New insights into the structural transition from UO₂₊ to U₃O₇ by quantitative Raman spectroscopy

The study of uranium oxides at different conditions is of paramount importance in the nuclear field, especially regarding characterization of the spent nuclear fuel behavior in dry storage scenarios. This paper reports results of XRD and Raman analysis on four powdered samples prepared in order to cover a specific stoichiometry range in UO₂₊, i.e. x = 0.24, 0.26, 0.28 and 0.30. XRD results reveal a clear increase of the average tetragonal distortion with the increase in oxidation degree, with the main phase detected for all the samples being a weakly tetragonal phase identified as U₃O_7- (c/a ≪ 1.032). U₄O₉ has not been detected in any sample. The Raman study carried out consists of both qualitative and quantitative analysis. The former, where a profile analysis has been performed on the acquired spectra, shows that the most intense bands (centered at ∼455 and ∼635 cm^-1) are actually a doublet each, in agreement with a previous experimental study. Moreover, this work shows, for the first time, that the band at ∼160 cm^-1 is also a doublet, which makes its classical assignment no longer obvious. The most important and original results from this study are obtained by applying Quantitative Raman Spectroscopy (QRS). This analysis shows that the second contribution at ∼475 cm^-1 to the known T_2g mode increases its relative intensity with the oxidation degree. This contribution may be related to the tetragonal distortion occurring in the cubic UO₂ lattice due to the addition of interstitial oxygen, based on its comparison with the obtained XRD outcomes. In addition, the so-called "defects band" (centered at around 600 cm^-1) presents a remarkable kink, of around 20 cm^-1, in its Raman shift between UO_2.26 and UO_2.28. Such behavior might be directly associated with the observed appearance of the stoichiometric U₃O₇ phase (c/a = 1.032) for UO_2.28 and UO_2.30.