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

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 spectro-scopic signatures of the phonon bands (1LO and 2LO) associated with oxidation of (U1-y My)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 0/M ratio up to (U1-y My)O-2 after oxidation was also examined using Raman spectroscopy, revealing that the 'defect band', including a component attributed to oxygen vacancies (similar to 540 cm(-1)) and the 1LO phonon (similar to 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 similar to 180 degrees C. Thermogravimetric analyses reveal an early oxidation feature that may be related to influx of 0 to satisfy hypostoichiometry up to (U1-yMy)O-2, possibly stabilizing a U4O9 or U3O7 intermediate, delaying oxidation to U3O8. (C) 2019 Elsevier B.V. All rights reserved.