Heterogenous mixed-valence and compound formation in ordered Yb/Ni(100) overlayers

The Yb valence state for ordered Yb/Ni(100) overlayers has been studied by use of photoelectron spectroscopy. At coverages between 50% and 100% of a densely packed Yb layer a $c(10\mathrm{}\times 2)$ surface structure is seen. The ultraviolet photoemission spectrum for this situation reveals both divalent and trivalent $4f$ emission features, indicating a mixed-valent state. From the observed $4f$ binding energies it is concluded that the mixed valence is of a heterogenous nature. The results are interpreted in terms of the formation of an epitaxial surface intermetallic compound with Yb distributed over several atomic layers. The results demonstrate the importance of surface compound formation in the present type of systems and offers an alternative explanation to the homogenous mixed valence recently proposed for related Sm overlayers.