SYNTHESIS AND CRYSTAL-STRUCTURE OF TETRAGONAL LNMG(2)H(7) (LN=LA, CE), 2 LAVES PHASE HYDRIDE DERIVATIVES HAVING ORDERED HYDROGEN DISTRIBUTION

LaMg2H7, CeMg2H7 and their deuterides were synthesized by hydrogenatio n (deuteration) of binary LnMg(2) at 373 K (Ln=La) and 423 K (Ln=Ce) a nd 100 bar hydrogen (deuterium) pressure and characterized by X-ray an d neutron powder diffraction at room temperature. Their metal substruc ture represents a new tetragonal distortion variant of the cubic C15 L aves 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) Ang strom; Z=4. The hydrogen (deuterium) distribution consists of four sym metry independent, fully occupied sites having triangular [Ln(2)Mg] an d tetrahedral [Ln(2)Mg(2)] and [Mg-4] metal coordinations with distanc es 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 rati o, H/M=7/3, is the highest reported so far for Laves phase hydride der ivatives.