Structural and electronic properties of U4O9 from ab initio and empirical potential calculations

Abstract

We address here the description of the different phases of ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ with ab initio density functional theory and molecular dynamics empirical potential calculations. These phases are built from the $\mathrm{U}{\mathrm{O}}_{2}$ fluorite by the addition of oxygen interstitials which assemble in clusters named cuboctahedra. Due to the unit cell size and complexity, their simulation represents a formidable task for numerical approaches. With $\mathrm{DFT}+U$, we study in detail two potential structures for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{U}}_{4}{\mathrm{O}}_{9}$, however, these are just two representatives among the numerous coexisting structures we found. The role of the different valence uraniums (${\mathrm{U}}^{4+}, {\mathrm{U}}^{5+}$, and possibly ${\mathrm{U}}^{6+}$) is highlighted thanks to our quantum-mechanical approach. Temperature effects are then appraised with empirical potentials to describe the evolution of the ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ structure with temperature. We observe a continuous symmetrization of the structure with increasing temperature. Indeed, below 800 K, it gradually turns cubic and the cuboctahedral oxygen clusters tend to symmetrize. Beyond 800 K, the cuboctahedra start to disappear up to 1400 K, where none can be found in our simulated structures.