Hydrogen vibrational modes and anisotropic potential inα-ScHx

The hydrogen vibrational spectra of single-crystal α-${\mathrm{ScH}}_{\mathrm{x}}$ (x=0.05, 0.16, and 0.25) have been measured using incoherent-inelastic-neutron-scattering methods. Alignment of the momentum-transfer vector Q either perpendicular or parallel to the c axis permitted the selective enhancement in scattering sensitivity of the different normal modes of the tetrahedrally bound hydrogen. The results show that the c-axis vibration peak (at ca. 103 meV) is over 30% lower in energy than the doubly degenerate basal-plane modes (at ca. 148 meV) and possesses considerable anharmonicity, as reflected by the ratio (1.76) of the second- to first-excited-state energies. High-resolution neutron spectra for the c-axis vibrations revealed a peak at all hydrogen concentrations that is much broader (ca. 14 meV full width at half maximum intrinsic linewidth, x=0.25) than the instrumental resolution, yet devoid of any resolved fine structure indicative of a distinct local-mode splitting as was observed previously for α-${\mathrm{YH}}_{\mathrm{x}}$. This suggests that although local ordering via hydrogen pairing across a Sc atom exists within α-${\mathrm{ScH}}_{\mathrm{x}}$ at low temperature, ordering of the hydrogen pairs themselves along the c axis is less extensive than in α-${\mathrm{YH}}_{\mathrm{x}}$.