Synthesis and crystal structure of tetragonal LnMg(2)H(7) (Ln=La, Ce), two Laves phase hydride derivatives having ordered hydrogen distribution

LaMg2H7, CeMg2H7 and their deuterides were synthesized by hydrogenation (deuteration) of binary LnMg(2) at 373 K (Ln=La) and 423 K (Ln=Ce) and 100 bar hydrogen (deuterium) pressure and characterized by X-ray and neutron powder diffraction at room temperature. Their metal substructure represents a new tetragonal distortion variant of the cubic C15 Laves phase structure, space group P4(i)2(1)2 (No. 92); cell parameters for LaMg2H7: a=6.4054(2) Angstrom, c=9.5994(3) Angstrom; LaMg2D7: a=6.3900(2) Angstrom, c=9.5782(4) Angstrom; CeMg2H7: a=6.3663(3) Angstrom, c=9.5226(5) Angstrom; CeMg2D7: a=6.3501(3) Angstrom, c=9.4957(5) Angstrom; Z=4. The hydrogen (deuterium) distribution consists of four symmetry independent, fully occupied sites having triangular [Ln(2)Mg] and tetrahedral [Ln(2)Mg(2)] and [Mg-4] metal coordinations with distances in the range La-D=2.39-2.55 Angstrom; Ce-D=2.36-2.51 Angstrom; Mg-D=1.92-2.13 A (La), 1.91-2.18 Angstrom (Ce). The hydrogen-to-metal ratio, H/M=7/3, is the highest reported so far for Laves phase hydride derivatives.