This paper discusses uranium oxide crystal structure modifications that are observed during the low-temperature oxidation which transforms UO(2) into U(3)O(8). The symmetries and the structural parameters of UO(2), beta-U(4)O(9), beta-U(3)O(7), and U(3)O(8) were determined by refining neutron diffraction patterns on pure single-phase samples. Neutron diffraction patterns were also collected during the in situ oxidation of powder samples at 483 K. The lattice parameters and relative ratios of the four pure phases were measured during the progression of the isothermal oxidation. The transformation of UO(2) into U(3)O(8) involves a complex modification of the oxygen sublattice and the onset of complex superstructures for U(4)O(9) and U(3)O(7), associated with regular stacks of complex defects known as cuboctahedra, which consist of 13 oxygen atoms. The kinetics of the oxidation process are discussed on the basis of the results of the structural analysis.

International audienceCharacterisation of uranium oxides in different conditions is a challenge both in nuclear and environment sciences. We focus here on U4O9, which is a superstructure of UO2. Homogeneous U4O9 powder was fabricated and characterised by Raman spectroscopy. The Raman spectrum of a pure-phase U4O9 is, for the first time, completely described and interpreted. U4O9 Raman spectrum derives from the well characterised one of UO2. Besides reminiscent modes of UO2, a band at 630 cm-1, which has a specific response to polarised light, was interpreted as characteristic of clusters of interstitial oxygen atoms. These experimental data will help to rationalise the interpretation of UO2 damaged samples