INVESTIGATION OF THE CHEMICAL BONDING IN 3D(8) NICKEL(II) CHARGE-TRANSFER INSULATORS (NIO, OXIDIC SPINELS) FROM LIGAND-FIELD SPECTROSCOPY, NI 2P XPS AND X-RAY-ABSORPTION SPECTROSCOPY

High-resolution L-2,L-3 X-ray absorption spectra are presented from ni ckel(II) oxides (NiO, NiFe2O4 and NiGa2O4) and compared with those obt ained from X-ray photoelectron spectroscopy. The analysis of the X-ray Ni K beta emission, Ni 2p XPS and X-ray absorption spectra may be cle arly interpreted by a charge transfer effect during the photoionizatio n of one of the core electrons. The ligand-field spectroscopy shows th at 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 tripl er and singlet states of this configuration. The p-d type optical gap is observed at 4 eV in NiO and mixed oxides NiM2O4 (M = Al, Ga). Magne tic interactions influence strongly the optical properties of the Ni2 ion octahedrally coordinated in oxides. (C) 1997 Elsevier Science Ltd .