Density functional study of lithium vacancy in Li4SiO4: Trapping of tritium and helium

Li4SiO4 is a solid breeder material that has important applications in future fusion reactors. The interaction of tritium/helium with lithium vacancy is investigated implementing pseudopotential plane wave method within density functional theory. Models of all types of lithium vacancies and vacancy−tritium/helium defect complexes are created. Possible tritium trap sites in lithium vacancies are examined and the formation energies, vacancy−tritium/helium interaction energies and electronic structures of the defects are calculated. The results indicate that the tritium atom trapped in the lithium vacancy bonds with one of the surrounding oxygen atoms. The formation energies of the vacancy−tritium complexes are in the range of 0.41-1.28 eV under oxygen-rich condition. The interaction energy calculation shows the lithium vacancy has strong tritium trapping capabilities. Moreover, the vacancy−helium complex formation energies are in the range of 1.97-3.43 eV under O-rich condition. The vacancy−helium interaction is relatively weaker, suggesting the helium atom may escape the lithium vacancy more easily.