Hydrogen capacity in Li4SiO4 crystal in presence with vacancy

Tritium retention is of major importance to the performance of solid breeder in nuclear fusion. Hydrogen capacity, which represents the tritium capacity in the solid breeder materials, has attracted much attention in recent years. In this work, the accumulation behavior of H atoms in defect Li4SiO4 models with vacancy is systematically investigated by first-principles calculations. Different Li4SiO4 defect models with Li vacancy, Si vacancy, and O vacancy are built up and optimized, respectively. And H atoms are added into the vacancy one by one and fully relaxed. The Bader charge and the density of states analysis show the H atoms bond with O dangling bonds in the vacancy and the Li vacancy serves as a nucleation center for H2 molecules. Moreover, by calculating the trapping energy, the Voronoi volume and the lattice distortion, the trapping and accumulation behavior of H atoms inside and outside different vacancies are obtained and compared. The results reveal that the defect models with vacancy present stronger trapping abilities than that of the perfect crystal, while defect models with a Li vacancy is able to accommodate more H atoms than that with a Si vacancy. 14 pairs of H2 molecules are observed maximally in the defect model with a type of VLi6 vacancy when the number of trapped H atoms can achieve 1.96 × 1022 atom/cm3. This work quantitatively determines the hydrogen capacity of the Li4SiO4 crystal and the obtained results can help explain and understand the related tritium release and retention behaviors in Li4SiO4 crystal.