Eur. Phys. J. B 70, 157-162 (2009)
https://doi.org/10.1140/epjb/e2009-00199-x

Charge transfer and dd excitations in transition metal oxides

q-dependent dielectric function study by inelastic X-ray scattering and optical reflectivity measurements

¹ National Synchrotron Radiation Research Center (NSRRC), 30076 Hsinchu, Taiwan
² Graduate School of Science and Technology, Kobe University, 657-8501, Kobe, Japan

Corresponding author: ^a hiraoka@spring8.or.jp

Received: 1 February 2009
Revised: 12 May 2009
Published online: 16 June 2009

Abstract

Electronic excitations in transition-metal oxides MnO, FeO, CoO and NiO are investigated by inelastic X-ray scattering and optical reflectivity measurements. The dielectric functions are derived from the experimental data as a function of the momentum transfer, q. Based on the derived q-dependent dielectric functions, two types of the charge transfer excitations, i.e., dipolar and non-dipolar charge transfer, are clearly identified. We show that the Mott gaps around 5 eV are defined by the former whereas the latter occurs at higher energies of 8–12 eV. Based on a molecular orbital analysis, we associate the dipolar and the non-dipolar excitations with non-local charge transfer and conventional charge transfer, respectively. These types of excitations are shown to be common for the 3d metal monoxides. On the other hand, the dd excitations observed in NiO and CoO at energy <4 eV do not appear in FeO and MnO. The reasons are addressed in this report.