THE AMPLITUDE WEIGHTED DENSITY OF BULK AND SURFACE VIBRATIONS IN A WELL DISPERSED NICKEL

The results are given of the experimental study and theoretical calculations for the spectra of the amplitude weighted density of states of the bulk and surface phonon modes in a Ni crystal. The neutron inelastic scattering spectra at 80 K are measured for well dispersed Ni with the mean particle size of 200 angstrom and for Ni macrograins of 1 mm in diameter. Due to enhancing the surface vibrations in Ni particles via hydrogen atoms of water adsorbed at its surface the amplitude weighted densities of bulk and surface states are obtained separately. The same characteristics are calculated in the slab approximation for the 30 and 50 layer slabs and for three crystallographic configurations of their planes as (111), (100) and (110). It is found that the amplitude weighted density of states varies slightly from the third layer and deep far into the crystal and it gains the features typical for the density of bulk phonons. As the density of states for the atoms of the first two layers depends on the surface structure and details of the force fields near it. The experimental amplitude weighted density of Ni bulk phonons is well described by the calculated value. The experimental results for the surface modes are in a qualitative agreement with the calculations for the (111) surface configuration that favours the faceting of the Ni particle surface with the (111) plane, predominantly.