Oxygen x-ray emission and absorption spectra as a probe of the electronic structure of strongly correlated oxides

Abstract

We present synchrotron-excited oxygen x-ray $K$-emission spectroscopy ($\text{O}\text{ }K\ensuremath{\alpha}$ XES) and oxygen x-ray absorption spectroscopy ($\text{O}\text{ }1s$ XAS) spectra of transition-metal (TM) oxides MnO, CoO, and NiO. The comparison of oxygen $K$-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that $\text{O}\text{ }1s$ XAS is not strongly influenced by the core hole effect, whereas the $\text{TM}\text{ }2p$ XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.