First-principles study of helium incorporation, clustering, and migration in Gd2Zr2O7 pyrochlore

Helium produced by α-decay is a key factor controlling the long-term radiation tolerance of ceramic waste forms. Here, density functional theory calculations with a Hubbard U correction were used to examine He incorporation, clustering, and migration in Gd 2 Zr 2 O 7 pyrochlore. The fully relaxed calculations show that He is most strongly trapped by Gd vacancies among the vacancy defects considered, with a formation energy of 0.150 eV. For interstitial configurations, He atoms initially located at Gd-Gd interstitial positions do not remain there; instead, local oxygen displacement drives relaxation toward a previously unreported tetrahedral-like site coordinated by two Gd and two Zr atoms. This relaxation lowers the apparent formation energy to 0.200 eV and explains the discrepancy with constrained calculations reported previously. Binding-energy analysis further indicates that Gd vacancies and the new tetrahedral-like site mainly trap isolated He atoms, whereas the octahedral interstitial region can accommodate small clusters of up to three He atoms before He-He repulsion becomes dominant. Structural relaxation of these clusters shows an octahedral-centered motif with neighboring interstitial occupation rather than simple on-site filling. The calculated migration barrier between adjacent tetrahedral-like sites via the octahedral transition state is 0.83 eV, indicating limited intrinsic He mobility in the perfect lattice. These results identify the local relaxation mechanism governing He stabilization and provide atomic-scale guidance for assessing He retention and bubble nucleation in irradiated Gd 2 Zr 2 O 7 .