Final-state effects and surface valence in Eu—transition-metal compounds

A systematic x-ray photoemission study of Eu $3d$ and Eu $4d$ core levels in the Eu—transition-metal compounds Eu${\mathrm{Rh}}_5$, Eu${\mathrm{Ni}}_5$, Eu${\mathrm{Pd}}_5$, Eu${\mathrm{Pt}}_5$, Eu${\mathrm{Cu}}_5$ and Eu${\mathrm{Ag}}_5$ is presented. The spectra in the divalent and trivalent compounds exhibit splittings into two components, caused by the occurrence of $4f^6$ and $4f^7$ configurations. For the divalent compounds Eu${\mathrm{Cu}}_5$ and Eu${\mathrm{Ag}}_5$, this effect is due to final-state shakeup. For the trivalent compounds, the presence of both configurations is discussed in terms of both initial- and final-state effects. An estimate of the $4f$-configurational stability at the surface of the ${\mathrm{Eu}}^{3+}$ intermetallics indicates surface valence transition of the Eu ions to the lower valent state in these compounds. Experimental evidence for this behavior is given.