| Abstract Scope |
Lanthanide-incorporated uranium dioxide system has intriguing dopant chemistry, defect compensation, and oxidation behavior are strongly coupled under extreme environments. Here, we combine X-ray absorption fine structure and high-temperature X-ray diffraction to determine how fabrication atmosphere controls the local structure and oxidation response of Ce-, Nd-, and Gd-doped UO₂. Ce readily incorporates into the fluorite lattice with limited structural distortion, whereas trivalent Nd and Gd produce stronger local lattice perturbations that depend sensitively on oxygen chemical potential during synthesis. Reduced Ar–H₂ conditions favor oxygen-vacancy compensation and vacancy association near Ln guest atoms, while non-reduced Ar conditions promote U(V)-based charge compensation. These defect-chemistry differences directly influence high-temperature oxidation resistance, with (Gd, U)O2 showing greater stability than (Nd, U)O2. This work demonstrates how coupled synchrotron-based local-structure probes and in-situ thermal diffraction can reveal defect-mediated degradation pathways in nuclear fuel materials exposed to chemically and thermally extreme conditions. |