Surface self-diffusion on an fcc crystal: An atomic view

Abstract

Migration of individual atoms self-adsorbed on different low-index planes of a face-centered cubic metal has been studied for the first time. Diffusion coefficients and activation energies for the motion of rhodium on perfect planes of the rhodium lattice, in the absence of high fields, have been derived from direct observation of atomic positions, using a field ion microscope. Correlation effects arising from interactions with other rhodium atoms are observed; these may be important even at interatomic distances greater than 7.5 Å. All diffusion parameters have therefore been derived from experiments with only a single atom on a plane. On the close-packed (111), motion is apparent even at cryogenic temperatures, with an activation energy Vm of only 3.6 kcal/mole. On the (311), (110), and (331) planes, atom movement is strictly one dimensional along close-packed [110] rows, with Vm at 12.4, 13.9, and 14.8 kcal/mole, respectively. The highest barrier, 20.2 kcal/mole, is found on the (100). On all surfaces the pre-exponential factor Do is normal. These results are at variance with previous measurements on similarly structured planes of tungsten; however, further observations of single atoms are needed to establish the exact role of surface structure.