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Fig. 9.37 Temperature
dependence of the observed
(
circle
) and corrected (
plus
)
magnetic susceptibilities of
8 (a), 9 (b), and 10 (c) and the
Bonner-Fisher fits (
dotted
lines
)[
38
]. The estimated
impurity Curie spin
contributions are subtracted
in the corrected data
suggests the possibility of a spin-Peierls (SP) transition [
86
-
94
]. Ikeuchi and Saito
et al. have reported that, for each compound, a small broad thermal anomaly due to a
spin-Peierls transition was observed around 40 K in the relaxation calorimetry. [
80
].
A spin-Peierls system undergoes a phase transition at
T
sp
, where for
T < T
sp
a
temperature-dependent structural dimerization takes place, creating an opening of a
spin gap between the singlet and triplet spin states. To date, numerous spin-Peierls
materials have been reported, as represented by the organic system TTF-CuBDT [
87
]
and the inorganic system CuGeO
3
[
88
,
89
]. The exchange constants and transition
temperatures are |
J
|/
k
B
¼
77K and
T
sp
¼
12 K for TTF-CuBDT and |
J
|/
k
B
¼
120 K
a
0
-NaV
2
O
5
wasalsoconsidereda
possible inorganic spin-Peierls system with a large exchange coupling constant of |
J
|/
k
B
¼
and
T
sp
¼
14 K for CuGeO
3
. On the other hand,
35.3 K [
90
-
92
], but more
recent studies have revealed that this phase transition may be a novel cooperative
phase transition associated with valence ordering, lattice dimerization, and spin
gap formation [
93
,
94
]. The spin Hamiltonian of the spin-Peierls phase below
T
sp
may be described by the alternating AF chain:
560 K and a high-transition temperature
T
c
¼
H ¼ J
0
S½S
2
i
S
2
i
1
þgS
2
i
S
2
iþ
1
:
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