Synthesis, crystal structure and hydrogenation properties of the ternary compounds LaNi4Mg and NdNi4Mg

The title compounds have been prepared by induction melting and investigated with respect to structure and hydrogenation properties. Both crystallise with the cubic MgCu4Sn type structure (space group F (4) over bar 3m, LaNi4Mg: a = 7.17-7.18 Angstrom, NdNi4Mg: a = 7.09875(1) Angstrom) and absorb reversibly up to 4 hydrogen atoms per formula unit at 7-8 bar and similar to50 degreesC. Synchrotron and neutron powder diffraction data on deuterated NdNi4Mg indicate an orthorhombic lattice distortion (NdNi4MgD3.6, space group Pmn2(1), a = 5.0767(2), b = 5.4743(2), c = 7.3792(3) Angstrom, DeltaV/V = 14.6%) and three almost fully occupied deuterium sites of which two are coordinated by a trigonal metal bipyramid ([A(2)B(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 I bar hydrogen pressure, but desorbs rapidly at 80 degreesC under vacuum. Under air it decomposes by catalytic water formation. (C) 2002 Elsevier Science B.V. All rights reserved.