Electronic structure of lanthanum transition-metal oxyarsenides La MAsO ( M = Fe, Co, Ni) and LaFe 1− xM′ xAsO ( M′ = Co, Ni) by X-ray photoelectron and absorption spectroscopy

Abstract

The electronic structures of the quaternary oxyarsenides La MAsO ( 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 M 2+ and As 3−) 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 3d 5/2 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 La MAsO yielded electron populations of states that support the formal charge assignment [La 3+O 2−][ M 2+As 3−]. The mixed-metal series LaFe 1− x M′ 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.