and temperature-dependent phase transitions known to involve tetragonal lattice distortion and o-site hydrogen Neutron powder diffraction (NPD) measurements of the rare-earth deuteride LaD 2.50 were undertaken between 10 and (H o ) ordering. For example, we recently performed a neu-430 K. Rietveld refinements indicated that, above Θ385 K, the tron powder diffraction (NPD) study (5) of LaD 2.25 con-LaD 2.50 structure is cubic (Fm3m) with deuterium fully occu-firming the presence of a high-temperature cubic Fm3m pying the tetrahedral (t) interstices of the fcc La lattice and structure (with full D o disorder) which undergoes tetragothe excess deuterium occupying a portion of the octahedral (o) nal distortion below Θ345 K to form a low-temperature interstices with full statistical disorder. As the temperature is I4/mmm structure (with long-range D o ordering). Similar decreased below Θ385 K, the LaD 2.50 structure undergoes a structural behavior was also observed for CeD 2.26 (6) and tetragonal distortion concomitant with the onset of deuterium the heavy-rare-earth deuterides TbD 2Ο©x for 0.1 Υ
x Υ
0.25 long-range order (I4 1 /amd) in the o sublattice, similar to previ- . Considering all o sites in the fcc metal lattice as ous structural results for the light-rare-earth deuterides RD 2Ψx residing in a family of ( ) C planes, perfect I4/mmm order for 0.3 Ο½ x Ο½ 0.5. Fully developed long-range order is estabcorresponded to full o-site occupation for every fourth lished in the vicinity of 200 to 230 K. Considering the o sublat-(042) plane with all other (042) planes empty. (In this tice as residing in a family of (042) Cubic planes, perfect longpaper, cubic indices are designated by either the subscript range order can be described as a repeating four-plane sequence comprised of two consecutive planes possessing fully occupied C or no subscript; tetragonal indices are designated by the o sites followed by two consecutive planes possessing fully vasubscript T). cant o sites. This ordering is accompanied by correlated basal-Various NPD studies have indicated that a different plane displacements of the t-site deuterium atoms along either RD 2Ο©x ordered structure exists for Θ0.3 Ο½ x Υ
0.5. Roomthe a or b axes in response to repulsive interactions with the temperature NPD measurements (9) of LaD 2.30 , PrD 2.37 , nearest-neighbor D o atoms. Moreover, the c-directed La-D o CeD 2.29 , and CeD 2.51 pointed to the presence of D o longbond distances are decreased by a displacement of the La atoms range order with tetragonal I4 1 md symmetry for the former toward the D o atoms.