Effects of lattice defects on hydrogen absorption–desorption pressures in LaNi5

The relationship between microstructures and hydrogen absorption-desorption properties described in terms of pressure-composition (P–C) isotherms has been investigated as a function of the number of hydrogen sorption cycles. While the desorption pressure does not significantly depend on the number of cycles, the absorption pressure decreases as the number of cycles increases with the most significant decrease occurring between the first and second cycles. TEM observations reveal that numerous a-type edge dislocations are introduced during the first absorption cycles. Severe cracking occurs also during the first absorption cycle in such a way that the size of sub-particles delineated by cracks is comparable to that of particles observed after five cycles. The introduction of numerous lattice defects, such as a-type edge dislocations as well as the formation of severe cracks, is responsible for the significant decrease in absorption pressure between the first and second cycles, indicating that reproducible P–C characteristics of LaNi5 can be achieved only when such numerous dislocations and cracks are formed in the bulk of LaNi5 particles.