Synthesis, crystal structure and hydrogenation properties of the ternary compounds LaNi 4Mg and NdNi 4Mg

Abstract

The title compounds have been prepared by induction melting and investigated with respect to structure and hydrogenation properties. Both crystallise with the cubic MgCu 4Sn type structure (space group F 4 ̄ 3m, LaNi 4Mg: a=7.17–7.18 Å, NdNi 4Mg: a=7.09875(1) Å) and absorb reversibly up to 4 hydrogen atoms per formula unit at 7–8 bar and ∼50 °C. Synchrotron and neutron powder diffraction data on deuterated NdNi 4Mg indicate an orthorhombic lattice distortion (NdNi 4MgD 3.6, space group Pmn2 1, a=5.0767(2), b=5.4743(2), c=7.3792(3) Å, Δ V/ V=14.6%) and three almost fully occupied deuterium sites of which two are coordinated by a trigonal metal bipyramid ([A 2B 3] apices: A=2Nd, base: B=2Ni,Mg) and one is coordinated by a metal tetrahedron ([AB 3] A=Nd, B=3Ni). The hydride is stable at room temperature under 1 bar hydrogen pressure, but desorbs rapidly at 80 °C under vacuum. Under air it decomposes by catalytic water formation.