Lattice defects introduced into LaNi5-based alloys during hydrogen absorption/desorption cycling

LaNi5-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 < 11<(2)over bar>0 > and < 0001 > in the {01 (1) over bar0} slip plane, were introduced into LaNi5, LaNi4.5Cu0.5 and LaNi4.5Fe0.5 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 {01 (1) over bar0} plane. In contrast, neither dislocations nor stacking faults were observed in LaNi4.5Si0.5, LaNi4.5Al0.5 and LaNi4.75Sn0.75, 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. (C) 2011 Elsevier B. V. All rights reserved.