Chemistry Reference
In-Depth Information
Fig. 9.36 Temperature
dependence of the electrical
conductivity of 4 (
plus
), 5
(
circle
), and 6 (
square
)[
38
]
9.3.6 Magnetic Properties
Yamashita et al. have reported that the magnetic susceptibility of [Ni
2
(MeCS
2
)
4
I]
1
(7)
is essentially temperature independent down to about 50 K (ca. 10
6
emu g
1
)andthen
rapidly increases at lower temperature [
29
]. The observed temperature independent
behavior has been ascribed to the very strong antiferromagnetic coupling between the
unpaired electrons on the Ni
3+
sites. The temperature dependences of the magnetic
susceptibilities
Et (8),
n
-Pr (9),
n
-Bu (10)) are shown in
Fig.
9.37
[
38
]. The common feature of the three compounds is that the susceptibility
at room temperature is considerably lower than that expected for one
S ¼
w
M
of [Ni
2
(RCS
2
)
4
I]
1
(R
¼
1/2 spin
per dimer. Furthermore, there are distinct differences between those of 8 and 9 and
that of 10 regarding the appearance of a gradual drop around 50 K for the former
two. In addition, 9 exhibits an anomaly in the temperature range 200-212 K, which
is associated with the first-order phase transition from the RT phase to the LT phase,
as discussed already. Assuming an
S ¼
1/2 1D Heisenberg AF chain model, the
Bonner-Fisher equation (
H ¼ J S S
i
·
S
i
+1
)[
85
,
100
] has been applied to fit the
susceptibility data above 100 K for 8 and68Kfor9. The fitting gives exchange
coupling constant of
jj=k
B
¼
898(2) K for 9.These
J
values are in fair agreement with the theoretical
J
value of
1,160 K calculated for
[Ni
2
(MeCS
2
)
4
I]
1
(7)usingtheUB3LYPmethod[
42
]. The very large
J
value
indicates a very strong AF interaction between the spin of the Ni
3+
ions through
the bridging iodide ion, justifying considerable electronic overlap between the d
z
2
(Ni) and p
z
(I) orbitals. The key feature of the magnetic susceptibility is that it
decreases more rapidly below 47 K for 8 and 36 K for 9.The
936(2) K for 8 and
jj=k
B
¼
w
M
values of 8 and 9
take a minimum at about 30 and 16 K, respectively,andthenshowanincrease
due to the presence of paramagnetism originating from impurities or lattice and end-
of-chain defects. Assuming that the impurities have have an
S
= 1/2 spin, their
concentrations are estimated to be 0.35 % for 8 and0.07%for9. By subtracting
these contributions from the experimental data, the corrected
w
M
values of 8 and 9
are found to decrease to a small constant value. This rapid decrease in
w
M
strongly
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