Electronic structure of lanthanum transition-metal oxyarsenides LaMAsO (M = Fe, Co, Ni) and LaFe1-xM ' xAsO (M ' = Co, Ni) by X-ray photoelectron and absorption spectroscopy

The electronic structures of the quaternary oxyarsenides LaMAsO (M = Fe, Co, Ni) were examined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Interpretation of the metal 2p(3/2) and arsenic 3d(5/2) binding energies, as well as a satellite feature in the Co 2p XPS spectrum, suggests charges that are much less extreme than expected (i.e., not M2+ and As3-) because of the strong covalent character within the M-As bonds. As M is varied, the differing degrees of charge transfer from M to As atoms within these bonds are manifested by shifts in the As 3d5/2 binding energies and changes in the As K-edge intensities. This charge transfer is isolated within the [MAs] layer and does not influence the O 1s and La 3d XPS spectra. Fitting the experimental valence band spectra of these oxyarsenides LaMAsO yielded electron populations of states that support the formal charge assignment [La3+O2-][M2+As3-]. The mixed-metal series LaFe1-xM'xAsO (M' = Co, Ni) was examined by XANES; analysis of the metal K- and L-edges, as well as of the Co 2p XPS satellite feature, revealed that no metal-metal charge transfer takes place. (C) 2009 Elsevier Masson SAS. All rights reserved.