Stability mechanism of cuboctahedral clusters in U O2+x: First-principles calculations

The stability mechanism of cuboctahedral clusters in nonstoichiometric uranium dioxide is investigated by first-principles local density approximation with Hubbard correction method. Calculations reveal that the structural stability is inherited from U6 O12 molecular cluster, whereas the energy gain through occupying its center with an additional oxygen makes the cluster win out by competition with point oxygen interstitials. Local displacement of the center oxygen along the 111 direction also leads the cluster eightfolded degeneracy and increases relatively the concentration at finite temperatures. However, totally, elevation of temperature, i.e., the effect of entropy, favors point interstitial over cuboctahedral clusters.