Determination of the Insulation Gap of Uranium Oxides by Spectroscopic Ellipsometry and Density Functional Theory

Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U–O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting...