Ab initio study of H and He migrations in β-phase Sc, Y, and Er hydrides

Ab initio calculations based on the density functional theory have been performed to investigate the migrations of hydrogen (H) and helium (He) atoms in β -phase scandium (Sc), yttrium (Y), and erbium (Er) hydrides with three different ratios of H to metal. The results show that the migration mechanisms of H and He atoms mainly depend on the crystal structures of hydrides, but their energy barriers are affected by the host-lattice in metal hydrides. The formation energies of octahedral-occupancy H (H_oct) and tetrahedral vacancy (V_mtet) pairs are almost the same (about 1.2 eV). It is of interest to note that the migration barriers of H increase with increasing host-lattice atomic number. In addition, the results show that the favorable migration mechanism of He depends slightly on the V_mtet in the Sc hydride, but strongly on that in the Y and Er hydrides, which may account for different behaviours of initial He release from ScT₂ and ErT₂.