Stability mechanism of cuboctahedral clusters in UO2: First-principles calculations

TL;DR

This study uses first-principles calculations to uncover how cuboctahedral clusters in nonstoichiometric uranium dioxide are stabilized, highlighting the role of molecular clusters and temperature effects on their stability.

Contribution

It reveals the stability mechanism of cuboctahedral clusters in UO2, emphasizing the importance of U6O12 molecular clusters and temperature-dependent entropy effects.

Findings

Structural stability inherited from U6O12 molecular cluster

Energy gain from oxygen at cluster center over point interstitials

Temperature favors point interstitials over clusters

Abstract

The stability mechanism of cuboctahedral clusters in nonstoichiometric uranium dioxide is investigated by first-principles LSDA+U method. Calculations reveal that the structural stability is inherited from U6O12 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 <111> direction also leads the cluster 8-folded degeneracy and increases relatively the concentration at finite temperatures. But totally, elevation of temperature, i.e., the effect of entropy, favors point interstitial over cuboctahedral clusters.