Covalent versus localized nature of 4f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory

Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping

“…1(g) and 1(h)]. The observed divergence between the Ce 4+ and Ce 3+ ions is in line with recent theoretical 26 and experimental 37 examination of the cation-anion interactions in ceria, where the former is found to be more sensitive to the anion shell.…”

“…1(j)], which is indicative of the contribution of the Ce 4+ ions to those states. 37,51 It is important to note that the resonant enhancement follows a Fano-type profile, 52 with a sharp rise in intensity below the absorption threshold and a Fig. 1(i)].…”

“…Our previous work on the subject contains a comprehensive explanation of the resonant photoemission enhancement at the Ce 4d → Ce 4f photoabsorption threshold. 37 For further details regarding the Ce 4d → Ce 4f resonant enhancement in ceria, we refer the reader to this work. We begin the discussion of the RPES spectra by comparing the samples containing virtually only Ce 3+ ions, viz.…”

“…RPES is conducted via tuning the primary photon energy so as to induce a constructive interference of electrons generated by a direct photoemission process and those generated via an Auger decay. We have previously shown that the resonant photoemission process in (nominally) pure ceria depends on the (initial) charge state of the photoionized cation in the following way: 37 For a Ce 3+ ion, it is an intra-atomic process that greatly enhances the spectral gain from 4f electrons, allowing detection of such species when present even in very small concentrations, Ce 4d 10 4f 1 O 2p 6 + hν → Ce 4d 9 4f 2 O 2p 6 → Ce 4d 10 4f 0 O 2p 6 + e − .…”

“…As such, RPES allowed us to directly demonstrate that the resonant enhancement of Ce 4+ is brought about by covalent interaction of the Ce 4f and O 2p states, indicating a partially covalent character of the Ce-O bond. 37 In addition to the results obtained with RPES, we use the results of hybrid density functional theory calculations to link the measured electronic structure with the atomic scale structure of the surface. We show how the fluorine contribution to the valence band changes in relation to its depth distribution and identify a specific configuration of fluoride ions bonded to Ce 4+ at the surface that gives rise to an F 2p band which is shifted toward the top of the O 2p band and exhibits an appreciable degree of covalency.…”