Crystal Structure Investigations of LaNi₂Hx (x = 0 and 3.08 with Metal Atoms in AlB₂ Units) and LaNiH1.01

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, LaNi2D3.08 with metal atoms in an AlB2-type structure, and LaNiD1.01 with metal atoms in a TlI-type structure. Although LaNi2 and LaNi2D3.08 had different crystal structures, LaNi2 contained a distorted AlB2 unit. The distorted AlB2 unit in LaNi2 became an undistorted AlB2 unit in LaNi2D3.08 due to the insertion of deuterium (hydrogen) atoms above 500 °C and 6 GPa. In the crystal structure of LaNi2D3.08, 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 NiD2–3 units for [NiD4]4– in complex hydrides with higher hydrogen storage capacities.