We studied the vibrational modes of H-doped nanocrystalline Pd $\mathrm{H}/\mathrm{Pd}\le 0.04)$ by neutron spectroscopy, using the small H content as a probe for a separate determination of the vibrations of Pd atoms at surfaces and/or grain boundaries. We find that these atoms are responsible for the additional low-frequency modes which are characteristic for nanocrystalline materials, and that their local vibrational density of states is essentially linear in frequency. The Pd atoms within grains do not noticeably contribute to the additional low-frequency modes.

• Received 10 November 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.1449