Isotopic effect of oxygen on the Raman mapping of a polycrystalline uranium dioxide UO2

Abstract

Uranium dioxide (UO₂) is a widely studied material due to its use as fuel in pressurised water reactors (PWR), and Raman spectroscopy is a technique of choice to characterise the evolution of its microstructure. UO₂ crystallises in a fluorite CaF₂ (space group Fm-3m) structure that gives rise to a unique Raman signature, the T_2g band. However, several other bands are often detected whose attribution remains unclear. The present study gives new insights on the Raman spectrum of UO₂ thanks to the combination of isotopic labelling with ¹⁸O and Raman imaging. In addition to the expected T_2g, U₂ (LO), 2LO and U₃ bands, we have detected a doublet at 885 and 925 cm⁻¹, a U* band at 555 cm⁻¹ in some specific areas and two bands located at 367 and 1196 cm⁻¹. All Raman bands shift under the effect of the replacement of ¹⁶O by ¹⁸O, excepting for the U* band that could not be detected anymore. The isotopic shift ratio could be determined for 20% and 30% ¹⁸O labelling. No discrepancy in band position is observed between grains and grain boundaries, except for the U₂(LO) band. We also evidence a difference between the U defect bands and the 885 and 925 cm⁻¹ doublet bands evolution under labelling, although the latter also seems to be connected to the presence of defects in the material.