Chemistry Reference
In-Depth Information
Chapter 8
Theory of MX Chain Compounds
Kaoru Iwano
8.1 Theoretical Framework
8.1.1 Overview
MX chains are fascinating materials not only for experimentalists but also for
theorists. They are almost ideal examples of one-dimensional systems and become
very good stages for us to check and deepen our knowledge and experiences about
the latter. Of course, we know famous polyacetylene and other conjugate polymers,
which had already been known when we started the studies of MX chains. In fact,
the models applied for polyacetylene are similar, in some sense, to those models for
MX chains that we introduce from here on. However, we have more varieties in
both the kinds of compounds and the types of electronic and lattice states. Here, we
state the most important classification:
Pd and Pt complexes
!
Charge
density
wave (CDW) systems,
Ni complexes ! Mott insulator systems :
In Fig. 8.1 , these two categories are very schematically illustrated. Readers
may see apparent differences between them. Namely, in Pt and Pd complexes, the
5
Þd z 2 orbital of Pt (Pd) that extends along the chain direction is doubly occupied or
empty, leading to a so-called mixed valence of +2 and +4. On the other hand, in Ni
complexes, the corresponding 3 d z 2 orbital contains one electron univocally, leading
to the valency of +3. Here, it will be important to comment why we think about d z 2
electrons. Assuming a formal valency of +3, the d-electron configuration is d 7 .
ð
4
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