Local modes in dilute metal-hydrogen alloys

We report on measurements of the local modes of H in the transition metals V, Nb, and Ta with particular emphasis on low-concentration alloys. The excitations appear as very broad peaks in the neutron scattering spectrum even at a level of <1 at. % H, in contrast to the narrow density-of-states peaks normally expected for such interstitial defects. The lower vibration peak in ${\mathrm{NbH}}_{0.0055}$ reveals, within the α phase between 295 and 210 K, an unexpected continuous shift from 106 to 118 meV. Several possible mechanisms to explain these unusual observations are discussed and evaluated.