Behavior of vacancy formation and recovery during hydrogenation cycles in LaNi_4.93Sn_0.27

To clarify the effect of substitution of Sn for Ni of LaNi₅ on cyclic durability of hydrogen storage, the behavior of lattice defects during the hydrogenation cycles at 288 K in LaNi_4.93Sn_0.27 was investigated by in situ positron lifetime measurements. Mean positron lifetime increased to 175 ps by vacancy formation during initial hydrogenation and then decreased to 135 ps by vacancy recovery during dehydrogenation. Vacancies were reversibly introduced and removed at 288 K, although quenched-in vacancy cannot migrate below 448 K in LaNi_4.93Sn_0.27. Reversible change of the positron lifetime was observed in subsequent cycles, and dislocation density and vacancy concentration remained almost constant at around 6 × 10⁹ cm^-2 and 10 ppm, respectively, which are two orders of magnitude lower than those in binary LaNi₅. These results indicate that the degradation of hydrogen capacity during a cycle closely relates to the concentration of lattice defects accumulated with hydrogenation.