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Fig. 9.38 X-ray diffraction photographs of [Ni 2 (EtCS 2 ) 4 I] 1 (8) taken at different temperatures.
b * (1D chain direction) is vertical in these photographs [ 38 ]. The diffuse circular lines originate
from the carbon fiber mounting a single crystal
of the lattice periodicity and the increase of the superlattice reflections on lowering
the temperature strongly support the spin-Peierls transition at 47 K. The Ni-Ni and
Ni-I distances indicate that the valence-ordered state in the LT phase is the ACP
state of -Ni (2.5 d )+ -Ni (2.5+ d )+ -I -Ni (2.5+ d )+ -Ni (2.5 d )+ -I -(
0.5). This phase
transition is a spin-Peierls transition that accompanies the rearrangement of the
valence ordering. In diplatinum compounds [Pt 2 (RCS 2 ) 4 I] 1 (R
d
Et (2), n -Bu (4)),
the difference between the long and short Pt-I distances, that characterizes the
degree of the lattice distortion, is 0.029 and 0.066 ˚ , respectively [ 33 ]. The lattice
distortion of 8 is therefore smaller than those of the diplatinum compounds. The
diplatinum compounds can be largely polarized due to the larger span of the 5d z 2
orbitals, whereas the degree of charge polarization of the nickel compound is small
since this material tends to retain a valence state close to the AV state due to the
strong electron-electron correlation effect on the 3d z 2 orbitals of the nickel atoms.
Compound 9 also exhibits superlattice reflections at the positions of 1/2 b *
corresponding to the twofold -Ni-Ni-I- period at temperatures below T sp ¼
¼
36 K.
Attempts to index the superlattice reflections have been, however, unsuccessful
because of their very weak intensities [ 38 ].
On the basis of the crystal structure analyses of the superstructure, the valence-
ordered states of [M 2 (EtCS 2 ) 4 I] 1 (M
Ni (8), Pt (2)) are assigned to the ACP state.
In contrast, the spin-Peierls transition has not been observed in [Ni 2 (MeCS 2 ) 4 I] 1 (7),
and the superlattice reflections due to the development of the superstructure
associated with the twofold periodic valence ordering have not been reported in
[M 2 (MeCS 2 ) 4 I] 1 (M
¼
¼
Ni (7), Pt (1)). The reasons for these results are discussed
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