Adsorption-induced surface valence changes in europium intermetallics

Abstract

The trivalent compounds EuPd₃ and EuNi₅ have been studied with X-ray excited Photoelectron Spectroscopy (XPS) and Bremsstrahlung Isocromat Spectroscopy (BIS). Due to the very small contribution from empty Pd or Ni d states in these compounds, BIS gives a clearer view of the Eu 4f states than the XPS valence electron spectra. In the BIS spectra as well as in the core level spectra we find contributions from a divalent surface layer. It is concluded that there is a substantial surface disproportionation of the compounds with an increased Eu concentration at the surface. Controlled oxidation showed that the divalent spectral features disappear at 1-2 L of oxygen, indicating that trivalent oxides are formed. Furthermore, we conclude that shake-down processes are not important in core level spectra from Eu compounds, in contrast to previous findings.