NEUTRON SPECTROSCOPIC EVIDENCE OF CONCENTRATION-DEPENDENT HYDROGEN ORDERING IN THE OCTAHEDRAL SUBLATTICE OF BETA-TBH2+X

The octahedrally coordinated hydrogen (H0) of the superstoichiometric rare-earth dihydrides beta-TbH2+x (0.03 < x < 0.25) has been probed by incoherent inelastic neutron scattering. The H0 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 approximately 76.7 and 83.1 meV is evident, consistent with the type of long-range order that is known to occur in the H0 sublattice at this higher concentration. Increasing the temperature above the long-range ordering transition leads to a more disordered H0 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.