured using
Cu Ka radiation employing Rigaku RINT 2100S. A home-
made pulsed NMR spectrometers was used to determine 1H
NMR spin–lattice relaxation time T1. A Jeol JES-FE2XG
ESR spectrometer equipped with an Air products LTD-3-
110 liquid helium transfer refrigerator was used for the ESR
measurements in the Institute for Molecular Science.
3. Results and discussion
Fig. 1. Temperature dependences of integrated intensity of ESR
lines observed in dark-brown [Pd(chxn)2][PdBr2(chxn)2]Br4 and the
bromine-adsorbed light-green sample. Data were normalized by use
of the room temperature value of the dark-brown compound. Solid
curves were obtained by the least-squares fitting by use of the
The bromine-adsorbed compound (light-green) was
stable in a sealed tube. It liberates bromine gas and gives
the original compound [Pd(chxn)2][PdBr2(chxn)2]Br4 (dark-
brown) when ground in a mortar by exposing to air. This
was confirmed by X-ray powder diffraction (XRD). It was
intended to measure XRD patterns of the bromine-adsorbed
compound using the sample covered by a thin film to pre-
vent losing bromine. Although diffraction peaks from a film
made it difficult to locate true peaks from the sample,
several sharp diffraction peaks were detected indicating that
the light-green compound keeps crystalline state. From the
elemental analysis, it is expected that adsorption of about
two Br2 molecules per [Pd(chxn)2][PdBr2(chxn)2]Br4 for-
mula unit gives the light-green compound.
equation consisting of Curie and thermally activated terms: xESR
C=T þ A expð2D=kTÞ=T:
¼
in the whole temperature range studied by bromine-
adsorption implies that fixed paramagnetic sites (Curie
spin, mainly Pd(III)) were partly oxidized into Pd(IV) in the
Br-adsorbed complex. A part of the movable neutral solitons
will also be oxidized to the charged solitons. The increase in
the concentration of the thermally activated spins in the Br-
adsorbed sample at high temperatures can be explained by
the formation of Pd(III) from impurity sites. This will also
account for the increase of the activation energy D by
bromine-adsorption.
In the light-green and the original dark-brown com-
pounds, analogous ESR spectra with each other showing a
small anisotropy were observed and the spin density in the
light-green compound was estimated to be about 70% of that
of the dark-brown compound at 295 K. The temperature
dependence of the integrated intensity of ESR lines, which
is a measure of spin susceptibility, xESR, is shown in Fig. 1
where the intensity was normalized by use of the room
temperature value of the dark-brown compound. The tem-
perature dependence can be fitted by Eq. (1) consisting of
Curie and thermally activated terms [1].
1
We have investigated the H magnetization recovery in
the both compounds at 125 K as shown in Fig. 2. Both data
giving nonexponential decays of 1H magnetizations as
reported in the previous study [3] were roughly separated
into two contributions of the rapid and the slow relaxation
components. T1 values roughly evaluated from the slope of
these decay curves were long in the Br-adsorbed complex
compared with the non-adsorbed complex. This result
implies the decrease in paramagnetic sites in the adsorbed
complex by oxidation in agreement with the decrease of the
spin susceptibility by bromine-adsorption as shown in the
above ESR results. In accordance with the previous
assignment [3], the rapid component will be attributed to
the relaxation from moving spins, i.e. mostly kink-type spin
solitons in this temperature range in the both complexes.
Although the moving spins is expected to be the minor
component from the ESR intensity, the proton coupled to
the moving spins may become major component since the
spins can travel along the bromine-bridged Pd-chain. The
xESR ¼ C=T þ A expð2D=kTÞ=T
ð1Þ
In Eq. (1), we assumed a thermal activation model for the
second term for the thermally activated spins. It is shown
from the fitting calculations that the Curie component is less
important in the light-green compound. The best-fit
parameters C, A and D/k are as follows: C ¼ 109,
A ¼ 793, D/k ¼ 449 K for the dark-brown compound and
C ¼ 37.7, A ¼ 1889, D/k ¼ 702 K for the light-green
compound. The fact that the spin susceptibility decreased