Beyond Ce-U Charge Transfer: Strain and Polarons in Epitaxial Ce_{1- 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 Ce_{1- 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 Ce_{1- 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 O^2- 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 Ce_{1- x} U _x O₂(111), with important implications for ceria-based redox materials used in catalysis and thermochemical fuel production.