Crystal and magnetic structure of TbNiAl-based deuterides, TbNiAlD0.30 and TbNiAlD1.04, studied by neutron diffraction and synchrotron radiation

The crystal structure of the two lower deuterides TbNiAlD0.30 and TbNiAlD1.04 formed by intermetallic TbNiAl have been studied by neutron and synchrotron X-ray powder diffraction at room temperature. In addition, neutron diffraction at low temperature has been used to study the magnetic properties of TbNiAlD0.30. Both deuterides were prepared by deuterium desorption from the saturated deuteride TbNiAlD1.4. For TbNiAlD0.30, superstructure reflections in the room temperature neutron pattern suggest space group P (6) over bar 2c and a doubling of the c-axis [a=7.0433(1), c=7.7826(1) Angstrom] relative to the intermetallic compound. Deuterium occupies every second trigonal bipyramidal Tb3Ni2 interstice along c (91% occupancy), and the compound orders antiferromagnetically below T-N = 42 K. Its magnetic structure at 7 and 28 K has two terbium sublattices, both with the same propagation vector k = (1/2, 1/2, 1), but with significantly different magnetic moments of 8.0(1) and 1.3(1) mu(B) per Tb atom (at 7 K). The structure of TbNiAlD1.04 is described in space group Amm2 [a=3.8257(1), b=12.4668(5), c=7.2964(3) Angstrom; orthorhombic distortion b/c=0.986 root 3]. Three different sites are occupied by deuterium, corresponding to one trigonal bipyramidal Tb3Ni2 type (88% occupancy) and two tetrahedral Tb2NiAl-type interstices (50 and 18% occupancy). (C) 1999 Elsevier Science S.A. All rights reserved.