Structural observation of nano-structured and amorphous hydrogen storage materials by neutron diffraction

Neutron diffraction is a powerful tool to elucidate the structure of hydrogen storage materials. H/D isotopic substitution was employed to observe the location of deuterium atoms because the coherent scattering length of deuterium is large enough to observe in comparison with that of the other atoms forming hydride materials. This work focused on two kinds of hydrogen storage materials to investigate the location of deuterium atoms. One is the deuterated nano-graphite prepared by mechanical milling under deuterium gas atmosphere and the other is a hydrogen-induced amorphized material. The structure of graphite was changed by a creation of dangling bonds and deuterium was absorbed by the solid-gas reaction when the milling proceeds. The RDF(r) observed by neutron diffraction indicates two kinds of locations of deuterium atoms, that is, some deuterium atoms bond with the carbon atoms and others exist between layers of the graphite. The structure around a deuterium atom in the crystalline intermetallic compound TbFe₂D_3.8 and amorphous TbFe ₂D_x (x = 3.0,2.0) informs us not only the location of the deuterium atoms but also rearrangement of metal atoms with transformation from a crystalline to an amorphous phase. In crystalline TbFe₂D _3.8 deuterium atoms occupy the tetrahedral unit consisting of 2Tb+2Fe. On the other hand, a tetrahedral unit consisting mainly of Tb atoms becomes a stable site of D atoms in the amorphous TbFe₂D_x.