OXIDATION AND ANION LATTICE DEFECT SIGNATURES OF HYPOSTOICHIOMETRIC DOPED UO2

A series of sintered UO₂ pellets doped with lanthanide elements (Ce, Nd, Yb), Th and Zr were investigated using powder X-ray diffraction, Raman spectroscopy, thermogravimetric analyses and differential scanning calorimetry. A combination of electron microprobe analyses and thermogravimetric analysis, for oxygen content, enabled precise determination of the hypostoichiometry for 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 oxidation of (U_1-yM_y)O_2-x and the anion defects introduced by dopant element 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 (U_1-yM_y)O₂ after oxidation was also examined using Raman spectroscopy, which revealed that the relative area of the LO Raman band at ~540 cm^-1, attributed to oxygen vacancies, increased relative to the T_2g band with increasing dopant concentration, and decreased upon oxidation. The chosen dopants inhibited oxidation to U₃O₈, most prominently for Yb 5 at%, having been delayed by ~180° C. Many of the doped samples exhibit an early oxidation feature that may be related to influx of O to satisfy hypostoichiometry up to (U_1-yM_y)O₂, and subsequently stabilizing a U₄O₉ or U₃O₇ intermediate, delaying oxidation to U₃O₈.