Charge-transfer and Mott-Hubbard Excitations in FeBO3: Fe K edge Resonant Inelastic X-ray Scattering Study

Momentum-resolved resonant inelastic x-ray scattering (RIXS) spectroscopy has been carried out at the Fe K edge, successfully for the first time. The RIXS spectra of a FeBO3 single crystal reveal a wealth of information on 1-10 eV electronic excitations. The IXS signal resonates both when the incident photon energy approaches the pre-edge (1s-3d) and the main-edge (1s-4p) of the Fe K edge absorption spectrum. The RIXS spectra measured at the pre-edge and the main-edge show quantitatively different dependences on the incident photon energy, momentum transfer, polarization, and temperature. Electronic excitations observed in the pre-edge and main-edge RIXS spectra are interpreted as Mott-Hubbard (MH) and charge-transfer (CT) excitations, respectively. The charge-transfer gap = 3.81 (0.04) eV and the Mott-Hubbard gap = 3.96 (0.04) eV, are determined, model independently from the experimental data. The CT and MH excitations are assigned using molecular orbitals (MO) in the cluster model and multiplet calculation in the many-electron multiband model, respectively.