ELECTRONIC AND MAGNETIC-PROPERTIES OF A CARBON-ATOM CHEMISORBED ON MODEL CLUSTERS SIMULATING THE (100) SURFACE OF NICKEL

Both spin-polarized and unpolarized linear combinations of Gaussian-type orbitals-model core potential-local spin density (LCGTO-MCP-LSD) calculations have been performed for clusters representing the three possible high symmetry chemisorption sites for carbon on the (100) surface of nickel. We found that the most stable chemisorption site is the fourfold hollow, in agreement with the experimental evidence. For this site, the computed equilibrium NiC distances are 1.79 and 1.77 Å at the spin-polarized and unpolarized levels, very close to the most recent experimental measurements. The calculated spin-polarized vibrational frequency perpendicular to the surface is found to be 407 cm -1 (410 cm-1 expt). The values of the binding energy are 11.5 and 11.8 eV at polarized and unpolarized levels, respectively (∼7 eV, expt); the carbon atom is strongly bound, essentially by a triple bond formed by interaction of the px, py, and pz orbitals of carbon with, primarily, the d orbitals of the four nearby surface nickel atoms. The effect of carbon chemisorption on the nickel magnetism has also been studied. The addition of the carbon atom reduces the spin magnetic moment of pure nickel by 2 or 4 μB depending on which of the two nearly degenerate nickel cluster states is taken as reference. The reduction of atomic spin magnetic moments is clearly larger on the 4 nickel atoms nearest to the carbon. The global and local (atomic) reduction in spin magnetic moments originate from some up-spin d density of states being pushed above E F, through antibonding interactions with the carbon 2p orbitals, and hence emptied. © 1990 American Institute of Physics.