Phase transformation and crystal structure of La₂Ni ₇H_x studied by in situ X-ray diffraction

The phase transformation of La₂Ni₇ during hydrogenation was investigated by in situ X-ray diffraction. We found two hydride phases, La₂Ni₇H_7.1 (phase I) and La₂Ni₇H_10.8 (phase II), during the first absorption cycle. The metal sublattice of phase I was orthorhombic (space group Pbcn) with lattice parameters a = 0.50128(6) nm, b = 0.8702(1) nm, and c = 3.0377(1) nm. The sublattice for phase II was monoclinic (space group C2/c) with lattice parameters a = 0.51641(9) nm, b = 0.8960(1) nm, c = 3.1289(1) nm, and β = 90.17(1)°. The lattice parameter c increased with the hydrogen content, while a and b decreased in the formation of phase I from the alloy. Phase transformation from phase I to phase II was accompanied by isotropic expansion. The La₂Ni₄ and LaNi₅ subunit expanded by 48.9% and 6.0% in volume, respectively, during hydrogenation to phase I. They expanded an additional 14% and 5.8%, respectively, in the formation of phase II. The obtained volume expansion suggested different hydrogen distribution in the La₂Ni₄ and LaNi₅ subunit during hydrogenation.