Huang, Qing. Zhen. Rush, J. J. Udovic, T. J. (NIST CENTER FOR NEUTRON RESEARCH - 610) Karmonik, C. (NIST CENTER FOR NEUTRON RESEARCH - 856)
Structural Ordering and Dynamics of LaH^d3-x^
Journal of Alloys and Compounds - September 01, 1999

The structural arrangements, optic-vibrational density of states, and diffusive motions of hydrogen (and deuterium) in the light-rare-earth hydride LaH^d3-x^ (for small hydrogen-vacancy fractions x) have been measured by netron powder diffraction, neutron vibrational spectroscopy, and quasielastic neutron scattering. Diffraction measurements of LaD^d3-x^ indicate preparation-dependent changes in structural ordering below room temperature. Vibrational spectra of both pure and isotopically mixed samples reflect the presence of significant H-H interactions involving the hydrogen located in the octahedral and tetrahedral interstices of the fcc metal sublattice. Quasielastic scattering measurements above room temperature reveal two types of hydrogen motions: a localized "rattling" motion of the octahedrally coordinated hydrogen and a long-range diffusional motion via the hydrogen vacancies. A sample color change from black to copperish-orange observed in the vicinity of the highest hydrogen concentrations likely reflects a transition from optical opacity to optical transparency, as reported for LaH^d3-x^ thin films.
Keywords: hydrogen motions , lanthanum hydride , neutron diffraction , quasielastic neutron scattering , vibrational spectroscopy
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