Electronic structure of lanthanum transition-metal oxyarsenides LaMAsO (M = Fe, Co, Ni) and LaFe_(1_x)M'_xAsO (M'= Co, Ni) by X-ray photoelectron and absorption spectroscopy

摘要

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 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 0 Is 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 [La~（3+）0~（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.