Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions: Lack of miscibility gap

Abstract

The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for the U4+ initial position is responsible for creating local structural disorder and changes in the oxidation states. In this scenario, the lattice parameter is affected and the concentration of U5+ and formation of oxygen vacancies as well, since these are the mechanisms necessary to maintain the charge neutrality. The systematic oxidation of U4+ as predominant charge compensation mechanism over the formation of vacancies can be reduced by performing a thermal treatment under reducing conditions. This paper presents an experimental characterization of the uranium oxidation state mixture and local structure using XAS techniques in samples with chemical formula (U1-yNdy)O2-x, with y = 0.04, 0.17 and 0.25 (0<x < 0.038). In all cases, the deviation from the ideal oxygen stoichiometry of a perfect fluorite is small and the average long-range structure is not affected because also the cation substitution occurs randomly onto the metal sites of the ideal fluorite. However, the local distances of the first atomic shell depend on the actual local chemical composition. The atomic arrangement of the Nd neighbours differs from those of U, which is also sensitive to the overall concentration of Nd and the amount of vacancies.