Ordered defects in the oxides of uranium

Abstract

Like some other oxides of the actinide elements, uranium dioxide has a fluorite structure and a propensity towards deviation from simple stoichiometry. Because oxygen can be transferred with ease between the UO 2 lattice and the ambient gas phase, gross departures from stoichio­metry occur readily. Diffraction data have shown that structural models of the hyperstoichiometric phases U 4 O 9 and U 3 O 7 cannot be based on a random distribution of point defects. Instead, interaction between interstitial defects produces microscopic substructures or clusters intro­ducing specific local properties. Spectroscopic evidence is presented together with the alternative cluster arrangements which are currently proposed as the basic structural units occurring in the hyperstoichiometric oxides of uranium. From this base a new relationship for the structure of the uranium-oxygen system has been developed for the compositional range UO 2 -UO 3 .