摘要:
Three Th2Al deuterides with compositions Th2AlDx, x = 3.9, 2.7 and 2.3, were studied by powder neutron diffraction (PND). The lower deuterides were obtained by desorption under dynamic vacuum at 338 and 443 K, respectively. Th2AlD4 has been reported to violate the "rule of 2 Angstrom", however Rietveld refinement of high quality PND data for saturated Th2AlD3.9, a = 7.6360(4) Angstrom, c = 6.5150(5) Angstrom, gave an acceptable D-D separation of 1.97 Angstrom. Space group 14/mcm of the intermetallic with Al2Cu-type atomic arrangement is preserved and deuterium occupies one interstice. 16l, almost completely. For an intermediate D content, Th2AlD2.75, the deuterium sublattice becomes highly ordered giving a supercell with triple c-axis, a = 7.6796(3) Angstrom, c = 19.073(1) Angstrom. This superstructure is described in space group P (4) over bar 2m. Deuterium is situated in nine interstices, i.e. eight 4n sites and one Id site. On further deuterium removal towards the composition Th2AlD2.3, a = 7.7014(5) Angstrom, c = 6.2816(5) Angstrom, the deuterium sublattice becomes disordered. The structure is again described in space group 14/mcm. Deuterium partly occupies two different interstices, 16l and 4b. For all compositions, solely Th, tetrahedral interstices are occupied by deuterium. (C) 2000 Elsevier Science S.A. All rights reserved.