Surface and bulk properties of cerium atoms in several cerium intermetallic compounds: XPS and X-ray absorption measurements

The authors present the results of XPS (X-ray photoemission spectroscopy) and XRA (X-ray absorption) studies performed on several intermetallic cerium compounds (CeRh3, CeCo5, CeNi5, CeNi2, CeRu4B4, CePd3 and CeBe13). In CePd3 and CeBe13, the cerium is known to be in the so-called intermediate valence state; in CeRh3 and CeCo5, the cerium is thought to be in the tetravalent state: Ce4+ (4f05d2). Finally for CeNi5, CeNi2 and CeRu4B4 the situation is unclear. The main purpose of this paper is to show that the valence of cerium in these compounds cannot be deduced from XPS and XRA measurements in the same way as it is for other rare-earth compounds (Tm or Sm for instance). Indeed pure tetravalent compounds like CeRh3, CeCo5 or even insulating CeO2, present XPS and XRA spectra which are similar to those obtained for intermediate valence compounds like TmSe, SmB6, etc.... If one supposes that the final states starting from a Ce4+ initial state are essentially made of configurations which contain one more electron in the 4f shell, the authors show that the valencies of cerium at the surface and in the bulk, deduced from their XPS and XRA experiments, are in approximate agreement with those deduced from lattice parameter measurements. Then, the direct observation of replicates in the XPS or XRA spectra of these cerium intermetallic compounds cannot be considered as a proof of the existence of an intermediate valence state.