Structural ordering and dynamics of LaH_3-x

The structural arrangements, optic-vibrational density of states, and diffusive motions of hydrogen (and deuterium) in the light-rare-earth hydride LaH_3-x (for small hydrogen-vacancy fractions x) have been measured by neutron powder diffraction, neutron vibrational spectroscopy, and quasielastic neutron scattering. Diffraction measurements of LaD_3-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_3-x thin films.