HYDROGEN SORPTION PROPERTIES OF INTERMETALLIC TBNIAL AND CRYSTAL-STRUCTURE OF TBNIALD1.1

Hydrogenation (deuteration) of intermetallic TbNiAl with ZrNiAl type c rystal structure has been studied by differential thermal analysis and X-ray and neutron powder diffraction. The compound absorbs up to 1.4 hydrogen (deuterium) atoms per formula unit at room temperature and at mospheric pressure, and desorbs hydrogen under vacuum in a multistep r eaction at about 105 degrees C, 160 degrees C and 320 degrees C, and d euterium at about 190 degrees C and 450 degrees C. Hydrogenation (deut eration) leads to an orthorhombic distortion of the hexagonal host str ucture; TbNiAlH1.4: a(similar to c(hex))=3.7204(7); b(similar to a(hex )root 3)=17.353(2) Angstrom; c (similar to a(hex))=7.636(1) Angstrom; TbNiALD(1.1): a=3.70187(8); b=12.4011(3); c=7.6075(7) Angstrom; space group Amm2. Three sorts of deuterium sites exist of which two have alu minium in their co-ordination polyhedra: trigonal bi-pyramidal [Tb3Ni2 ] (88% occupancy) and two tetrahedral [Tb2NiAl] (67% and 10% occupancy ). The metal-deuterium bond distances are Tb-D=2.18-2.42 Angstrom, Ni- D=1.50-1.90 Angstrom, and Al-D=1.64-1.85 Angstrom. The deuterium-deute rium contact distances are all greater than 1.98 Angstrom. (C) 1998 El sevier Science S.A. All rights reserved.