SURFACE EFFECTS IN RARE-EARTH MATERIALS

Recent experimental results concerning the surface-related electronic structure of rare-earth (RE) compounds are presented. Special attentio n is paid to the occurrence of surface-valence transitions for trivale nt and mixed-valent compounds of Sm, Eu, Tm and Yb which are due to an energetical lowering of unoccupied 4f states caused by the reduced at omic coordination at the surface. Similar phenomena are observed for s o-called alpha-like Ce compounds which are characterized by a strong h ybridization of the 4f levels with valence-band states. Here, the redu ced atomic coordination causes mainly a decrease in hybridization lead ing to an enhanced localization of the 4f states in the outermost atom ic surface layer. The importance of this phenomenon for the correct in terpretation of electron-spectroscopic data of Ce systems is discussed .