AbstractThe aim of this experiment is a structural characterization of hyper-stoichiometric uranium oxides containing different proportions of Gadolinium at different temperatures in order to improve our description of oxygen defects. Gadolinium is introduced to simulate the presence of lanthanides which make up an abundant class of fission products. The neutron diffraction patterns corresponding to several samples with different deviations from stoichiometry and doping concentrations will be collected in a temperature range that covers the phase transition between the two-phase region (UO_(2+x) and U_4 O_9) to single-phase region (UO_(2+x))of the U-O phase diagram . The corresponding results (i.e. the PDF analysis and Rietvield refinement) will be compared to the reference data obtained under identical experimental conditions to understand how and to what extent the dopants disrupt the regular arrangement of cuboctahedral clusters that exist in hyperstoichiometric uranium dioxide.