Hydrogen vibration and phonon dispersion inα-ScDxsystems

Abstract

The phonon dispersion relations in ${\mathrm{ScD}}_{\mathrm{x}}$ were determined by inelastic neutron scattering for x=0.19 and x=0.33. Predominantly, an increase of the acoustic-phonon frequencies is observed after hydrogen absorption. The hardening is most pronounced for the transverse-acoustic TA(002) phonon branch which in addition shows a shallow dip near q=(2/3 especially at higher hydrogen concentration. An investigation of the deuterium local mode showed a strong vibrational anisotropy similar to recent results in ${\mathrm{YD}}_{0.19}$ and ${\mathrm{LuD}}_{0.19}$. However, in contrast to ${\mathrm{YD}}_{0.19}$ and ${\mathrm{Lu}}_{0.19}$ no peak splitting was observed for the deuterium local mode polarized in the hexagonal direction especially at low temperatures where the formation of D pairs is fully developed. This indicates that a direct D-D interaction is not essential for the pair formation.