Theory of MX Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 137

a b Mott-Hubbard

Ins.

c

CT Ins.

CDW Ins.

UHB CB

UHB

X band

LHB VB

LHB

X band X band

Fig. 8.25 Three type of insulators. In (a) and (b), UHB and LHB mean the lower and upper
Hubbard bands, respectively. In (c), CB and VB are the conduction and valence bands of the
CDW, respectively. All these bands consist of the metal d z 2 orbitals. The X band common to the
three figures is the halogen p z band

Upon these experimental findings, a theoretical calculation was performed
within the HF framework [ 46 ]. In Fig. 8.24 , we show one result of the simulation,
in which 20%-Ni concentration is assumed in a 200-site system. In this calculation,
the effects of metal mixing are incorporated by changing the site energy and the on-
site repulsion energy from site to site. With this Ni distribution specified by the
vertical dotted lines in Fig. 8.24a , we notice that some regions become a Mott-
insulator state even involving a finite number of Pd sites. For example, the region
from the 150th to 200th sites loses lattice dimerizations as shown in Fig. 8.24a , and
simultaneously finite amplitude of spin density appears (Fig. 8.24b ), in spite of
relatively large number of Pd sites in it. At that time, it was interpreted as a kind of
proximity effect, namely, that the Pd sites were involved in the Mott-insulator state
forcibly by the Ni sites nearby. As another example, the cases with 50% Ni
concentrations were also investigated to find that the most stable states were always
Mott insulators with inhomogeneous spin densities in accordance with the Pd
inclusion, which finding was consistent with the experimental tendency.

8.4.2 Pd Compound as a Mott-Hubbard Insulator

We now proceed our discussion about the metal mixing further to the next point.
Although we have not made it clear so far, there are two types of Mott-insulator
states. One is the CT insulator, while the other is the Mott-Hubbard insulator. The
Ni compound belongs to the former category [ 47 ]. In Fig. 8.25 , we show the three
type of the insulators, adding the CDW insulator to the two Mott insulators.
Regarding the ground state properties, the two Mott insulators are not so different.
Meanwhile, the excited states have an important difference; the lowest-energy
optical transition is the CT transfer from the halogen to the metal in the CT
insulator, while it is the metal-metal transition from the lower Hubbard band
(LHB) to the upper Hubbard band (UHB) in the Mott-Hubbard insulator. Since
the pure Pd compound, [Pd(chxn) 2 Br]Br 2 (hereafter PdBr), as one of the CDW
insulators has the band scheme of Fig. 8.25c , it is natural to expect a band scheme as