New insights into the structural transition from UO2+x 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+x, 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 U3O7−z (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 T2g 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 UO2.26 and UO2.28. Such behavior might be directly associated with the observed appearance of the stoichiometric U3O7 phase (c/a = 1.032) for UO2.28 and UO2.30.