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

The electronic structure of the formally Cu3+ (d8) compound NaCuO2 has been studied by photoemission spectroscopy and subsequent cluster configuration-interaction calculations. We find that the d8 d9L (L: ligand hole) charge-transfer energy is negative and the ground state is dominated by the d9L configuration. The band gap corresponds to charge fluctuations of the type d9L+d9L d9+d9L2 (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.