Beyond Ce–U Charge Transfer: Strain and Polarons in Epitaxial Ce1– x U x O2(111)

Abstract The redox chemistry of mixed ceria-based oxides is commonly interpreted in terms of direct charge transfer between neighboring cations. In the Ce1–xUxO2 system, this corresponds to the oxidation of U4+ into U5+ accompanied by the reduction of Ce4+ into Ce3+. In this work, epitaxial Ce1–xUxO2(111) thin films with different uranium contents (with x ranging from 0.05 to 1) were grown under ultra-high vacuum conditions and investigated using angle-resolved X-ray photoelectron spectroscopy. The measurements reveal a substantial excess of Ce3+ relative to U5+, particularly at low uranium fractions and in the topmost surface region, indicating that the reduction of Ce4+ cations cannot be explained solely by direct U4+ to Ce4+ charge transfer. Quantitative analysis further shows deviations in oxygen stoichiometry when lattice oxygen (Ol) is assumed to remain entirely in the O2– state. The effect is stronger in epitaxial films and largely absent in structurally disordered counterparts. These observations support a picture in which oxygen ions participate in the redox process through lattice distortion and polaron-mediated charge redistribution. The results suggest that (i) oxygen-mediated electronic coupling, (ii) strain, and (iii) local structural order collectively govern the reduction behavior in Ce1–xUxO2(111), with important implications for ceria-based redox materials used in catalysis and thermochemical fuel production.