CHARACTERIZATION OF THE STRUCTURE OF TBD2.25 AT 70 K BY NEUTRON POWDER DIFFRACTION

A neutron-powder-diffraction pattern of the superstoichiometric rare-e arth dideuteride TbD2.25 was measured at 70 K. Profile refinements ind icated that the TbD2.25 structure possesses I4/mmm symmetry with deute rium fully occupying the tetrahedral (t) interstices of the nominally fee Tb lattice and the excess deuterium occupying one-fourth of the oc tahedral (o) interstices with long-range order. Ideal order correspond s to the occupation of only those o sites within every fourth (042)(Cu bic) plane, in agreement with the results of a previous TbD2+x study. Moreover, in line with recent LaD2.25 results, refinements indicated t hat the o-site deuterium (D-0) ordering in TbD2.25 is accompanied by a slight tetragonal distortion, an outward expansion of the cubic ensem ble of eight t-site deuterium atoms surrounding each D-0 atom, and a d ecrease in the c-directed Tb-D-0 bond distances by a displacement of t he Tb atoms toward the D-0 atoms.