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Fig. 9.39 1D chain structure of [Ni 2 (EtCS 2 ) 4 I] 1 (8) at 26 K with an atomic numbering scheme
and relevant interatomic distances (thermal ellipsoid set at the 50 % probability level) [ 38 ]
here based on the relative arrangement of 1D chains in the crystal, which minimizes
the Coulomb repulsion between them. As shown in Fig. 9.40 , there are two types for
the arrangements of 1D chains [ 38 ].
Adjacent 1D chains in arrangement type (a) shift about a half period from each
other, whereas the 1D chains in arrangement type (b) are aligned with the same
phase. Here I discuss the twofold periodic valence ordering which minimizes the
Coulomb repulsion between the 1D chains. The type (b) arrangement is capable of
taking both valence-ordered ACP and CDW states. [M 2 (MeCS 2 ) 4 I] 1 (M
Ni (7),
Pt (1)) have relatively short interchain S
S contacts in the type (a) arrangement,
and these compounds are considered to have two-dimensional interactions [ 28 , 37 ].
When a 1D chain takes the ACP state in type (a), adjacent 1D chains should take the
CDW state to minimize the Coulomb repulsion. As a result, it is presumed that these
compounds would have difficulty adopting the superstructure associated with the
twofold periodic valence ordering, and consequently, [Ni 2 (MeCS 2 ) 4 I] 1 (7) would
be difficult to show the spin-Peierls transition associated with the ACP state. On the
other hand, it is possible for [Ni 2 (RCS 2 ) 4 I] 1 (R
Et (8), n -Pr (9)) to adopt twofold
periodic valence ordering, since the one-dimensionality of these compounds
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