X-ray photoemission study of Ce-pnictides

All the Ce pnictides crys allize in the NaCl structure with lattice constant increasing systematically from CeN to CeBi. Since Ce has the largest $4f$-orbital radius in the entire lanthanide series, it is quite interesting to follow the evolution of his level as the distance between the Ce atoms is varied. The core levels and the valence-band region of CeN, CeP, CeAs, and CeSb have been studied by high-resolution x-ray photoemission spectroscopy. The $4d$ and $3d$ core-level spectra of Ce show clearly that the $4f$ state remains strictly localized in all compounds. In CeN a superposition of lines corresponding to two valences is found which reveals the intermediate valence character of this compound. In the valence-band spectrum, the $4f$ level is found to be pinned at the Fermi level and superimposed on the $5d$ states. The extended valence states originating from the anion $p$ states are located at higher binding energies. In the compounds of heavier rare-earth elements the $4f$ state moves away from the Fermi level and gradually overlaps in energy with $p$ states, so that the probability for interatomic Auger processes in the photoemission final state increases rapidly. The natural width of the $4f$ peak is already 0.8 eV in CeP, and becomes so large in CeAs and CeSb that this line can no longer be unambiguously identified in the other valence-band structure. Finally, it can be concluded from the similarity of the $4d$ spectrum of metallic Ce and CeN that the $4f$ electron remains localized in the $\alpha$ phase of this metal.