Ab initio investigation of properties and mobility of helium defects in La₂Sn₂O₇ pyrochlore

The accumulated helium atoms mitigate the physical properties of nuclear waste forms through long-time radionuclide decay and thus cause the safety concern. In this work, first-principles calculations were carried out to investigate the effects of helium atoms on the structural, electronic, and mechanical properties of La₂Sn₂O₇ pyrochlore. He atoms cause small volume swelling but no significant influence on its mechanical properties. The formation energy analysis suggests that octahedral interstitial sites are preferred. He interstitial may drive phase change from pyrochlore to the fluorite structure. The local electronic properties have also been analyzed to assess the impact of He clusters. The minimum energy pathway for helium to migration is identified as via O-O interstitial sites with a barrier of 1.39 eV. This work might offer a theoretical foundation for the He damage of the nuclear waste forms.