Tetragonal Structure of Neodymium Deuteride NdD2.27 Revisited.

Abstract The tetragonal structure of NdD 2.27 at room temperature has been investigated by a joint refinement of neutron and synchrotron powder diffraction data. In contrast to a previous report on tetragonal NdD 2.36 (P. Knappe, H. Muller and H.W. Mayer, J. Less-Common Met., 95 (1983) 323) the structure was found to be centrosymmetric (space group I 4 1 / amd , a =5.42545(9), c =10.8819(4) A, c /2 a =1.00286(6)) rather than non-centrosymmetric. The deuterium atoms fill tetrahedral interstices with nearly full occupancy (96%) and one subset of octahedral interstices with partial occupancy (72%). The occupied metal atom octahedra are smaller (Nd–Nd=3.80, 3.84 A) than the empty ones (Nd–Nd=3.84, 3.89 A). This effect is attributed to deuterium displacements towards energetically more favourable off-centre positions that allow the strengthening of metal–deuterium interactions. The metal–deuterium bond lengths and deuterium site symmetries differ significantly from those reported previously.