VIBRATIONS OF HYDROGEN AND DEUTERIUM IN SOLID-SOLUTION WITH LUTETIUM

The vibrational spectroscopy of hydrogen isotopes in alpha-phase solid solution with Lu was probed by incoherent-inelastic-neutron-scatterin g methods. Low-resolution spectra of H and D located at tetrahedral in terstices are qualitatively similar to those of other rare-earth/hydro gen alpha phases, indicating a vibrational mode along the c direction that is approximately 30% softer and significantly broader than the do ubly degenerate vibrational modes in the basal plane. High-resolution spectra of these broad c-axis vibrations reveal a temperature- and con centration-dependent line shape believed to consist of local acoustic and optic bands associated with the dynamical coupling of hydrogen ato ms paired on either side of a metal atom along the c direction. Previo us diffuse-elastic-neutron-scattering studies indicate the existence o f short-range ordering of these hydrogen pairs into c-axis-oriented ch ains possessing interchain correlations. The shape and width of these bands at low temperature near the alpha/beta-phase boundary compared t o those of the analogous Sc and Y alpha-phase systems suggest that the extent of ordering in alpha-LuH(x) is intermediate with respect to th at in the (less ordered) alpha-ScH(x) and (more ordered) alpha-YH(x) s olid solutions.