Origin of the band gap in the negative charge-transfer-energy compound NaCuO2.

Abstract

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 ${\mathit{d}}^{8}$\ensuremath{\rightarrow}${\mathit{d}}^{9}$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\ensuremath{\rightarrow}${\mathit{d}}^{9}$+${\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.