Characterization of the structure of LaD{sub 2.50} by neutron powder diffraction
- National Inst. of Standards and Technology, Gaithersburg, MD (United States)
Neutron powder diffraction (NPD) measurments of the rare-earth deuteride LaD{sub 2.50} were undertaken between 10 and 430 K. Rietveld refinements indicated that, above {approximately} 385 K, the LaD{sub 2.50} 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 {approximately} 385 K, the LaD{sub 2.50} structure undergoes a tetragonal distortion concomitant with the onset of deuterium long-range order (I4{sub 1}/amd) in the o sublattice, similar to previous structural results for the light-rare-earth deuterides RD{sub 2+x} for 0.3 < x < 0.5. Fully developed long-range order is established in the vicinity of 200 to 230 K. Considering the o sublattice as residing in a family of (042){sub Cubic} planes, perfect long-range order can be described as a repeating four-plane sequence comprised of two consecutive planes possessing fully occupied o sites followed by two consecutive planes possessing fully vacant o sites. This ordering is accompanied by correlated basal-plane displacements of the t-site deuterium atoms along either the a or b axes in response to repulsive interactions with the nearest-neighbor D{sub o} atoms. Moreover, the c-directed La-D{sub o} bond distances are decreased by a displacement of the La atoms toward the D{sub o} atoms.
- OSTI ID:
- 263734
- Journal Information:
- Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 1 Vol. 122; ISSN 0022-4596; ISSN JSSCBI
- Country of Publication:
- United States
- Language:
- English
Similar Records
Effect of short-range ordering on the electrical conductivity and superconducting properties of Nb{sub 6}C{sub 5} single crystals
Deuterium site occupation in the oxygen-stabilized eta-carbides Zr/sub 3/V/sub 3/OD/sub x/. I. Preparation and neutron powder diffraction