Degradation of hydrogen absobing capacity in cyclically hydrogenated TiMn₂

Degradation of the hydrogen absorbing capacity in cyclically hydrogenated TiMn₂ (Ti-60 at.% Mn) with Laves structure was studied by measurement of pressure-composition-isotherm (PCT) curves, X-ray diffraction and microstructure observation. Characteristic changes with cyclic hydrogenation are observed as follows. The hydrogen absorbing capacity remarkably decreases, the lattice constant increases, the amount of hydrogen retained in TiMn₂ increases and it approaches almost zero by dehydrogenation at 673 K in vacuum. Nano-sized regions with Moiré patterns are produced in TiMn₂ with hydrogenation and Debye rings corresponding to titanium hydride 5-TiH appear in the diffraction pattern. Based on these observations it is concluded that the degradation of hydrogen absorbing capacity after cyclic hydrogenation is attributable to the introduction of retained hydrogen, heterogeneous strain and/or nano-sized regions.