It was recalled that positron lifetime measurements revealed that huge numbers of excess vacancies were formed, in addition to dislocations, during the initial H absorption process in H storage materials. Here, first-principles calculations were made of the positron lifetimes of vacancies as well as the bulk state in LaNi5-H and Pd-H systems in order to identify the lattice defects formed during H absorption and during isochronal annealing after H desorption. By comparing experimental and theoretical positron lifetimes, one of the defect components which formed during H absorption could be attributed to annihilation at vacancy clusters composed of 2 or 3 vacancies in both LaNi5–H and Pd–H. During isochronal annealing after H desorption in LaNi5, the positron lifetime of the vacancy-cluster component in LaNi5 increased from 190 to over 400ps. This indicated the formation of 3-dimensional vacancy clusters. On the other hand, the vacancy-cluster component in Pd decreased from 200 to 160ps during isochronal annealing. This suggested that not 3-dimensional, but 2-dimensional, vacancy clusters formed during the recovery process of vacancies in Pd.

First-Principles Calculations of Positron Lifetimes of Lattice Defects Induced by Hydrogen Absorption. M.Mizuno, R.Tonomori, K.Sakaki, H.Araki, Y.Shirai: Solid State Ionics, 2004, 172[1-4], 149-53