Crystal structural studies of AB₅-type, BCC and Zintl phase hydrogen absorbing alloys

Our progress on crystal structural studies on hydrogen absorbing alloys including intermetallics and solid solutions is reviewed. LaNi₅ shows an extremely high equilibrium pressure at the first hydrogen absorption, but not at the second one. The structural change in the first absorption was followed by in situ X-ray diffraction. We found that the crystalline size did not change but the anistropic strain was introduced to the lattice of the hydride formed. The strain introduced was not relaxed in the following dehydrogenation and hydrogenation. However, the behavior of LaNi_4.75Al_0.25 for the first absorption was completely different from LaNi₅. The strain observed for LaNi₅ was not found at the first absorption but a reversible stain parallel to the c-axis was observed in desorption. The crystal structure of Laves phase related BCC alloys and their hydrides was refined by the Rietveld method using in-situ X-ray and neutron powder diffraction data. It was confirmed that these alloys had BCC structure, which means constituent elements occupy the BCC sites randomly. A new class of hydride with Zintl anion was found and the crystal structure of hydride/deuteride was refined using neutron diffraction.