Nature of Ce 4f electronic states from 4d excitations in metals and insulators

We have investigated a variety of metallic and nonmetallic Ce materials by soft-X-ray-absorption spectroscopy at the N4,5 thresholds employing the total-electron-yield method. All metallic systems, irrespective of their γ-like or α-like properties, exhibit similar multiplet structures in the prethreshold region, corresponding to 4d94f2 final states. In contrast, 4d94f1 multiplets are observed for CeF4 and CeO2, as in LaF3, in accordance with the absence of localized 4f electrons in the ground states of these compounds. We conclude that tetravalent Ce compounds occur among insulators, but do not exist as metals