Electronic structure of the local-singlet insulatorNaCuO2

The insulating oxide ${\mathrm{NaCuO}}_2$ has been studied by x-ray photoemission spectroscopy and subsequent cluster-model analysis. It is found that the ${\mathit{d}}^8$→${\mathit{d}}^9$L charge-transfer energy (L:ligand hole) is negative and the ground state is dominated by the ${\mathit{d}}^9$L configuration. Using the Anderson impurity model, it is shown that strong 3d-ligand hybridization opens a band gap for a negative charge-transfer energy. This band gap corresponds to charge fluctuations mainly of the p-p type, ${\mathit{d}}^9$L+${\mathit{d}}^9$L→${\mathit{d}}^9$+${\mathit{d}}^9$ ${\mathit{L}}^2$, with a considerable mixture of d character into the p states, and not of the conventional Mott-Hubbard (d-d) type nor of the charge-transfer (p-d) type. The magnitude of the gap is strongly affected by the geometrical arrangement of metal-oxygen local units, giving a natural explanation for the difference between the insulating ${\mathrm{NaCuO}}_2$ and metallic ${\mathrm{LaCuO}}_3$. The electronic structures of ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Ni}}^{3+}$ oxides and their insulating versus metallic behaviors, which are expected to resemble those of the ${\mathrm{Cu}}^{3+}$ oxides, are also discussed. To generalize the above conclusions, a modification of the metal-insulator boundaries in the Zaanen-Sawatzky-Allen diagram is proposed to include compounds with small or negative charge-transfer energies.