Comprehensive First-Principles Investigation of Tritium Migration in γ-LiAlO2

Abstract In this work, we employ density functional theory (DFT) calculations to investigate the atomic-scale mechanisms of hydrogen (H) migration (using H as a model substitute for tritium) in both pristine and lithium-vacancy-containing γ-LiAlO2. In the pristine lattice, three interstitial configurations were identified: oxygen-bonded (BI), tunnel (TI), and cage (CI) interstitial configurations. Charged-defect calculations show that protonic H (H+) at the BI site is the lowest-energy H configuration for Fermi levels near the valence-band maximum, EF < 2.82 eV. H+, however, is not structurally stable at the TI and CI sites and instead relaxes toward a BI-like O–H bonded configuration. Neutral H (H0) is structurally stable at CI and TI sites and has lower formation energy than anionic H (H–) for Fermi levels near the valence-band maximum up to approximately midgap, whereas H– becomes lower in energy toward the conduction-band minimum and is the most thermodynamically stable species under high-Fermi-level conditions (EF > 3.80 eV). When all relaxed configurations and charge states are compared, H0 is not the lowest-energy isolated H species over the Fermi-level range considered. H+ migrates between adjacent BI sites with a migration barrier of 0.66 eV, which is comparable to the previously reported vacancy-mediated migration barrier [ Paudel, H. P.; et al.. J. Phys. Chem. C2018, 122, 9755–9765. ]. H– migrates along the tunnel direction with a relatively low barrier of 0.13 eV, suggesting that H– may contribute to tunnel-directed transport under electron-rich conditions. In contrast, higher barrier along the cage-involving pathway (0.72 eV) suggests that H– migration through the cage is less favorable. For locally stable H0 configurations, even though the TI–TI migration barrier is exceptionally small (0.021 eV), this pathway is best interpreted as a kinetically fast but thermodynamically limited migration channel. Furthermore, in the intermediate Fermi-level range 2.82 eV < EF < 3.80 eV, an interstitial H2 molecule is thermodynamically favorable within the tunnel and retains a gas-phase-like H–H bond length and dissociation energy; once present, it migrates along the tunnel with a barrier of ∼0.36 eV. Finally, a negatively charged lithium vacancy can bind up to two H+ ions, whereas the addition of a third H+ is thermodynamically unfavorable. The estimated detrapping energies indicate that the bound H+ ions are strongly trapped at Li-vacancy-associated configurations.