First principles modeling of stability mechanism of nonstoichiometric uranium dioxide

To understand the stability mechanism of defects in the nonstoichiometric uranium dioxides, first-principles calculations have been performed by PAW-LSDA + U method for various defects clusters formed from interstitial oxygen atoms and the lattice vacancies. Calculations revealed that the cuboctahedron cluster embedded into the crystal UO₂ with one O-atom at the center is the most stable configuration among all known clusters including point oxygen interstitials at the ground state. This picture clarified the ambiguity remaining for long in structure of nonstoichiometric UO_2+x. By incorporating the temperature effect, concentrations of different types of defects clusters are evaluated, then a pseudo phase diagram of temperature and the oxygen concentration has been constructed, which led to a new physical model of the thermodynamic competition between cuboctahedron and point oxygen interstitials in UO_2+x. It shows that at low temperature, the cuboctahedral clusters dominate the stability, whereas at elevated temperature, point interstitial is more favorite over the cuboctahedral clusters.