VIBRATIONAL BEHAVIOR OF NANOCRYSTALLINE NI

The phonon density of states (DOS) of nanocrystalline Ni has been inve stigated by means of inelastic neutron scattering. Compacted as well a s non-compacted samples were prepared by inert gas condensation. With respect to a reference sample of coarse grained polycrystalline Ni, th e DOS of the compacted sample is strongly enhanced at frequencies belo w 15 meV. The specific heat calculated from this DOS reproduces the ex cess specific heat well, as measured in similar samples. Attributing t entatively the observed enhancement of the DOS to the grain-boundary ( GB) component, its own GB-DOS is obtained. This GB-DOS shows an almost linear slope at low frequencies in contrast to what is found e.g. in amorphous metals. The non-compacted nanocrystalline powder shows a les s pronounced enhancement of the low energy DOS. This supports our view that the excess vibrations originate from the grain-boundaries and ar e not due to a large fraction of inner surfaces.