Tetragonal structure of neodymium deuteride NdD2.27 revisited

The tetragonal structure of NdD2.27 at room temperature has been investigat ed by a joint refinement of neutron and synchrotron powder diffraction data . In contrast to a previous report on tetragonal NdD2.36 (P. Knappe, H. Mul ler and HW Mayer, J. Less-Common Met., 95 (1983) 323) the structure was fou nd to be centrosymmetric (space group 14, lamd, a == 5.42545(9), c = 10.881 9(4) Angstrom, c/2a = 1.00286(6)) rather than non-centrosymmetric. The deut erium atoms fill tetrahedral interstices with nearly full occupancy (96%) a nd one subset of octahedral interstices with partial occupancy (72%). The o ccupied metal atom octahedra are smaller (Nd-Nd = 3.80, 3.84 Angstrom) than the empty ones (Nd-Nd = 3.84, 3.89 Angstrom). This effect is attributed to deuterium displacements towards energetically more favourable off-centre p ositions that allow the strengthening of metal-deuterium interactions. The metal-deuterium bond lengths and deuterium site symmetries differ significa ntly from those reported previously. (C) 2001 Elsevier Science BV All right s reserved.