Stability of 4f configurations in rare-earth-metal compounds

Abstract

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. © 1986 The American Physical Society.