Insulator Charge transfer

Unlike the Mott-Hubbard insulator MnO described above the band-gap in the isostruc-tural oxide NiO is much smaller than expected from intrasite Coulomb repulsion. Fujimori and Minami showed that this is owing to the location of the NiO oxygen 2p band - between the lower and upper Hubbard sub-bands (Fujimori and Minami, 1984). This occurrence can be rationalized by considering the energy level of the d band while moving from Sc to Zn in the hrst transition series. 

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Figure 6.52 Schematic electron addition and removal spectra representing the electronic structure of transition-metal compounds for different regimes of the parameter values (a) charge-transfer insulator with U > A (b) Mott-Hubbard insulator A> U (From Rao et al, 1992).

Figure 7.1. The band gap is determined by the d-d electron correlation in the Mott-Hubbard insulator (a), where A > I/. By contrast, the band gap is determined by the charge transfer excitation energy in the charge transfer insulator (b), where U > A.

A systematic semiempirical study of the core-level photoemission spectra of a wide range of 3d transition-metal compounds has been carried out (Bocquet et al., 1992,1996). The values for U and A obtained from a simplified Cl cluster model analysis are demonstrated in Figure 7.2. As can be inferred from the graphs, the heavier 3d transition metal compounds shown in the figure are expected to be charge-transfer insulators, whereas the compounds of the fighter metals are generally expected to be of the Mott-Hubbard type. 

We considere in this section one particle excitations, i.e. electron removal (hole creation) and electron addition processes. We will restrict our considerations to charge transfer insulators [245]. 

Fig. 11.4. A simple model of the band structure shows that for a nominally fully oxygenated sample of YBa2Cu307 <5 (i.e. d 0) the holes responsible for superconductivity reside in the predominantly oxygen 2p valence band (a). As the oxygen deficiency, (5, is increased, the number of holes in the valence band is reduced until at d 0.6 superconductivity is lost and a charge transfer insulator is created (b). Also shown are the corresponding oxygen K-edge spectra.

Charge-transfer insulators, W < Act < U. such as La2Cu04, LaMnOs, LaFeOs, and La2Ni04... 

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