Origin of the band gap in the negative charge-transfer-energy compoundNaCuO2

The electronic structure of the formally ${\mathrm{Cu}}^{3+}$ (${\mathit{d}}^8$) compound ${\mathrm{NaCuO}}_2$ has been studied by photoemission spectroscopy and subsequent cluster configuration-interaction calculations. We find that the ${\mathit{d}}^8$→${\mathit{d}}^9$L (L: ligand hole) charge-transfer energy is negative and the ground state is dominated by the ${\mathit{d}}^9$L configuration. The band gap corresponds to charge fluctuations of the type ${\mathit{d}}^9$L+${\mathit{d}}^9$L→${\mathit{d}}^9$+${\mathit{d}}^9$ ${\mathit{L}}^2$ (p-p type), being neither of the Mott-Hubbard (d-d) type nor of the charge-transfer (p-d) type. The gap is stabilized and destablized by the intracluster and intercluster hybridizations, respectively.