Thermodynamics and crystal chemistry of the RE₂MgNi₉H_12-13 (RE = La and Nd) hydrides

Chem. Met. Alloys 7 (2014) 1-8

https://doi.org/10.30970/cma7.0243

Volodymyr YARTYS, Roman DENYS

Ternary RE-Mg-Ni intermetallics are promising negative electrode materials for high-energy/high-power Nickel-Metal Hydride (Ni-MH) batteries. These compounds belong to a family of hybrid layered structures (AB₃, A₂B₇ and A₅B₁₉; A = RE, Mg; B = Ni), composed of stacked Laves-type layers, RE_2‑xMg_xNi₄, and Haucke-type RENi₅ layers. In the present study structural and hydrogen storage properties of a new compound, Nd₂MgNi₉ (PuNi₃ type; a = 4.9783(1), c = 24.1865(6) Å), are reported and compared with those of the isostructural La₂MgNi₉ intermetallic. RE₂MgNi₉ (RE = La and Nd) were found to easily form hydrides containing 13 (La) or 12 (Nd) H/f.u. As for La₂MgNi₉H₁₃, formation of the Nd₂MgNi₉H₁₂ hydride proceeds via isotropic expansion of the unit cell (a = 5.3234(2), c = 26.506(2) Å; ΔV/V = 25.3 %). In situ neutron diffraction studies of the saturated deuterides La₂MgNi₉D₁₃ and Nd₂MgNi₉D₁₂, performed at SINQ, PSI, Switzerland, revealed: (a) nearly equal distribution of H atoms within the REMgNi₄ and RENi₅ layers; (b) preferred filling of the Mg- and Ni-surrounded sites within the REMgNi₄ layers; (c) local hydrogen ordering with the H-sublattice built from stacking of MgH₆ octahedra and NiH₄ tetrahedra, indicating directional metal–hydrogen bonding. In spite of the similarity of the crystal structures and hydrogenation capacities, Nd₂MgNi₉H₁₂ shows a significantly lower thermodynamic stability (DH_des = 29 kJ/mol H₂) than La₂MgNi₉H₁₃ (DH_des = 36 kJ/mol H₂).

Hydrogen sublattice in the structures RE₂MgNi₉D_12-13deuterides, formed by stacking of the MgD₆octahedra and NiD₄tetrahedra

Keywords

Metal hydrides / Magnesium / Neodymium / Nickel / Powder neutron diffraction / Crystal structure / Hydrogen