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

The redox chemistry of mixed ceria-based oxides is commonly interpreted in terms of direct charge transfer between neighboring cations. In the Ce1- x U x O2 system, this corresponds to the oxidation of U4+ into U5+ accompanied by the reduction of Ce4+ into Ce3+. In this work, epitaxial Ce1- x U x O2(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 (O l ) 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- x U x O2(111), with important implications for ceria-based redox materials used in catalysis and thermochemical fuel production.