Amplitude weighted density of bulk and surface vibrations; ultrafine nickel particles

Experimental and computation studies of the amplitude weighted density of states for bulk and surface phonon modes of Ni crystal are presented. The experiments were performed at. 80 K by the inelastic neutron scattering technique for a micro sample consisting of highly divided Ni particles with an average size of 200 Å and macro granules of more than 1 mm in average size. The bulk and surface weighted densities of states could be distinguished using an amplification effect caused by hydrogen atoms "riding" on the surface Ni atoms. Slab calculations have been performed for the (111), (11.0) and (100) surface configurations with up to 50 layers. A bulk-like behavior of the amplitude weighted density of states is starting at the third layer whereas its behavior shows strong deviation and a pronounced dependence on the surface configuration for the first two layers. The observed amplitude weighted density of states of the bulk modes is described fairly well by the computations. A qualitative similarity between the experimental and calculated results for the surface modes is obtained when we consider the (111) face configuration. This is the evidence of a predominant (111) faceting of the microparticles. © 1990.