Surface compounds of divalent rare-earth metals with palladium

We report an angle-resolved photoemission (PE) study of ordered Yb/Pd(111), Eu/Pd(111), and Ba/Pd(111) interfaces. The low-energy electron-diffraction patterns reveal a 2×{}2 overstructure with respect to Pd(111). From these results and an analysis of the PE spectra, the formation of YbPd₃, EuPd₃, and BaPd₅ compounds is concluded. While in the compounds both Eu and Yb are found to be trivalent in the bulk, Eu becomes divalent at the surface. Although this valence transition is accompanied by a large increase of the ionic radii, the hexagonal in-plane order of the surface atoms is not affected, implying an outward relaxation of the divalent Eu atoms. It is suggested that, in analogy to the Ba compound, the space between the divalent Eu surface atoms is filled by additional Pd atoms.