Thermodynamics and crystal chemistry of the RE2MgNi9H12-13 (RE=La and Nd) hydrides

Ternary RE-Mg-Ni intermetallics are promising negative electr ode 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, RE2-xMgxNi4, and Haucke-type RENi5 layers. In the present study structural and hydrogen storage properties of a new compound, Nd2MgNi 9 (PuNi3 type; a = 4.9783(1), c = 24.1865(6) Å), are reported and compared with th ose of the isostructural La2MgNi 9 intermetallic. RE2MgNi 9 (RE = La and Nd) were found to easily form hydrides containing 13 (La) or 12 (Nd) H/f.u. As for La2MgNi 9H13, formation of the Nd2MgNi 9H12 hydride proceeds via isotropic expansion of the unit cell ( a = 5.3234(2), c = 26.506(2) Å; ∆V/V = 25.3 %). In situ neutron diffraction studies of the saturated deuterides La 2MgNi 9D13 and Nd2MgNi 9D12, performed at SINQ, PSI, Switzerland, revealed: (a) nearly equal distribution of H atoms within the REMgNi 4 and RENi5 layers; (b) preferred filling of the Mgand Ni-surrounded sites within the REMgNi 4 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, Nd2MgNi 9H12 shows a significantly lower thermodynamic stabilit y (∆Hdes = 29 kJ/mol H2) than La2MgNi 9H13 (∆Hdes = 36 kJ/mol H2).