Valence instabilities in rare earth systems as revealed by photoemission

We used high resolution (δE < 150 meV) photoemission with synchrotron radiation in the range 10 eV < hv < 130 eV to study archetypal mixed valent and related cerium-, samarium-, europium-, thulium- and ytterbium-based systems. Generally all mixed valent systems (except the cerium-based system which is not yet fully characterized) exhibit an integer valent surface layer which has the lower value of the two bulk valence configurations. Temperature-dependent changes in bulk valence were observed in the europium and samarium compounds. Mixed valence was observed to persist down to at least 5 at.% dilution of thulium in TmxY1-xSe while thulium appears to remain trivalent in all TmxY1-xS compounds studied. © 1983.