Stability of 4fconfigurations in rare-earth-metal compounds

Surface effects of the 4f and 5p binding energies as well as on the valence state were investigated for the dialuminides of the heavy rare-earth (RE) metals by high-resolution photoemission (PE) spectroscopy using synchrotron radiation. For comparison, Eu, Yb, and Lu metals as well as YbZn₂ were also studied. Complementary bremsstrahlung isochromat spectroscopy measurements were performed on GdAl₂, ErAl₂, and TmAl₂. A pronounced difference in the magnitude of the surface core-level shifts and the intensity of the surface signals was observed between trivalent and divalent (including mixed-valent) RE compounds. The considerably larger shifts of the divalent (mixed-valent) compounds as compared to the trivalent ones are shown to be due to the larger heat of compound formation for the latter. The high surface-to-bulk intensity ratio in the PE spectra of divalent and mixed-valent compounds is discussed alternatively in terms of either a reduced mean free path due to the 4f scattering or a possible surface segregation of RE ions. On the basis of Johanssons's stability diagram for RE metals and the measured 4f binding energies for compounds, a simple scheme is presented which allows a prediction of 4f configurational stability in a whole series of RE compounds. In agreement with this model, a divalent surface layer was observed for trivalent SmAl₂, but not for SmPd₃.