The octahedrally coordinated hydrogen (${\mathrm{H}}_{\mathit{o}}$) of the superstoichiometric rare-earth dihydrides β-${\mathrm{TbH}}_{2+\mathit{x}}$ (0.03<x<0.25) has been probed by incoherent inelastic neutron scattering. The ${\mathrm{H}}_{\mathit{o}}$ sublattice arrangements and associated optical vibrations are sensitive to the value of x. For x=0.03 at low temperature, the majority of the low-concentration o-site hydrogens are isolated in a local cubic environment and exhibit a relatively sharp vibrational density of states at 80.8 meV, in accord with the presence of triply degenerate normal modes. In contrast, for x=0.25, a broad bimodal band peaking at ∼76.7 and 83.1 meV is evident, consistent with the type of long-range order that is known to occur in the ${\mathrm{H}}_{\mathit{o}}$ sublattice at this higher concentration. Increasing the temperature above the long-range ordering transition leads to a more disordered ${\mathrm{H}}_{\mathit{o}}$ sublattice and a density of states similar to that at low values of x but broadened, most likely by the persistence of some short-range order.

• Received 4 April 1994

DOI:https://doi.org/10.1103/PhysRevB.50.7144