Crystal structure of Th 2Al deuterides

Abstract

Three Th 2Al deuterides with compositions Th 2AlD x , 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. Th 2AlD 4 has been reported to violate the “rule of 2 Å”, however Rietveld refinement of high quality PND data for saturated Th 2AlD 3.9, a=7.6260(4) Å, c=6.5150(5) Å, gave an acceptable D–D separation of 1.97 Å. Space group I4/mcm of the intermetallic with Al 2Cu-type atomic arrangement is preserved and deuterium occupies one interstice, 16 l, almost completely. For an intermediate D content, Th 2AlD 2.75, the deuterium sublattice becomes highly ordered giving a supercell with triple c-axis, a=7.6796(3) Å, c=19.073(1) Å. This superstructure is described in space group P4̄2m. Deuterium is situated in nine interstices, i.e. eight 4 n sites and one 1 d site. On further deuterium removal towards the composition Th 2AlD 2.3, a=7.7014(5) Å, c=6.2816(5) Å, the deuterium sublattice becomes disordered. The structure is again described in space group I4/mcm. Deuterium partly occupies two different interstices, 16 l and 4 b. For all compositions, solely Th 4 tetrahedral interstices are occupied by deuterium.