Structural and electronic properties of U$_4$O$_9$ from ab initio and empirical potential calculations
Résumé
We address here the description of the different phases of U$_4$O$_9$ with ab initio Density Functional
Theory and molecular dynamics empirical potential calculations. These phases are built from the
UO$_2$ 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 DFT+U, we study in details two potential structures for α-U$_4$O$_9$, however these are just 2 representative among the numerous coexisting structures we found. The role of the different valence uranium (U$^{4+}$, U$^{5+}$, and possibly U$^{6+}$) is highlighted thanks to our quantum-mechanical approach. Temperature effects are then appraised with empirical potentials to describe the evolution of U$_4$O$_9$ structure with temperature. We observe a continous 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
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