Investigation of the chemical bonding in 3 d8 nickel(II) charge transfer insulators (NiO, oxidic spinels) from ligand-field spectroscopy, Ni 2 p XPS and X-ray absorption spectroscopy

Abstract

High-resolution L 2,3 X-ray absorption spectra are presented from nickel(II) oxides (NiO, NiFe 2O 4 and NiGa 2O 4) and compared with those obtained from X-ray photoelectron spectroscopy. The analysis of the X-ray Ni Kβ emission, Ni 2 p XPS and X-ray absorption spectra may be clearly interpreted by a charge transfer effect during the photoionization of one of the core electrons. The ligand-field spectroscopy shows that the nickel(II) oxides (NiO and inverse spinels) and fluorides are 3 d 8 systems in the sense that the levels below 3.4 eV belong to triplet and singlet states of this configuration. The p- d type optical gap is observed at 4 eV in NiO and mixed oxides NiM 2O 4 (M = Al, Ga). Magnetic interactions influence strongly the optical properties of the Ni 2+ ion octahedrally coordinated in oxides.