Other Types of Electronic Transitions

for Lai-xTiOs, both disorder and correlation are probably important. These phases are best considered disordered Mott-Hubbard insulators. 

The unit-cell volume expands with increasing lanthanum content. However, the metallic behavior appears with the larger unit cells. This is contradictory to bandwidth control, in which greater atomic orbital overlap would be expected to overcome correlation effects with contractions in the unit cell. 

For Lao.eeTiOs, titanium is fully oxidized to Ti + (d°). Hence, the conduction band is empty. As the Ti + (d ) content increases with increased lanthanum deficiency, however, the conduction band becomes occupied with electrons. Thus, the M-NM transition appears to be under filling control. 

In this chapter, the focus has been on the Mott (Mott-Hubbard) and Anderson transitions. When charge ordering is present, other types of transitions are also possible. A classic example is the mixed-valence spinel Fe203. There are two types of cation sites in Fc203, denoted as A and B. The A sites are tetrahedrally coordinated Fe 

A CDW is a periodic modulation of the conduction electron density within a material. It is brought about when an applied electric field induces a symmetry-lowering lattice modulation in which the ions cluster periodically. The modulation mechanism involves the coupling of degenerate electron states to a vibrational normal mode of the atom chain, which causes a concomitant modulation in the electron density that lowers the total electronic energy. In one-dimensional systems, this is the classic Peierls distortion (Peierls, 1930, 1955). It is analogous to the JT distortion observed in molecules. 

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