Crystal structure of TbNiSiD1.78

A hydride of TbNiSi has been synthesized and studied by neutron and synchro tron X-ray diffraction. TbNiSiD1.78 (space group P6(3)/mmc, a = 4.03078(10) Angstrom, c = 7.97563(11) Angstrom) takes the ZrBeSi-type structure with D in 4f sites (1/3, 2/3, 0.0466(3)), surrounded by Tb3Ni in tetrahedral coor dination. The metal atoms of TbNiSiD1.78 occupy 2a, 2c and 2d sites, respec tively. Each D-filled Tb3Ni tetrahedron shares three edges (via Tb-To bonds ) and one corner (via Ni) with equivalent Tb,Ni tetrahedra. During deuterat ion a displacive transition of the metal lattice take place, which increase s the symmetry from orthorhombic (TiNiSi-type structure) to hexagonal. TbNi SiD1.78 has the geometry of an AA stacking of close-packed Tb layers. The s ame transition has earlier been observed for LaNiSn deuteride. (C) 2001 Els evier Science B.V. All rights reserved.