Phase Structure of (La, Mg, Nd)_5Ni_(19) Hydrogen Storage Alloy

摘要

The crystal structure of (La, Mg, Nd)_5Ni_(19) alloy has been investigated using powder x-ray diffraction. A Rietveld refinement is used to determine the phase structures for tying to find out the correlations of lattices among the phases in this hydrogen storage alloy. The results show that the alloy consists of three phases, which correspond to (La, Mg)_3Nd_2Ni_(19) with a Pr_5Co(19)-type structure, La(Mg, Nd)Ni_7 phase with a Ce_2Ni_7-type structure and (La, Mg)_3Nd_2Ni_(19) with a Ce_5Co_(19)-type structure. Mg and Nd substitute for La atom at the 4f position for Pr_5Co_(19)-type and Ce_2Ni_7-type structures and the 6c position for Ce_5Co_(19)-type structure, respectively. It induces a good lattice match along the a-axisand b-axis among the three phases. The three kinds of structures might form a rod-like shape structural unit by stacking along the c-axis. This phase constitution seems to be beneficial for the cyclic stability of the hydrogen storage alloys.