Crystal Structure Investigations of LaNi ₂ H _<i>x</i> ( <i>x</i> = 0 and 3.08 with Metal Atoms in AlB ₂ Units) and LaNiH _1.01

Metal hydrides have typically been investigated as hydrogen storage materials with a focus on hydrogen absorption and desorption reactions at hydrogen pressures below 10-20 MPa. However, hydrogen absorption (hydrogenation) reactions above 1 GPa have attracted attention as a route to metal hydride formations because the chemical potential of hydrogen is higher above 1 GPa. In this study, we investigated the hydrogenation reaction of La₂Ni₃ and the crystal structures of the resulting products to explore La-Ni hydride above 1 GPa. Combining synchrotron radiation X-ray and neutron diffraction, we identified LaNi₂ with an Fe₂P-type structure, LaNi₂D_3.08 with metal atoms in an AlB₂-type structure, and LaNiD_1.01 with metal atoms in a TlI-type structure. Although LaNi₂ and LaNi₂D_3.08 had different crystal structures, LaNi₂ contained a distorted AlB₂ unit. The distorted AlB₂ unit in LaNi₂ became an undistorted AlB₂ unit in LaNi₂D_3.08 due to the insertion of deuterium (hydrogen) atoms above 500 °C and 6 GPa. In the crystal structure of LaNi₂D_3.08, some of the deuterium atoms were located around Ni atoms at a distance of 1.40 Å, which is similar to Ni-D distances and atomic arrangements in intermediate NiD_2-3 units for [NiD₄]^4- in complex hydrides with higher hydrogen storage capacities.