Distribution of hydrogen in metal hydrides studied by in situ powder neutron diffraction

Interstitial metal hydrides were studied by in situ powder neutron diffraction for their use in hydrogen storage. The main focus was the distribution of hydrogen atoms in the materials. In full hydrides of LaNi5−xAlx, hydrogen occupation depended strongly on the amount of Al. The symmetry varied from P63mc (x=0) to P6mm (x=0.1) and P6/mmm (x⩾0.25) with change in occupancy of each site. Variation in hydrogen occupation during absorption and desorption was analyzed for LaNi4.78Sn0.22. Redistribution between three tetrahedral sites was observed during desorption. La5Ni19-based hydrides with a structure consisting of stacking La2Ni4 and LaNi5 cells were also studied. Rietveld refinement of La4MgNi19D∼24 showed that hydrogen distributed almost evenly in LaMgNi4 and LaNi5 cells (0.8–1.0 H/M), which was consistent with the uniform lattice expansion of those cells. This result indicated that substitution of Mg for La changed the preferred sites for hydrogen occupation in the La2Ni4 and LaNi5 cells.