Characterization of the structure of TbD2.25 at 70 K by neutron powder diffraction

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