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Three Th 2 Al deuterides with compositions Th 2 AlD 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 2 AlD 4 has been reported to violate the “rule of 2 A”, however Rietveld refinement of high quality PND data for saturated Th 2 AlD 3.9 , a =7.6260(4) A, c =6.5150(5) A, gave an acceptable D–D separation of 1.97 A. Space group I4/mcm of the intermetallic with Al 2 Cu-type atomic arrangement is preserved and deuterium occupies one interstice, 16 l , almost completely. For an intermediate D content, Th 2 AlD 2.75 , the deuterium sublattice becomes highly ordered giving a supercell with triple c -axis, a =7.6796(3) A, c =19.073(1) A. This superstructure is described in space group P42m . 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 2 AlD 2.3 , a =7.7014(5) A, c =6.2816(5) A, 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. |
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