Oxidation and anion lattice defect signatures of hypostoichiometric lanthanide-doped UO2

Abstract A series of sintered UO2 pellets doped with lanthanide (Ce, Nd, Yb) elements were investigated using powder X-ray diffraction, Raman spectroscopy, thermogravimetric analyses and differential scanning calorimetry. A combination of electron microprobe and thermogravimetric analyses, for oxygen content, enabled precise determination of the hypostoichiometry for lanthanide-doped samples at 1 and 5 atom percent. Two Raman laser wavelengths (785 and 455 nm) have afforded greater sensitivity to spectroscopic signatures of the phonon bands (1LO and 2LO) associated with oxidation of (U1-yMy)O2-x and the anion defects introduced by lanthanide substitution. Oxygen hypostoichiometry forces a reduction in the average coordination number surrounding (U,M) sites, which is compensated by a decrease in U–O bond length, and concomitantly the lattice parameter, consistent with the obtained Raman spectra. The evolution of O/M ratio up to (U1-yMy)O2 after oxidation was also examined using Raman spectroscopy, revealing that the ‘defect band’, including a component attributed to oxygen vacancies (∼540 cm−1) and the 1LO phonon (∼575 cm−1) increased in intensity with increasing dopant concentration and upon oxidation. The lanthanide dopants inhibited oxidation to U3O8, most prominently for Yb 5 at%, having been delayed by ∼180 °C. Thermogravimetric analyses reveal an early oxidation feature that may be related to influx of O to satisfy hypostoichiometry up to (U1-yMy)O2, possibly stabilizing a U4O9 or U3O7 intermediate, delaying oxidation to U3O8.