A Mott-Hubbard insulator

Matsumoto [98] tried to summarize the work done on the electronic structure of iron oxides and concluded that the about 2 eV in the bandgap of the hematite semiconductor measured in photoelectrochemistry indeed is based on the 3d band transition between the Fe3+ ions, which supports a Mott-Hubbard insulator. 

The near-infrared reflectance provides the response to plasmon oscillations of the electron gas (which are uniform excitations). This region of the spectrum is, however, not sensitive to the strength of the short-range coulombic interactions, which prevent conductivity in a Mott-Hubbard insulating state. This is illustrated by the frequency-dependent conductivity cx((o) measured in various salts exhibiting very different values of the conductivity at room temperature (Fig. 27). The peak of the conductivity at the frequency w0 correlates with the metallic character namely, a low frequency of the peak position corresponds to a high dc conductivity and vice versa. The structures below 0)o are attributed to the coupling with intramolecular modes. 

Correlation effects are likely to be quite important in the compound 0(ET)2I3 since they also are narrowband conductors. However, the reason why these strong interactions do not materialize in a Mott-Hubbard insulator could be attributed to the absence of one-dimensional character for this system, which precludes establishment of a Mott-Hubbard localized state. 

Instabilities in a 1-D system, driven by a strong on-site electron-electron Coulomb repulsion U, lead to a Mott-Hubbard insulator [161], particularly for p = 1 systems this causes charge localization, and the crystal becomes insulating. For a chemist, a Mott-Hubbard insulator is like a NaCl crystal, where the energy barrier to moving a second electron onto the Cl site is prohibitively high, as is the cost of moving an electron off a Na site. 

When p = 1 (one electron per molecule) the electrons (or holes) are localized [23] any transport of charge puts two electrons on the same site, at a huge cost in energy thus a p=l system is a Mott-Hubbard insulator [161]. If Em + Id Aa 0 then interesting properties become possible [48]. 

In the case of the CP state, where an unpaired electron exists at the M " site per dimer unit, no cell doubling occurs and so either a Mott-Hubbard insulator or a one-dimensional metal is expected. Conversely, the electronic structures of the CDW state and the ACP state are regarded as Peierls and spin-Peierls states, respectively [178,184,185]. The four states of the MMX complexes are distinguished by the valence states of the metal and the bond distances in the crystal structure. 

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