Lattice defects introduced into LaNi 5-based alloys during hydrogen absorption/desorption cycling

Abstract

LaNi 5-based intermetallic compounds before and after hydrogen absorption/desorption cycling were observed by transmission electron microscopy. Both a-type and c-type dislocations, which had respective Burgers vectors of 1 / 3 < 1 1 2 ¯ 0 > and <0 0 0 1> in the { 0 1 1 ¯ 0 } slip plane, were introduced into LaNi 5, LaNi 4.5Cu 0.5 and LaNi 4.5Fe 0.5 after the first absorption/desorption cycle. These alloys exhibited large difference in the equilibrium pressure between the first absorption and first desorption. Dislocations were introduced during hydrogenation in order to accommodate lattice mismatch between the metal-hydrogen solid solution and the metal hydride; furthermore, the interface between the solid solution and hydride was suggested to be the { 0 1 1 ¯ 0 } plane. In contrast, neither dislocations nor stacking faults were observed in LaNi 4.5Si 0.5, LaNi 4.5Al 0.5 and LaNi 4.75Sn 0.75, which showed little or no hysteresis in the hydrogen pressure–composition isotherms. Excess pressure was needed in the absorption process for the formation of misfit dislocations.