Chemistry Reference
In-Depth Information
Table 9.7 Parameters defining some spin-Peierls systems
Compound
|
J
|/
k
B
D
(0)/K
T
sp
/K
0
2
D
(0)/
k
B
T
sp
References
TTF-CuBDT
77
21
12
0.195
3.50
[
87
]
MEM-(TCNQ)
2
106
28
17.7
0.209
3.16
[
86
,
97
]
CuGeO
3
120
24.5
14
0.146
3.50
[
88
,
89
]
a
0
-NaV
2
O
5
560
113.7
35.3
0.079
6.44
[
90
-
94
]
8
936(2)
95(7)
47
0.063
4.04
[
38
]
9
898(2)
77(2)
36
0.050
4.28
[
38
]
rapidly increases with a lowering of the temperature. Assuming that the increase of
w
M
below 50 K is originated from the impurity spin, the impurity spin concentration
is estimated to be 1.8 %. However, the origin is considered not to be impurity spin
but another factor, since this value is much larger than those of 8 and 9. Saito et al.
observed a broad thermal anomaly in the heat capacity measurement of 10, which is
attributed to a higher order phase transition around 135 K [
81
]. Therefore, it is
reasonable to assume that the structural modulation occurs with the higher order
phase transition, and the magnetic defects giving free spins would be formed in the
1D chain below 135 K. As a consequence, a spin-Peierls transition appears to be
suppressed in 10.
9.3.7 Synchrotron Radiation X-ray Crystallography
A spin-Peierls system undergoes a lattice instability at
T
sp
, and when
T < T
sp
, the
system dimerizes and the spin gap opens. Figure
9.38
shows the synchrotron X-ray
diffraction photographs of 8 [
38
]. In contrast to the diplatinum compounds 2-5 [
32
,
34
,
35
,
54
,
73
], the dinickel compounds 8 and 9 do not show any X-ray diffuse
scattering, demonstrating that the dinickel compounds do not exhibit valence
fluctuation. Whereas, new reflections clearly appear in the photographs of 8 taken
below 35 K, indicating a twofold superstructure. The number and intensities of the
superlattice reflections increase as the temperature is lowered. This fact indicates
that the unit cell along the
b
axis has doubled, which strongly implies that the lattice
dimerizes along the -Ni-Ni-I- chain. When the superlattice reflections are
included, the supercell can be indexed by
a
super
¼
0.5
a
0.5
b
,
b
super
¼
0.5
a
0.5
b þ c
, and the space group changes from
C
2/
c
to
P
¯
. Figure
9.39
shows the crystal structure of the superstructure of 8 at 26 K [
38
].
Two crystallographically independent [Ni
2
(EtCS
2
)
4
I] units exist in a unit cell of
8 in the LT phase, and the periodicity of the crystal lattice is twofold of a -Ni-Ni-I-
period. The two Ni-Ni distances are almost the same (Ni1-Ni2
1.5
b
, and
c
super
¼
0.5
a þ
¼
2.5387(9) and
2.5402(9)
˚
), but there are two different Ni-I distances. The short Ni-I
distances (Ni2-I2
Ni3-Ni4
¼
2.8839(7)
˚
) are about 0.013
˚
¼
2.8773(7) and Ni3-I2
¼
less
2.8888(7)
˚
).
than the long Ni-I distances (Ni1-I1
¼
2.8975(7) and Ni4-I1*
¼
The twofold superstructure originates from the different Ni
I distances since the
structures of the two crystallographically independent [Ni
2
(EtCS
2
)
4
] units are
almost the same and twisting between dinuclear units does not occur. The doubling
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