Abstract
UO2.07 was characterized using neutron total scattering in order to elucidate defect morphology in the low oxygen-to-metal regime (x<0.125 for UO2+x). Data were collected at temperatures (600 and 1000 °C) coinciding with the single-phase UO2+x region of the established phase diagram, and results were compared with data of stoichiometric UO2 collected at near-identical temperatures. Experimental data were modeled and interpreted using a holistic approach employing a combination of analyses that characterized multiple spatial length scales. Preferential modeling of long-range atomic arrangements with Rietveld refinement suggests the existence of primarily monointerstitials in UO2.07, whereas preferential modeling of short-range atomic structures with small-box pair distribution function (PDF) refinement indicates the presence of defect clusters in UO2.07. Simultaneous modeling of multiple length scales using complementary reverse Monte Carlo (RMC) and molecular dynamics (MD) methods confirms that excess oxygen atoms in UO2.07 exist as small defects, such as monointerstitials and di-interstitials. RMC and MD results agree with diffraction analysis but differ significantly from small-box PDF refinements, which may be related to a lack of intermediate- and long-range structural information gained from the small-box PDF refinement procedure. Employing a combination of analysis methods with varying length-scale sensitivities enabled more accurate assessment of the UO2+x defect structure. Our findings provide experimental support for previously predicted di-interstitial defect morphologies in UO2+x that highly influence the accurate prediction of bulk physiochemical properties of UO2+x, such as oxygen diffusivity.
