NEUTRON-POWDER-DIFFRACTION STUDY OF THE LONG-RANGE ORDER IN THE OCTAHEDRAL SUBLATTICE OF LAD2.25

Neutron-powder-diffraction patterns of the superstoichiometric rare-earth dideuteride LaD2.25 were measured between 15 and 400 K. Profile refinements indicated that, above ∼345 K, the LaD2.25 structure is cubic (Fm3m) with deuterium fully occupying the tetrahedral (t) interstices of the fcc La lattice and the excess deuterium occupying a portion of the octahedral (o) interstices with full statistical disorder. As the temperature is decreased below ∼345 K, the LaD2.25 structure undergoes a transformation to tetragonal symmetry concomitant with the onset of deuterium long-range order (I4/mmm) in the o sublattice. Fully developed long-range order is established near 230 K and ideally corresponds to the occupation of only those o sites within every fourth (042)cubic plane, in line with that reported for other rare-earth-deuterium systems possessing similar D/metal stoichiometric ratios. This ordering is accompanied by an outward expansion of the cubic ensemble of eight t-site deuterium atoms surrounding each o-site deuterium (Do) atom. Moreover, the c-directed La-Do bond distances are decreased by a displacement of the La atoms toward the Do atoms. © 1995 The American Physical Society.