NICKEL CLUSTER STRUCTURE DETERMINED FROM THE ADSORPTION OF MOLECULAR NITROGEN - NI-49-NI-71

The geometrical structures of nickel clusters in the size range from 4 9 to 71 atoms are studied by the chemical probe method. Saturation cov erages of molecular nitrogen are determined for each cluster and from this data specific structures are proposed (except for Ni-66 and Ni-67 ). The results indicate that icosahedral packing is the dominant struc tural configuration throughout this size range, in agreement with earl ier results based on water and ammonia adsorption. In addition, it see ms that for clusters larger than Ni-54 the excessive strain in the sur face of the 55-atom regular icosahedron often leads to rearrangements of the surface atoms to relieve that strain. Ni-55, in particular, is found to have two isomers, the regular icosahedron and a structure in which a single apex atom is displaced to the center of an opposite fac e. Ni-71 occurs as a 55-atom regular icosahedron with a 16-atom cap. T he results suggest that the atoms in the cap adopt an ABA configuratio n relative to the underlying icosahedron rather than an icosahedral ar rangement. For some clusters the saturation with nitrogen causes a sma ll degree of surface reconstruction that leads to the adsorption of ad ditional nitrogen molecules.