An investigation of defect structures in single-crystal UO2 +x by optical absorption spectroscopy

Abstract

Optical absorption spectra of single-crystal UO_{2 +x}(sample thicknesses ca. 20 µm) have been recorded in the visible and near-infrared regions as a function of excess oxygen, where x ranged between 0 and 0.06. As excess oxygen entered the lattice four main changes were identified in the spectra: (i) the absorption edge commencing around 2 eV shifted to higher energy, (ii) a reduction occurred in the contribution to the spectra due to U⁴⁺(5ƒ²) intracationic transitions, (iii) a small peak appeared at 0.78 eV and (iv) a very broad band arose, with a half-width > ca. 1 eV and a possible maximum around 2 eV. These changes are here attributed to specific changes in the charge state of the uranium sublattice associated with the formation of oxygen interstitial clusters. The origin of the main broad band is considered with reference to symmetric and unsymmetric intervalence transitions and their association with 2 : 1 : 2 and 2 : 2 : 2 defect clusters. The results are discussed in terms of the initial appearance of 2 : 1 : 2 clusters as excess oxygen enters the lattice, and that these only become 2 : 2 : 2 clusters when x in UO_{2 +x} reaches ca. 0.1.